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2021 43 Borja Muñiz Pardos Fuerza, potencia y entrenamiento vibratorio en nadadores adolescentes Departamento Director/es Fisiatría y Enfermería Vicente Rodríguez, Germán © Universidad de Zaragoza Servicio de Publicaciones ISSN 2254-7606 Borja Muñiz Pardos FUERZA, POTENCIA Y ENTRENAMIENTO VIBRATORIO EN NADADORES ADOLESCENTES Director/es Fisiatría y Enfermería Vicente Rodríguez, Germán Tesis Doctoral Autor 2019 UNIVERSIDAD DE ZARAGOZA Repositorio de la Universidad de Zaragoza – Zaguan http://zaguan.unizar.es Tesis Doctoral Internacional [International Doctoral Thesis] Curso Académico 2018-2019 FUERZA, POTENCIA Y ENTRENAMIENTO VIBRATORIO EN NADADORES ADOLESCENTES STRENGTH, POWER AND WHOLE-BODY VIBRATION TRAINING IN ADOLESCENT SWIMMERS Borja Muñiz Pardos Departamento de Fisiatría y Enfermería Facultad de Ciencias de la Salud y del Deporte Universidad de Zaragoza Strength, power and whole-body vibration training in adolescent swimmers - 2 - - 3 - FUERZA, POTENCIA Y ENTRENAMIENTO VIBRATORIO EN NADADORES ADOLESCENTES STRENGTH, POWER AND WHOLE-BODY VIBRATION TRAINING IN ADOLESCENT SWIMMERS Borja Muñiz Pardos Strength, power and whole-body vibration training in adolescent swimmers - 4 - - 5 - A mis directores y supervisores, Germán, José Antonio, Bruton y Yannis, y a mis compañeros del grupo GENUD, ejemplos a seguir A mi familia, especialmente a mi madre y a mis tíos Alejandro, Enrique, Luís, Andrés y María Muñiz, por un apoyo incondicional y por hacer esto posible A mi padre, mi fuerza Strength, power and whole-body vibration training in adolescent swimmers - 12 - Strength, power and whole-body vibration training in adolescent swimmers - 13 - Publicaciones incluidas en la Tesis Doctoral [Doctoral Thesis publications] Chapter 4.1 Muniz-Pardos, B, Gómez-Bruton, A, Matute-Llorente, A, Gonzalez-Aguero, A, Gomez-Cabello, A, Gonzalo-Skok, O, José A. Casajús, Vicente-Rodríguez G. Nonspecific resistance training and swimming performance: Strength or Power? A systematic review. J Strength Cond Res. [Accepted for publication]. Chapter 4.2 Muniz-Pardos, B, Gómez-Bruton, A, Matute-Llorente, A, Gonzalez-Aguero, A, Gomez-Cabello, A, Gonzalo-Skok, O, José A. Casajús, Vicente-Rodríguez G. Swimspecific resistance training and swimming performance: A systematic review. J Strength Cond Res. [Accepted for publication]. Chapter 4.3 Muniz-Pardos B, Gómez-Bruton A, Matute-Llorente A, González-Agero A, Olmedillas H, Gómez-Cabello A, Sutehall S, Pitsiladis Y, Casajús JA, VicenteRodríguez G. Lower-body strength and power contribution in sprint swimming performance in trained adolescent swimmers. J Strength Cond Res. [Submitted]. Chapter 4.4 Muniz-Pardos B, Gómez-Bruton A, Matute-Llorente A, González-Agüero A, GómezCabello A, Casajús JA, Vicente-Rodríguez G. Long-term effects of whole-body vibration in trained adolescent swimmers. Does it increase strength, power or swimming performance? Int J Sports Physiol Perf. [Submitted]. Chapter 4.5 Muniz-Pardos B, Lozano-Berges G, Marín-Puyalto J, González-Agero A, VicenteRodríguez G, Casajús JA, Garatachea N. Validity and reliability of an optoelectronic system to measure movement velocity during bench press and half squat in a Smith machine. Proc Inst Mech Eng P J Sport Eng Technol. [Submitted]. Strength, power and whole-body vibration training in adolescent swimmers - 14 - Strength, power and whole-body vibration training in adolescent swimmers - 15 - Tabla de contenidos Proyecto de Investigación ........................................................................................... - 19 - Resumen General ........................................................................................................ - 21 - Listado de Abreviaturas ............................................................................................. - 25 - Capítulo 1. Introducción y Justificación ................................................................... - 27 - 1.1. Natación ......................................................................................................................... - 29 - 1.1.1. Historia de la natación ............................................................................................ - 29 - 1.1.2. Características del medio acuático ......................................................................... - 31 - 1.2. Bases del entrenamiento de fuerza en el deporte .......................................................... - 37 - 1.2.1. Fundamentos biológicos .......................................................................................... - 37 - 1.2.2. Concepto de fuerza .................................................................................................. - 39 - 1.2.3. Manifestaciones de la fuerza ................................................................................... - 40 - 1.2.4. Evaluación de la fuerza ........................................................................................... - 42 - 1.3. La importancia de la fuerza en natación ...................................................................... - 44 - 1.3.1. Fuerza de las extremidades superiores y rendimiento en natación ........................ - 45 - 1.3.2. Fuerza de las extremidades inferiores y rendimiento en natación .......................... - 45 - 1.4. Entrenamiento vibratorio en el ámbito deportivo ......................................................... - 47 - Capítulo 2. Hipótesis y Objetivos .............................................................................. - 51 - Capítulo 3. Material y Métodos ................................................................................. - 57 - 3.1 Comité de ética ................................................................................................................ - 58 - 3.2 Características de la muestra y diseño del proyecto ...................................................... - 59 - 3.3. Pruebas y valoraciones .................................................................................................. - 60 - 3.3.1. Valoración de la fuerza y potencia .......................................................................... - 60 - 3.3.2. Valoración del rendimiento en natación ................................................................. - 62 - 3.3.3. Otros tests ................................................................................................................ - 62 - 3.4. Programa de entrenamiento vibratorio ......................................................................... - 64 - 3.4.1. Equipamiento ........................................................................................................... - 64 - 3.4.2. Diseño del estudio ................................................................................................... - 64 - 3.5. Análisis estadísticos........................................................................................................ - 66 - Capítulo 4. Resultados y Discusión [in English] ............................................................. - 69 - Capítulo 4.1. Entrenamiento de fuerza no específico y rendimiento en natación.............. - 73 - Capítulo 4.2. Entrenamiento de fuerza específico y rendimiento en natación ................. - 103 - Capítulo 4.3. Relación entre la fuerza y el rendimiento en nadadores adolescentes ....... - 131 - Capítulo 4.4. Efectos del entrenamiento vibratorio en la fuerza, la potencia y el rendimiento de nadadores adolescentes .................................................................................................. - 155 - Strength, power and whole-body vibration training in adolescent swimmers - 16 - Capítulo 4.5. Validación y fiabilidad de un sistema optoelectrónico para determinar la velocidad de desplazamiento en ejercicios de fuerza ......................................................... - 175 - Capítulo 5. Conclusiones y Aportaciones principales ............................................ - 197 - Referencias................................................................................................................. - 205 - Sobre el doctorando .................................................................................................. - 223 - Publicaciones del autor ............................................................................................. - 225 - Apéndice..................................................................................................................... - 227 - Agradecimientos [Acknowledgements] ................................................................... - 229 - Anexos ........................................................................................................................ - 237 - Strength, power and whole-body vibration training in adolescent swimmers - 17 - Table of contents Research Project ......................................................................................................... - 20 - General Abstract ......................................................................................................... - 23 - Chapter 1. Introduction and Justification ................................................................ - 27 - 1.1. Swimming ....................................................................................................................... - 29 - 1.1.1. History of swimming ................................................................................................ - 29 - 1.1.2. Characteristics of the aquatic environment............................................................. - 31 - 1.2. Basis of strength training in sport ................................................................................. - 37 - 1.2.1 Biological foundation ............................................................................................... - 37 - 1.2.2. Concept of strength .................................................................................................. - 39 - 1.2.3. Types of strength ...................................................................................................... - 40 - 1.2.4. Evaluation of strength ............................................................................................. - 42 - 1.3. The importance of strength in swimming ..................................................................... - 44 - 1.3.1 Upper-body strength and swimming performance ................................................... - 45 - 1.3.2. Lower-body strength and swimming performance .................................................. - 45 - 1.4. Whole-body vibration training in sport ......................................................................... - 47 - Chapter 2. Hypothesis and Aims ............................................................................... - 51 - Chapter 3. Material and Methods ............................................................................. - 57 - 3.1 Ethics committee .............................................................................................................. - 58 - 3.2 Sample characteristics and study design ......................................................................... - 59 - 3.3. Assessments ..................................................................................................................... - 60 - 3.3.1. Dry-land strength and power assessments .............................................................. - 60 - 3.3.2. Swimming performance records .............................................................................. - 62 - 3.3.3. Other tests ................................................................................................................ - 62 - 3.4. Whole-body vibration program ...................................................................................... - 64 - 3.4.1. Equipment ................................................................................................................ - 64 - 3.4.2. Study design ............................................................................................................. - 64 - 3.5. Statistical analyses ......................................................................................................... - 66 - Chapter 4. Results and Discussion [in English]........................................................ - 69 - Chapter 4.1. Non-specific resistance training and swimming performance ...................... - 73 - Chapter 4.2. Swim-specific resistance training and swimming performance .................. - 103 - Chapter 4.3. Relationship between strength, power and swimming performance in adolescent swimmers ............................................................................................................................. - 131 - Chapter 4.4. Long-term effects of whole-body vibration training on strength, power and swimming performance in adolescent swimmers .............................................................. - 155 - Strength, power and whole-body vibration training in adolescent swimmers - 18 - Chapter 4.5. Validity and reliability of an optoelectronic system to measure movement velocity during resistance exercises .................................................................................... - 175 - Chapter 5. Conclusions and Main contribution of the Thesis .............................. - 197 - References .................................................................................................................. - 205 - About the PhD student ............................................................................................. - 224 - Publications of the PhD student............................................................................... - 225 - Appendix .................................................................................................................... - 227 - Acknowledgements [Section in Spanish] ................................................................ - 229 - Annexes ...................................................................................................................... - 237 - Strength, power and whole-body vibration training in adolescent swimmers - 19 - Proyecto de Investigación La Tesis Doctoral que se presenta a continuación se enmarca dentro del siguiente proyecto de investigación: “Repercusión del entrenamiento y la práctica de la natación sobre el desarrollo metabólico y estructural del hueso en crecimiento. Beneficios de la incorporación de entrenamiento pliométrico o vibratorio. (Acrónimo: RENACIMIENTO)” Este proyecto nacional de 3 años de duración fue financiado por el Ministerio de Ciencia e Innovación (DEP2011-29093), cuyo investigador principal fue Germán Vicente Rodríguez. Strength, power and whole-body vibration training in adolescent swimmers - 20 - Research Project The present Doctoral Thesis is within the frame of the research project: “Swimming repercussion on metabolic and structural bone development; benefits of the incorporation of whole body vibration or plyometric training: the RENACIMIENTO project.” This three-year national project was funded by the Spanish Ministry of Science and Innovation (DEP2011-29093), whose principal researcher was Germán Vicente Rodríguez. Strength, power and whole-body vibration training in adolescent swimmers - 21 - Resumen General La fuerza y la potencia de las extremidades inferiores han demostrado tener un papel fundamental en el rendimiento en nadadores, principalmente durante las fases la salida y los virajes. No obstante, existe discrepancia en cuanto al tipo de variables de fuerza y potencia que mejor podrían explicar el rendimiento en pruebas de natación, así como al tipo de entrenamiento más adecuado para mejorar la fuerza y el rendimiento en nadadores. Por otro lado, el entrenamiento vibratorio de todo el cuerpo parece ser beneficioso para la mejora de la fuerza y potencia de las extremidades inferiores, aunque los efectos crónicos de este método en la fuerza y el rendimiento deportivo, a día de hoy, todavía no han sido examinados en nadadores. El principal objetivo de esta tesis doctoral es analizar la importancia de la fuerza en el rendimiento en natación, así como mostrar los efectos crónicos del entrenamiento vibratorio en la fuerza y el rendimiento en nadadores. En primer lugar, los capítulos 4.1 y 4.2 tratan de contextualizar y mostrar el estado actual de la literatura científica en cuanto a las asociaciones entre la fuerza y el rendimiento, así como los tipos de entrenamiento de fuerza existentes en natación. El capítulo 4.3 utiliza una muestra de 44 nadadores adolescentes para examinar la asociación entre variables de fuerza y potencia con el rendimiento en 50 y 100 m. En el capítulo 4.4 se exponen los resultados de un entrenamiento vibratorio de todo el cuerpo de 6 meses (3 veces por semana) en 20 nadadores adolescentes, mientras que un grupo control compuesto por 17 nadadores proseguían con su rutina de entrenamiento acuático. Por último, en el capítulo 4.5 se muestra un estudio de validación y fiabilidad de un sistema optoelectrónico para medir la velocidad de desplazamiento durante el ejercicios de fuerza con 22 sujetos con experiencia en entrenamiento de fuerza. Los resultados principales de la presente Tesis Doctoral muestran que la potencia de las extremidades inferiores guarda una gran relación con el rendimiento en competición de nadadores chicos adolescentes. En nuestro estudio longitudinal, el entrenamiento vibratorio de 6 meses no produjo efectos en la fuerza, potencia o rendimiento deportivo en nadadores. Finalmente, el aparato optoelectrónico demostró ser un sistema válido y fiable para medir la velocidad de desplazamiento durante ejercicios de fuerza. Muñiz-Pardos, B. International Doctoral Thesis - 28 - 1 Introduction - 29 - 1.1. Natación 1.1.1. Historia de la natación La Real Academia Española define el término “natación” como “la acción y efecto de nadar” 1, definiendo “nadar” como “la acción por parte de personas o animales de trasladarse en el agua, ayudándose de los movimientos necesarios, y sin tocar el suelo ni otro apoyo” 1. Sin embargo, entendiendo la natación dentro del ámbito deportivo, Arellano (1992) matizó: “práctica de un deporte olímpico reglamentado, con el objetivo de desplazarse de la forma más rápida posible en el agua, gracias a las fuerzas propulsivas que genera con los movimientos de los miembros superiores, inferiores y cuerpo, que le permiten vencer las resistencias que se oponen al avance del nadador” 2. Counsilman y Counsilman (1994) 3 citaban en su libro: “Los peces y otros animales marinos están equipados con aletas que son relativamente pequeñas en comparación con el tamaño de su cuerpo, los humanos tenemos unos miembros superiores e inferiores largos y delgados que proporcionan muy poca superficie con la que interactuar con el agua”. Esta cita pone de manifiesto el pobre diseño del cuerpo del ser humano para nadar de forma eficiente. Los primeros registros históricos relativos a la práctica de la natación por el ser humano se remontan a las pinturas rupestres realizadas sobre los acantilados de Gilf Kebir (Egipto) en el año ~ 5.000 a.C., también llamadas “cuevas de los nadadores” 4. Sin embargo, la práctica de la natación en este contexto se basaba en una acción de mera supervivencia, destacando la caza de alimentos, la búsqueda de materias primas y la fuga ante enemigos. No es hasta el s. XIX cuando la natación se populariza extensivamente como práctica deportiva. Este acontecimiento fue causado principalmente por las primeras Muñiz-Pardos, B. International Doctoral Thesis - 30 - grandes travesías a nado (especialmente la del Canal de la Mancha), por el papel que los militares le otorgan a la natación durante su preparación física, y por la aparición de los primeros clubes, piscinas y competiciones regladas 5. En referencia a este último acontecimiento, en el año 1837 tuvo lugar la fundación de la “National Swimming Association” en Inglaterra, lo que promovió la construcción de la primera piscina de la era moderna en 1845 5. La consolidación definitiva de la natación como práctica deportiva a nivel internacional culminó con la celebración de los primeros Juegos Olímpicos de la Era Moderna en Atenas (1896), donde se celebró una competición en mar abierto de 1200 m a nado libre. La fundación de la FINA se realizó tras la celebración de la IV edición de los Juegos Olímpicos de 1908 en Londres. El s. XX se caracterizó por una expansión masiva en el número de practicantes, piscinas, clubes y federaciones en los cinco continentes. Esta popularización a lo largo del último siglo se refleja claramente en la actual participación de 209 miembros federativos nacionales registrados por la FINA en enero del año 2018 6 (Figura 1). Figura 1. Mapa de los miembros nacionales de la FINA en relación a su confederación. Introduction - 31 - Por último, cabe destacar que durante esta etapa histórica también se extendió la noción del rendimiento deportivo en el deporte, entendido como la optimización de las capacidades físicas de una persona para maximizar el gesto deportivo. De hecho, el concepto de récord per se es relativamente moderno, ya que hasta 1870 solo se tenía en cuenta la clasificación; poco importaba el tiempo empleado en recorrer una determinada distancia, por lo que las marcas eran ignoradas. La palabra récord aparece por primera vez en el diccionario Oxford English Dictionary en 1880: “a record is a performance or occurrence remarkable among, or going beyond, others of the same kind: especially, the best recorded achievement in any competitive sport”. La primera utilización de la palabra récord aparece precisamente en una obra sobre natación en 1883 7. 1.1.2 Características del medio acuático Desde un punto de vista evolutivo, las características específicas del agua no permiten al ser humano desplazarse de forma eficiente en el medio acuoso 5. Mientras que la carrera a pie es un tipo de locomoción de naturaleza filogenética (i.e., medio en el que ha evolucionado), la natación es de naturaleza ontogenética (i.e., depende del aprendizaje del individuo) 8. Este importante matiz explica las diferentes progresiones de los records del mundo entre la carrera y la natación a mediados del s. XX (Figura 2). Como sugiere Miyashita apoyándose en esta figura 9, mientras que las mejoras en la carrera a pie se debían fundamentalmente a la mejora de la condición física, las mejoras en los records de natación se debieron a los continuos perfeccionamientos y modificaciones de la técnica de nado durante el último siglo, con el estilo de crol consolidándose como estilo libre más rápido en la década de los 20 y 30 del s. XX 5. Muñiz-Pardos, B. International Doctoral Thesis - 32 - Figura 2. Evolución de los records mundiales en la milla (carrera a pie) y en los 400 m libres de natación (Miyashita y col., 1981). La natación competitiva, al igual que otros deportes como el atletismo, el ciclismo o el patinaje de velocidad, se caracteriza por la capacidad del deportista de completar una distancia determinada en el menor tiempo posible. Sin embargo, la peculiaridad de que sea una práctica desarrollada en el medio acuático, dota a este deporte de unas características únicas 10. En primer lugar, el hecho de que el agua sea ~ 800 veces más densa que el aire incrementa considerablemente el gasto energético para desplazarse a través de ella 11. En segundo lugar, el nadador se encuentra inmerso en un medio inestable que provoca que solo una parte de la fuerza aplicada se traduzca en fuerza propulsiva 11, que es la fuerza que posibilita el avance del nadador en el agua. Mikel Izquierdo destaca la existencia de cuatro fuerzas principales que gobiernan el nado del ser humano 12 (Figura 3): Introduction - 33 - - Fuerza peso. Se trata de la fuerza vertical y descendente debida al peso del individuo. Esta fuerza junto con el empuje hidrostático determinan la flotabilidad. - Empuje hidrostático. Esta fuerza se basa en el principio de Arquímedes, según el cual “todo cuerpo sumergido en un fluido experimenta un empuje vertical (dirección) y ascendente (sentido) igual al peso del volumen de fluido desalojado”. Considerando que la densidad del agua es ~ 1000 kg·m-3, y que la densidad de los tejidos corporales, es mayor (hueso= ~ 1400-1800 kg·m-3; músculo, ligamentos y tendones= ~ 1020-1050 kg·m-3), el cuerpo humano debería hundirse siempre 12. Sin embargo, la baja densidad del aire en pulmones y vías respiratorias (~ 1.2 kg·m-3) permiten la flotación del cuerpo en el momento de la inspiración, por lo que la habilidad de flotación pasiva del individuo depende de su capacidad para expandir la caja torácica 12. - Fuerza propulsiva. La propulsión humana en el medio acuático durante el estilo libre (estilo de crol) proviene fundamentalmente de las fuerzas propulsivas de las extremidades superiores 13. La fuerza propulsiva que realiza el nadador con sus Figura 3. Fuerzas que intervienen durante el nado (Izquierdo, 2008). Muñiz-Pardos, B. International Doctoral Thesis - 34 - extremidades superiores es la suma vectorial de dos fuerzas cuya correcta interacción resultará en un eficiente desplazamiento hacia delante. Estas dos fuerzas responden a dos principios físicos fundamentales: la fuerza de arrastre y la fuerza de sustentación. La fuerza de arrastre se explica principalmente por el Principio de Acción y Reacción. La acción de empuje que realiza la mano a través del agua hacia atrás acelera y mueve el agua en dirección opuesta a la que uno desea moverse. Esta fuerza estará determinada por la resistencia total de la mano y el brazo contra el agua 2. Por otro lado, la fuerza de sustentación, explicada por el Principio de Bernoulli, es aquella que utiliza una hélice para propulsarse, cuyas palas no empujan agua hacia atrás sino que giran en un plano perpendicular a la dirección del movimiento, encontrando continuamente agua en reposo 14. En natación, esta fuerza se ve reflejada cuando la mano del nadador corta el agua generando una zona de altas presiones en la palma y otra zona de bajas presiones en el dorso de la mano. Esta diferencia de presiones genera una mayor velocidad en el flujo de agua que se desliza por el dorso de la mano resultando en una mayor fuerza propulsiva. - Fuerza resistencia. Durante la locomoción en el medio acuático, el nadador desplaza el agua que se encuentra en su camino, sufriendo una fuerza que se opone a su avance llamada fuerza resistencia o fuerza hidrostática. El nadador se enfrenta a tres tipos de fuerzas: la resistencia por fricción, la resistencia de presión y la resistencia por olas. La resistencia por fricción es la menos importante de las tres y depende de la cantidad de superficie en contacto con el agua, la viscosidad del agua, la fricción con la piel, pelo y bañador, y de la velocidad de nado. La resistencia de presión es la más influyente de las tres y se debe a que, durante el nado, se genera una zona de altas presiones frente al nadador y una zona de bajas presiones detrás Introduction - 35 - del mismo. Este gradiente de presiones genera unos flujos turbulentos que frenan el avance del nadador 15. Por último, la resistencia por olas es un tipo de fuerza que solo se produce cuando el cuerpo se mueve sobre la superficie del agua (entre el agua y el aire). A altas velocidades puede llegar a ser la resistencia más importante, ya que el choque del nadador con la masa de agua que generan las olas es proporcional a la velocidad de nado 16. Teniendo en cuenta las fuerzas externas a las que se enfrenta el nadador en este singular entorno, el rendimiento en competición estará influenciado por una compleja interacción de factores fisiológicos, morfológicos, neuromusculares, biomecánicos y técnicos en el nadador 17. Sin embargo, este apartado está focalizado en aquellos factores más determinantes para el rendimiento en pruebas de natación relacionadas con la fuerza, así como las características principales del entrenamiento vibratorio. De esta manera, se contextualizará de una forma más ajustada a los objetivos y contenidos de la presente Tesis Doctoral. Muñiz-Pardos, B. International Doctoral Thesis - 36 - Introduction - 37 - 1.2. Bases del entrenamiento de fuerza en el deporte 1.2.1 Fundamentos biológicos Una de las características básicas de los animales es que pueden moverse, y para generar movimiento es necesaria la producción de fuerza muscular. Los músculos se definen como máquinas moleculares capaces de convertir energía química en energía mecánica 18. Los mecanismos a partir de los cuales tiene lugar esta conversión son fruto de un complejo sistema cuyo funcionamiento sigue siendo una de las mayores cuestiones sin resolver en el área de la biología 18. El cuerpo humano dispone de más de 400 músculos esqueléticos voluntarios (i.e., utilizables voluntariamente) y, entre otras funciones, éstos se ocupan de generar fuerza sobre las extremidades para provocar movimiento 19. El tipo, la amplitud y la potencia de este movimiento depende de la dimensión y disposición de los músculos que actúan sobre las palancas óseas. El proceso anatómico de la contracción muscular se explica a partir de la teoría de los filamentos deslizantes en el sarcómero. Esta teoría establece que la contracción muscular resulta del deslizamiento de los filamentos delgados (moléculas de actina) sobre los filamentos gruesos (cabezas de miosina). La tensión producida por cada sarcómero depende del número de interacciones actina-miosina que acontecen en un momento dado, por lo que el sarcómero se acortará desarrollando la máxima tensión cuando Muñiz-Pardos, B. International Doctoral Thesis - 44 - 1.3. La importancia de la fuerza en natación Los beneficios del entrenamiento de fuerza en natación han sido estudiados previamente 34,35, siendo cuestionados por entrenadores debido a la creencia de que una mayor masa muscular y una menor flexibilidad podría conllevar un aumento en la fuerza resistencia a vencer en el medio acuático (fuerza previamente explicada en el apartado 1.1.2), causando un efecto negativo en el rendimiento 17. No obstante, los beneficios fisiológicos derivados del entrenamiento de fuerza son indiscutibles, incluyendo aumentos en los depósitos de fosfocreatina (PCr), aumento de proteínas contráctiles, mejora de la potencia anaeróbica, mejora de la arquitectura muscular, optimización de los ángulos de penación de las fibras musculares, aumento de la síntesis proteica o aumento de una hipertrofia de las fibras rápidas 36–38. El tipo de entrenamiento de fuerza aplicado para mejorar el rendimiento en natación es de muy diversa índole, aunque esta Tesis Doctoral se centra en aquellos tests o rutinas de entrenamiento de fuerza desarrollados “en seco”, o más conocidos en la literatura científica como “dry-land tests” o “dry-land training”. Aunque en los capítulos 4.1 y 4.2 se discute con mayor profundidad la literatura científica existente en relación a los tipos de tests de fuerza, así como los diferentes entrenamientos de fuerza en natación, a continuación se muestra una breve introducción y justificación de la necesidad de la presente Tesis Doctoral. Además, debido a que la fuerza de las extremidades superiores e inferiores tiene un rol desigual en el rendimiento en natación, esta parte introductoria se realizará acorde a estas dos categorías. Introduction - 45 - 1.3.1 Fuerza de las extremidades superiores y rendimiento en natación La fuerza desarrollada por los miembros superiores ha demostrado tener una gran asociación con la fuerza propulsiva en el agua 39 y, por lo tanto, con la velocidad de nado 40. En relación a esto, algunos autores han revelado que la fuerza ejercida por las extremidades superiores contribuye en un ~ 70% de la fuerza propulsiva total 13. En referencia a los tipos de tests más apropiados para relacionar la fuerza de las extremidades superiores con el rendimiento en natación, existe una amplia variedad en la literatura científica, tanto de naturaleza isométrica, dinámica o isocinética 41–43. Esta amplia variedad y la falta de una revisión de la literatura que aúne los diferentes tipos de tests y especifique aquellos que podrían ser más adecuados, dificulta la aplicación práctica de tests “en seco” por parte de entrenadores o preparadores físicos en la natación. Por otro lado, aunque la fuerza de las extremidades superiores ha demostrado contribuir notablemente en el rendimiento, no existe ningún consenso sobre el tipo de entrenamiento implicando la musculatura de las extremidades superiores (e.g., pliométrico, vibratorio, pesos libres, inercial, etc.) más adecuado para mejorar la fuerza y el rendimiento en nadadores. Por esta razón, una profunda revisión de la literatura podría clarificar o ayudar a seleccionar aquellas prácticas que potencialmente podrían mejorar la fuerza y el rendimiento en mayor medida. 1.3.2 Fuerza de las extremidades inferiores y rendimiento en natación La fuerza desarrollada por las extremidades inferiores también ha demostrado tener un papel fundamental durante las fases en las que el nadador tiene contacto con la pared o el suelo/poyete (i.e., durante virajes o en la salida), principalmente en el rendimiento en pruebas cortas de natación (25 m a 100 m). Por ejemplo, Cossor y Masson observaron que el rendimiento durante la fase de la salida desde el poyete contribuye en un ~ 26% en el Muñiz-Pardos, B. International Doctoral Thesis - 46 - tiempo de 50 m en estilo libre 44. La habilidad del nadador para desarrollar fuerza contra el poyete o contra la pared está relacionada con la fuerza y la potencia de las extremidades inferiores, que podría ser evaluada con numerosos tests 45–48. En este sentido, los saltos verticales han sido los tests más estudiados, revelando fuertes correlaciones con el rendimiento en la fase de la salida (típicamente el tiempo hasta alcanzar los 15 m) 49. A pesar de la estrecha relación entre la potencia de las extremidades inferiores con el rendimiento durante la salida, la relación con pruebas de natación cortas (i.e., 25, 50 o 100 m) ha sido estudiada en menor medida, con resultados controvertidos 50,51. Por otro lado, podemos distinguir diferentes programas de entrenamiento involucrando a la musculatura de las extremidades inferiores (e.g., pliometría, entrenamiento inercial, pesos libres con cargas altas, pliometría con cargas, etc.) aunque, al igual que lo expuesto con las extremidades superiores, no existe un consenso en cuanto al tipo de entrenamiento de fuerza más apropiado para maximizar el rendimiento en natación. Esta falta de claridad demanda de un profundo análisis de la literatura, así como examinar con mayor atención el papel de la fuerza y la potencia de las extremidades inferiores en el rendimiento en natación. Además, existe la urgente necesidad de desarrollar estudios de intervención que prueben la eficacia de diferentes métodos de entrenamiento para proporcionar a entrenadores y especialistas con protocolos útiles y válidos que puedan mejorar el rendimiento. Por ejemplo, el efecto del entrenamiento vibratorio en la natación se desconoce, por lo que sería de gran interés comprobar la eficacia de éste método en el rendimiento y en el desarrollo de la fuerza en nadadores. Por último, destacar que la presente Tesis Doctoral se centrará en el papel de la fuerza y potencia de las extremidades inferiores en el rendimiento en natación. Introduction - 47 - 1.4 Entrenamiento vibratorio en el ámbito deportivo El estímulo vibratorio, entendido como una oscilación mecánica aplicada al cuerpo de un determinado sujeto (whole-body vibration o WBV en inglés), ha sido implementado como método de entrenamiento deportivo desde hace unas tres décadas 52,53. El aparato utilizado para el entrenamiento WBV es la plataforma vibratoria, cuyo motor genera movimientos oscilantes, estimulando contracciones musculares involuntarias 54. En la actualidad, una visita a casi cualquier centro deportivo o gimnasio nos demostrará cuan populares son las plataformas vibratorias, a pesar de su elevado precio. Entre las diferentes plataformas existentes, podemos distinguir dos modos diferentes de transferir la energía vibratoria al cuerpo 55 (Figura 5): Modo simultáneo o sincrónico. El estímulo vibratorio se transfiere a ambos pies al mismo tiempo (i.e., el movimiento vertical de los pies se produce de forma simultánea) y una aceleración lineal se transfiere al tronco. Modo alterno. El estimulo se produce con una verticalidad opuesta de un lado al otro, es decir, mientras el pie derecho está en su punto mas bajo, el pie izquierdo está en su punto más alto. Este modo Transmisión del estimulo vibratorio Simultáneo Alterno Figura 5. Diseños de vibración en plataformas vibratorias (Cardinale y Wakeling, 2005). Muñiz-Pardos, B. International Doctoral Thesis - 48 - de vibración introduce un movimiento rotacional de la zona lumbar, lo que favorece una menor transmisión del estimulo vibratorio al tronco. En relación a esto, la evidencia científica sugiere que un sujeto puede tolerar aceleraciones más altas a través de este tipo de vibración 56. Numerosos estudios han examinando la efectividad (tanto crónica como aguda) de este método de entrenamiento en el desarrollo de la fuerza y el rendimiento deportivo. Sin embargo, existe una gran discrepancia en los resultados que a menudo se atribuye a la gran variedad en los protocolos de vibración aplicados. La intensidad del estímulo es una variable fundamental, y varía en función de la amplitud (A) y la frecuencia (f) utilizada durante el entrenamiento vibratorio 57. La amplitud depende de la magnitud del movimiento oscilatorio (desplazamiento “peak-to-peak”, en mm), mientras que la frecuencia está determinada por el número de ciclos de oscilación (expresada en Hz). A continuación se contextualizará el estado actual del conocimiento en referencia al efecto de tipo crónico o adaptativo del entrenamiento vibratorio, y no de tipo agudo, ya que será el tipo de intervención desarrollado en la presente Tesis Doctoral. Una revisión sistemática y meta-análisis publicada en 2015 ha examinado los efectos del entrenamiento vibratorio en el desarrollo de la fuerza y en el rendimiento en diferentes deportes (carrera de larga distancia, carrera de velocidad, baloncesto o rugby) 58. Los resultados de este estudio revelaron efectos reducidos en la fuerza (tamaño del efecto [TE]=0.44), potencia (TE=0.42) y rendimiento deportivo (TE= 0.45). No obstante, otro meta-análisis publicado previamente reportó mayores efectos del entrenamiento vibratorio en la potencia de las extremidades inferiores (altura de vuelo en el salto con contramovimiento; TE=0.77). Considerando este posible efecto positivo del entrenamiento vibratorio en las extremidades inferiores, y la gran relación entre la potencia de las piernas Introduction - 49 - y el rendimiento durante la salida y virajes, es posible que el entrenamiento vibratorio favorezca un incremento en el rendimiento en pruebas cortas de natación. Muniz-Pardos, B. International Doctoral Thesis - 50 - Hypotheses - 51 - Capítulo 2 Hipótesis y Objetivos Muniz-Pardos, B. International Doctoral Thesis - 52 - Hypotheses - 53 - Hipótesis La fuerza y la potencia desarrollada por las extremidades inferiores mostrará una relación positiva con el rendimiento en pruebas de natación en nadadores adolescentes. Los nadadores que realicen un programa de entrenamiento de 6 meses en plataformas vibratorias presentarán mayores incrementos en variables de fuerza, potencia y rendimiento deportivo en pruebas de velocidad que aquellos que no realicen la intervención. Un aparato optoelectrónico es un sistema válido y fiable para medir la velocidad de ejecución durante ejercicios de fuerza. Hypothesis The lower-body strength and power capabilities show a positive relationship with swimming performance in adolescent swimmers. Swimmers involved in a 6-month whole body vibration training will show greater improvements in strength, power and swimming performance than those swimmers who do not perform the intervention program. An optoelectronic device is a valid and reliable system to measure movement velocity during resistance exercises. Muniz-Pardos, B. International Doctoral Thesis - 60 - El capítulo 4.4 contiene datos longitudinales, y fue realizado con una muestra de 37 nadadores (23 chicos y 14 chicas). El capítulo 4.5 es un estudio transversal y de validación, y cuenta con los datos extraídos durante un experimento independiente al proyecto RENACIMIENTO. La muestra de este estudio estuvo compuesta por 22 sujetos sanos con experiencia en ejercicios de fuerza, ya que no necesitaban ser nadadores para estudiar la validación de un aparato para medir la velocidad de ejecución en ejercicios de fuerza. 3.3 Pruebas y valoraciones A continuación se muestran las pruebas de valoración funcional desarrolladas en la presente Tesis Doctoral, y cuyo tratamiento estadístico aparece reflejado en el capítulo 4. 3.3.1 Valoración de la fuerza y potencia “en seco” - Media sentadilla isométrica. La fuerza isométrica máxima partiendo de la posición de media sentadilla fue evaluada en una maquina Smith. Cada nadador se colocó de pie sobre una plataforma de fuerzas KISTLER (Kistler instruments Ltd., Hampshire, Reino Unido) centrada en la maquina Smith, flexionando rodillas a 90º. La barra se ajustó a la altura conveniente para el sujeto en esta posición, y fue anclada con cadenas al suelo para evitar cualquier tipo de movimiento. A la señal del investigador, el nadador debía empujar la barra con la máxima fuerza y tan rápido como fuera posible, registrando el pico de fuerza en N. Cada participante realizó 2 intentos, registrando únicamente el mejor. Material and methods - 61 - - Media sentadilla dinámica. En este caso, cada sujeto debía iniciar el movimiento de pie y, a la señal del investigador, realizaba 3 medias sentadillas a la máxima velocidad ascendente posible. Las cargas utilizadas para realizar este test fueron del 20%, 30% y 40% de la fuerza isométrica máxima previamente registrada. La velocidad media propulsiva de la mejor repetición fue registrada para el posterior análisis. - Extensión isométrica de rodilla. La fuerza isométrica máxima de los músculos extensores de cada rodilla fue evaluada usando una galga extensiométrica (MuscleLab, Force Sensor, Norway). Cada nadador adoptaba una posición de sentado con la rodilla a 90º, la espalda recta y los brazos cruzados en el pecho. A la señal del investigador, el nadador extendía la rodilla a la máxima velocidad y con la máxima fuerza posible, registrando el pico máximo de fuerza (N). Se les permitieron dos intentos a cada nadador, registrando únicamente el mejor. - Saltos verticales. Cada nadador realizó 3 tipos de saltos verticales diferentes sobre una plataforma de fuerzas. En primer lugar, realizaron un salto sin contramovimiento (squat jump [SJ]) partiendo desde una posición de 90º de flexión de rodilla y los brazos “en jarra” con las manos en la cintura. En segundo lugar, realizaron un salto con contramovimiento (countermovement jump [CMJ]) partiendo desde una posición de pie con los brazos en la misma posición que en el SJ. El nadador saltaba realizando un contramovimiento previo lo más rápido posible. El último salto era igual al CMJ, pero incorporando el movimiento de los brazos de forma coordinada para maximizar el rendimiento del salto (salto Muniz-Pardos, B. International Doctoral Thesis - 62 - Abalakov). Tres intentos fueron permitidos para cada salto con una recuperación de ~ 3 min, registrándose únicamente el mejor. - Salto horizontal. Cada nadador se posicionaba con los pies en paralelo tras una línea, saltando lo más lejos posible permitiendo aprovecharse de la acción de brazos para maximizar el salto. Como los otros saltos, el mejor de los 3 intentos fue registrado. - Sprint de carrera de 30 metros. Finalmente, cada nadador realizó 2 sprints corriendo 30 m a la máxima velocidad (con una recuperación de ~ 3 min), con el mejor intento siendo registrado. Un set de células fotoeléctricas (Byomedic photoelectric cells, Barcelona, España) fue colocado a cada extremo para medir el tiempo necesario para recorrer los 30 m. 3.3.2 Valoración del rendimiento en natación Las marcas en competiciones oficiales de 50 y 100 m fueron registradas para cada nadador. Como requisitos, los nadadores debían haber competido dentro de un margen de 15 días cercanos a las sesiones de los tests de laboratorio. 3.3.3 Otros tests La maduración sexual se evaluó facilitando a todos los participantes una planilla con distintas imágenes de maduración sexual para que se auto-evaluasen siguiendo los 5 estadios propuestos por Tanner y Whitehouse (Anexo III). Además, la técnica en el estilo de crol de cada nadador fue evaluada subjetivamente por un entrenador con más de 10 años Material and methods - 63 - de experiencia utilizando una escala de 10 puntos, con 1 siendo una técnica muy pobre y 10 una técnica perfecta. Muniz-Pardos, B. International Doctoral Thesis - 64 - 3.4 Programa de entrenamiento vibratorio Aunque el diseño y la metodología empleada en el desarrollo de este programa de entrenamiento se describe más exhaustivamente en el capítulo 4.4, a continuación se resumen las características principales sobre el equipamiento empleado y sobre el diseño del programa de entrenamiento. 3.4.1 Equipamiento Para el desarrollo de esta intervención, se utilizaron plataformas vibratorias Power Plate® Pro5 (PowerPlate; Amsterdam, Holanda). Esta plataforma es de tipo vertical o sincrónico, lo que significa que las vibraciones tienen lugar verticalmente y no oscilantes). Se colocó una plataforma en el centro deportivo de cada club de natación para que los nadadores no tuvieran que desplazarse al laboratorio para realizar el entrenamiento. En el club de natación con mayor muestra se colocaron dos plataformas vibratorias. 3.4.2 Diseño del estudio Después de una evaluación inicial, los nadadores fueron asignados aleatoriamente al grupo control o intervención, quedando una muestra final para este estudio de 37 nadadores (20 nadadores en el grupo intervención y 17 nadadores en el grupo control). Estos nadadores participaron en un programa de entrenamiento de WBV durante 6 meses y con una frecuencia de tres sesiones por semana. Material and methods - 65 - Los ejercicios incluidos en esta intervención se muestran gráficamente en la Figura 6 y constan de: 1) Sentadilla a 120º, 2) Sentadilla a 90º, 3) Desplazamiento de 120º a 90º a una velocidad de 2 segundos para subir y 2 para bajar, 4) Lunge con pierna derecha delante, y 5) Lunge con pierna izquierda delante. El entrenamiento fue siempre supervisado por un investigador para garantizar la seguridad del nadador, anotando también asistencias, faltas e incidencias. Los nadadores realizaban el protocolo en parejas y de forma alterna, (i.e., mientras uno entrenaba, su compañero descansaba). Debido a que las plataformas vibratorias estaban emplazadas en los centros de los clubes de natación, los nadadores empleaban muy poco tiempo en completar el entrenamiento (~ 15 min). En el caso de que un nadador no pudiera asistir a una sesión de entrenamiento, se ofrecía la posibilidad de recuperarla en otro momento. El protocolo de WBV se muestra en la Tabla 1. Debido a una falta de evidencia científica sobre las características de un protocolo vibratorio óptimo y a que el objetivo inicial de este proyecto fue mejorar la masa ósea, se utilizó un protocolo vibratorio que mostró previamente mejoras en la masa ósea de adolescentes normoactivos 60. Este protocolo fue Figura 6. Ejercicios realizados durante el protocolo de vibración (Gómez-Bruton y col., 2017) Muniz-Pardos, B. International Doctoral Thesis - 66 - de carácter progresivo y de características similares a las reportadas por una revisión sistemática previa examinando los efectos del WBV en la fuerza y el rendimiento en otros deportes 58. Los artículos incluidos en esta revisión utilizaron intensidades similares a las aplicadas en la presente Tesis Doctoral, con frecuencias entre 25 y 40 Hz, desplazamientos entre 1.5 to 6 mm y aceleraciones entre 5.4 y 29.6 g. Tabla 1. Protocolo de vibración de cuerpo entero Mes Nº Ses Frec. (Hz) Despl. (mm) Duración (s) Descanso (s) Rep. Vibración total (min) Entreno total (min) Acel. Pico (g) Mes 1 12 30 2 45 45 2 7.5 15 3.6 Mes 2 12 30 4 45 45 2 7.5 15 7.2 Mes 3 12 32 4 45 45 2 7.5 15 8.2 Mes 4 12 34 4 60 60 2* 8 16 9.3 Mes 5 12 36 4 60 60 2* 8 16 10.4 Mes 6 12 38 4 60 60 2* 8 16 11.6 NºSes=Número de sesiones al mes; Frec=Frecuencia; Despl.=Desplazamiento (De pico a pico); Rep=Repeticiones realizadas; Acel. Pico= Aceleración pico. *Sentadilla 120, 90º y dinámica se realizaron dos veces mientras que el lunge con cada pierna se realizó una vez. 3.5 Análisis estadísticos A continuación, se resumen brevemente las pruebas estadísticas que se efectuaron para obtener los resultados de esta Tesis Doctoral; no obstante, en el capítulo 4 aparece una descripción más detallada de todos los análisis estadísticos realizados. Dependiendo del estudio, los análisis se realizaron con el paquete informático SPSS (versión 22.0 para Mac OS X, SPSS Inc., Chicago, IL, EEUU), con el paquete estadístico R (versión 3.5.1) o utilizando las hojas proporcionadas por Hopkins 61 para realizar inferencia basada en la magnitud (o “magnitude-based inference”, en la literatura científica). En general, los datos se presentan como media ± desviación estándar, a menos que se indiquen otros estadísticos. En primer lugar, se estudió la normalidad en la distribución de las variables continuas mediante el test de Kolmogorov-Smirnov. Si la distribución de una variable era normal, las Material and methods - 67 - diferencias entre grupos se establecían mediante el test para muestras independientes (test t de Student) o con el test de análisis de la varianza (ANOVA). En algunas pruebas estadísticas se utilizaron covariables para ajustar variables que podían estar influenciadas por otras. En esos casos se efectuó el test de análisis de la covarianza (ANCOVA) junto con el test post hoc de Bonferroni. Las asociaciones entre variables se estudiaron mediante correlaciones bivariadas de Pearson y regresiones lineales. Las variables nominales como los estadios de maduración sexual de Tanner, se analizaron con tablas de contingencia aplicando el test de Chi-cuadrado. El nivel de significación estadístico fue tomado, como norma general como p < 0,05. Para analizar el efecto del entrenamiento vibratorio en la fuerza, potencia y rendimiento, se utilizaron las hojas de Hopkins para determinar el tamaño del efecto por medio del mínimo cambio apreciable (MCA). Se usaron los puntos de corte establecidos por Hopkins y col. 62: el tamaño del efecto de la d de Cohen puede ser trivial (0,0 – 0,2), pequeño (0,2 – 0,6), moderado (0,6 – 1,2), o grande (> 1,2 – 2,0). Para la comparación intrae inter-grupos, se determinaron las posibilidades de que el efecto de la intervención fuera beneficioso o perjudicial cuantitativa y cualitativamente. De esta manera, la valoración seguía la relación que se muestra en la Tabla 2: Tabla 2. Relación entre el porcentaje de la probabilidad de acontecimiento y su valoración cualitativa (Hopkins, 2009) Valoración cuantitativa Valoración cualitativa < 1% Casi seguro que no 1% to 5% Muy improbable >5% to 25% Improbable >25% to 75% Posiblemente >75% to 95% Probable >95% to 99% Muy probable >99% Casi seguro * En el caso de que las posibilidades de tener un efecto beneficioso o perjudicial fueran ambos >5%, el efecto se determinaría como “confuso”. Muniz-Pardos, B. International Doctoral Thesis - 68 - Por último, para el estudio de validación, se utilizó el paquete estadístico R ya que el SPSS no permitía la realización de las regresiones propuestas por Ludbrook (llamadas “OLP regression”) 63. Este modelo de regresión fue utilizado en lugar de regresiones lineales simples para estimar el error sistemático y el error proporcional, ya que es el más apropiado cuando ninguno de los aparatos es el verdadero “gold standard”. El error sistemático tenía lugar cuando el intervalo de confianza del intercepto incluía el valor 0, mientras que existía un error proporcional cuando el intervalo de confianza de la pendiente incluía el valor 1. Results and discussion - 69 - Chapter 4 Results and Discussion Muniz-Pardos, B. International Doctoral Thesis - 76 - INTRODUCTION Swimming performance is influenced by a multitude of factors such as flexibility, body composition or technique 41. Notably, most of the literature evaluating swimming performance is focused on swimmers’ strength and power development with the majority of studies presenting positive associations between strength and swimming success 41,51. Given this apparent association, a wide variety of strength training protocols have been examined to maximize swimming performance. These include free-weight training, swimming resistance training or plyometric training, amongst others. However, due to the large heterogeneity among these studies (i.e., different assessment tools, protocols, outcomes and conditions), making comparisons between studies remains difficult. Previous studies have shown that non-specific resistance training (NSRT) provides a foundation of fitness for all sports, allowing the development of a balanced neuromuscular system and serves as a base from which to train more specifically at later stages 64. Since NSRT requires limited equipment (e.g., bench press, squats or pull ups), and specific dry-land training demands exclusive well-maintained equipment only affordable for a few well-funded laboratories or elite swimming clubs (such as biokinetic benches or specific ergometers), NSRT often remains as the strength training method of choice. The American College of Sports Medicine (ACSM) 2009 Position Stand defines the term “progression” in resistance training as the pursuit of a specific target over time until the established goal is reached 65. NSRT would correspond to the first stage of this progression in resistance training. Improvements in non-specific parameters such as motor performance, vertical jump, sprint speed or agility are associated with athletic performance improvements in sport-specific activities 65 and consequently, these Results and discussion - 77 - practices have gained prominence in the athlete’s preparation phase. Despite its popularity, the effectiveness of this modality as a training stimulus is yet to be evaluated and optimized. Regarding the effects of resistance interventions on swimming performance, the present systematic review considered 2 previous systematic reviews of similar characteristics 17,34. Although the current review holds some alignments with Crowley et al.’s review 17, there are key differences that make this systematic review necessary and useful for coaches seeking the most recent non-specific strength training strategies. Crowley et al. limited their search to participants aged 16 and over, evaluating only the chronic effects of resistance training while failing to include cross-sectional studies, studies evaluating start performance and studies with acute effect interventions. Subsequently, a review providing up to date quantitative data on NSRT as a means to improve swimming performance, will be a valuable addition to an already established coaching strategy. Thus, the aims of this systematic review were to determine which type of NSRT are more suitable for the enhancement of swimming performance and to determine the non-specific strength and power variables that are better associated with swimming performance in adolescent and young adult swimmers. METHODS Literature search This study was performed following the systematic review methodology proposed in the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) statement 66. Studies were identified by searching within electronic databases (PubMed, SPORTDiscus and Cochrane Plus), reference lists and consultation with experts in the Muniz-Pardos, B. International Doctoral Thesis - 78 - field. The search was conducted up to and including the 1st of June 2018. The key words used in the search were “swimming”, “muscle strength” and “athletic performance”. The specific search strategy for PubMed was: ("Muscle Strength"[Mesh]) OR "Athletic Performance"[Mesh]) AND "Swimming"[Mesh]) with the additional filter of “Humans”. For SPORTDiscus the search was ((DE "MUSCLE strength") OR (DE "PERFORMANCE")) AND (DE "SWIMMING") and, finally, the search strategy for Cochrane Plus was ((Muscle Strength) OR (Athletic Performance)) AND (Swimming). Eligibility criteria For inclusion, studies included in this analyses were longitudinal, randomized or non-randomized controlled trials, studying the effects of NSRT programs on swimming performance. Cross-sectional studies evaluating the relationship between non-specific strength/power variables and swimming performance were also included. All participants were competitive swimmers between 13 and 19 years old (following PubMed criteria). Studies in languages other than English, unpublished data, and studies involving other athletes excluding competitive swimmers were excluded from the present systematic review. Studies focusing on variables other than strengthor power-related interventions/assessments (e.g., swimming technique, rehabilitation, physiological parameters or respiratory muscle training) and studies in which the assessments/interventions used specific exercises (e.g., biokinetic swim benches or resistance training from swim-specific positions) were also excluded. Additionally, studies only evaluating strength-related variables without including a measure of swimming performance were also excluded. Results and discussion - 79 - Quality assessment The methodological quality of the manuscripts included in this systematic review was assessed using 2 different tools. For cross-sectional studies, the Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies proposed by the National Heart, Lung and Blood Institute 67 was used, grading articles on a scale of 14 points. For experimental studies, the Physiotherapy Evidence Database (PEDro) scale 68 was used, classifying articles on a checklist composed of 11 items. Two separate researchers evaluated the quality of the studies independently. Type of studies The articles selected for this review were distributed into 4 categories: dry-land strength, dry-land power, combination of training methods and strength and power in start performance. 1. Dry-land strength (DLS). The articles included were those of which assessments/training programs comprise of non-specific dynamic, isometric or isokinetic strength exercises in addition to strength-related NSRT methods such as vibration interventions, electrical stimulation training or core training. 2. Dry-land power (DLP). The studies included in this category were those including jumps or ball throwing exercises. 3. Combination of training methods. This category comprised training interventions which include exercises typical of 2 or more of the previous categories (e.g., intervention training with jumps and weights training). 4. Dry-land strength and power in start performance. Since start performance strongly influences overall swim performance and some studies only examined Muniz-Pardos, B. International Doctoral Thesis - 80 - the potential effects of NSRT on the start phase performance (block start only or including the subaquatic phase), these articles were classified into an independent sub-section. RESULTS Included studies Searches identified 1844 potentially relevant articles. Following review of titles, abstracts and excluding the duplicates, the total was reduced to 91 relevant manuscripts for inclusion. Of these articles, 33 met the selection criteria and were included in this review (Figure 7). Figure 7. Flow chart of the studies including non-specific tests or protocols Results and discussion - 81 - Thirteen out of the 33 included manuscripts were cross-sectional studies. These studies focused on the relationship between swimming performance and DLS (isometric, dynamic, isokinetic) and DLP (jumps, throws) variables (Table 3). Some of the crosssectional studies assessed variables typical of more than 1 category (DLS or DLP), and therefore were included in both sections. A total of 20 out of the 33 included articles were intervention studies assessing the effect (acute or chronic) of different protocols on swimming performance (Table 4), such as postactivation potentiation (PAP), weights training, core training or electrical stimulation. One of these 20 intervention studies 69 also reported associations between performance and DLP and was therefore included in both Table 3 and Table 4. Muniz-Pardos, B. International Doctoral Thesis - 82 - Table 3. Cross-sectional studies included in the systematic review (n=13) Article Age (years) Level N (Males) Variables Outcomes Gomez-Bruton et al., 2016 (47) 14.3±2.2 Regional 67(38) S(50), DLS (ISOM KE, GS), DLP (horizontal jump) Positive relationship between DLP (horizontal jump) and S(50). García-Ramos et al., 2016a (69) 17.1±0.8 International 15(15) S(5),S(10),S(15), DLP (SJ, loaded SJ). Positive relationship between all distances and SJ and loaded SJ. Gracía-Ramos et al., 2016b (49) 15.3±1.6 International 20(0) S(5),S(10),S(15), DLP (SJ, CMJ, loaded SJ), DLS (ISOM KF, KE muscle torque). Positive relationship between SJ and CMJ and S(5). Positive relationship between Loaded SJ and S(5),S(10),S(15), although no correlation found with DLS. Loturco et al., 2016 (84) 17.0±0.7 National 10(10) S(50), S(100), S(200), DLS (ISOM: bench press, quarter squat), DLS (bench press) and DLP (SJ, CMJ). Positive relationship between Jump squat and S(50). Positive relationship between tethered SW and CMJ, SJ. Gola et al., 2014 (46) 23.0±1.2 PE University students 16(16) S(25), S(50), DLS (ISOM shoulder, elbow, knee and hip FLX and EXT). Positive relationship between the relative sum of the upper extremity muscle torque values and S(25), S(50). Beretić et al., 2013 (45) 21.1±4.3 International 27(27) S(10), DLS (ISOM leg EXT). Positive relationship between S(10) and Fmax, F rel, RFD50%, RFD 50%rel of the leg EXT. Results and discussion - 83 - Garrido et al., 2012 (71) 12.5-18.6 National 78(39) S(100), S(200), DLS (GS). Positive relationship between S(100) and grip strength in both males and females. Morouço et al., 2011 (50) 14.9±0.7 National 10(10) S(50), DLS (bench press, Squat and lat pull-down back), DLP (CMJ). Positive relationship between CMJ and squat. Positive relationship between bench press and lat pull down back. Positive relationship between S(50) and lat pull down back. West et al., 2011 (48) 21.3±1.7 International 11(11) S(15), DLS (3RM squat), DLP (CMJ). S(15) correlated with DLS and DLP. Positive relationship between DLS and DLP. Geladas et al., 2005 (41) 12-14 Young sprinters 263(178) S(100), DLP (Horizontal jump), DLS (GS). In boys: Positive relationship between S(100), GS and horizontal jump. In girls: Positive relationship between horizontal jump and S(100). Arellano et al., 2005 (70) 21.4±2.2 National 11(6) S(5), DLP (CMJ and simulated jump off the block). Positive relationship between horizontal force off the block and S(5). Dopsaj et al., 1999 (51) NR Collegiate 16(16) S(25), DLS (ISOM KE,TF,TE,SF and GS). Positive relationship between ISOM SF, TF and S(25). Miyashita, 1979 (72) Ms(16.7±2.5) Fs(14.6±1.3) Young swimmers 35(19) S(100), DLS ( ISOK peak torque of KE and arm pull exercise). Positive relationship between peak torque of arm-pull muscles and S(100) CMJ= countermovement jump; DLP= dry-land power; DLS= dry-land strength; EF: elbow flexion; EXT=extension; F=force; FLX=flexion; Fs=females; GS= grip strength; HT= height; ISOK=isokinetic; ISOM= isometric; KE=knee extension; KF=knee flexion; max=maximum; Ms=males; NR=not reported; PE= physical education; RFD=rate of force development; RM=repetition maximum; S(“number”)= sprint (meters); SF=shoulder flexion; SJ= squat jump; SW=swimmers or swimming; TE=trunk extension; TF=trunk flexion. Muniz-Pardos, B. International Doctoral Thesis - 84 - Table 4. Intervention studies included in the systematic review (n=20). Article Age (years) Level N (Males) CG [N] EG [N] Measurements Outcomes Effect McGowan et al., 2017 (89) 20.0±3.0 International 25(12) SW remained seated throughout the 30min transition 30-min combo transition (5-min dry-land circuit + heated jacket). S(100), S(15). Positive effect of combo transition on both S(100) and S(15). Acute Rejman et al., 2016 (92) 21.9±3.4 National 9(9) NA 6w, 2 sess/w. UB and LB PT. Time to water contact during the start phase. Positive effects of PT on start performance. Chronic Iizuka et al., 2016 (95) 20.2±1.0 National 9(9) NA PAP (Trunk stabilization exercises) S(5). Positive effects of PAP on S(5). Acute McGowan et al., 2016 (87) 16.0±1.0 National 16(11) SW remained seated throughout the 30min transition - 30-min passive transition (wearing a heated jacket while seated) - DLS transition (5-min DLS circuit) - 30-min combo transition (5-min dry-land circuit + heated jacket). S(100), S(15). Positive effects of DLS and combo transitions on S(100). Positive effects of Combo transition on S(15). Acute García-Ramos et al., 2016a (69) 17.1±0.8 National 15(15) NA 17 days DLS T (25 SW sess +10 sess of half squat and lunge) at moderate altitude (2,320 m). S(5), S(10), S(15), DLP (loaded SJ with 0, 25, 50, 75, 100% of the SW’s body mass). Positive effect on SJ height and SW performance (higher increment in jump height=greater reduction in start time). Chronic Sarramian et al., 2015 (79) 16.0±1.62 National 18(10) Traditional SW WU - UB PAP (3 RM pull-ups). - LB PAP (5 jumps to box). S(50). Individual r time. No effects found on S(50) after any PAP protocol. Traditional WU were Acute Results and discussion - 85 - - Combined PAP (UB PAP+LB PAP). significantly faster than those followed by an UB PAP. Cuenca-Fernandez et al., 2015 (94) 17-23 National 14(10) Traditional SW WU - 3RM lunge PAP WU. - 4 reps YoYo squat WU (position used in the block in a SW start). S(15) and S(5). r = 8 min. Positive effects of both PAP on S(5) and S(15), only for the YoYo squat PAP. Acute Sawdon-Bea et al., 2015 (78) 14-17 RegionalNational 32(16) [16] SW T only [16] 6w, 3sess/w. dry-land T (6 strength exercises focused on core and shoulder stabilizers). S(50), DLS (McGill Trunk Flexor test, GS). No effects on S(50) or UB strength. Positive effects on Core strength. Chronic Weston et al., 2015 (73) 16.0±1.0 National 20(10) [10] SW T only [10] 12w, 3sess/w. core-T program in addition to their normal SW routine. S(50). Prone-bridge and straight-arm pull-down test. Positive effect of core-T on S(50), prone-bridge and straight-arm pulldown test. Chronic Girold et al., 2012 (80) 21.8±3.9 National 24(12) [8] SW T only - [8] 4w, 3sess/w. dry land T group (pull-ups and draws). - [8] 4w, 3sess/w. ES group (latissimi dorsi muscles). S(50) (at w0,w4,w8), EXT peak torque of shoulder (ECC, CON and ISOM). Positive effects of both ES and Dryland T programs on S(50) and peak torque. No differences between programs. Chronic Kilduff et al., 2011 (93) 22.0±2.0 International 9(7) SW T only PAP stimulus (1x3 reps at 87%RM squat). S(15). r = 8 min. DLP (CMJ, PVF and PHF from the block) Positive effects of PAP on PVF, PHF and CMJ. No effects on S(15). Acute Potdevin et al., 2011 (83) EG (14.3±0.2) CG (14.1±0.2) Regional 23(10) [11] SW T only [12] 6w, 2sess/w. PT before SW T. S(50), S(400), DLP (CMJ and SJ). Positive effects of PT on CMJ, SJ, S(50) and S(400). Chronic Muniz-Pardos, B. International Doctoral Thesis - 92 - Pichon et al., 1995 (81) No No No Yes No No No Yes Yes Yes Yes 5 Trappe et al., 1994 (43) No No No Yes No No No No Yes Yes Yes 4 Romney et al., 1993 (86) No No No Yes No No No No Yes Yes Yes 4 Tanaka et al., 1993 (77) No No No Yes No No No No Yes Yes Yes 4 Strass, 1988 (76) No No No Yes No No No No Yes Yes Yes 4 1. Eligibility criteria were specified. 2. Subjects were randomly allocated to groups (in a crossover study, subjects were randomly allocated an order in which treatments were received). 3. Allocation was concealed. 4. The groups were similar at baseline regarding the most important prognostic indicators. 5. There was blinding of all subjects. 6. There was blinding of all therapists who administered the therapy. 7. There was blinding of all assessors who measured at least one key outcome. 8. Measures of at least one key outcome were obtained from more than 85% of the subjects initially allocated to groups. 9. All subjects for whom outcome measures were available received the treatment or control condition as allocated or, where this was not the case, data for at least one key outcome was analysed by “intention to treat”. 10. The results of between-group statistical comparisons are reported for at least one key outcome. 11. The study provides both point measures and measures of variability for at least one key outcome. Results and discussion - 93 - DISCUSSION Non-specific dry-land strength and swimming performance Over the last 40 years, many investigations have shown positive correlations between swimming velocity and DLS 45,46,51,71–73. Gola et al. showed that this correlation seems to vary with swimming distance, with the relative importance of strength increasing as the distance decreases 46. Accordingly, several DLS interventions have shown positive effects on sprint swimming performance 73–75. However, the wide variety of the protocols and the diversity of results make it necessary to examine the existing evidence in an attempt to reach a consensus of the most effective DLS practices. One of the pioneering studies investigating the chronic effects of a DLS training on swimming performance was performed by Strass et al. 76. These authors assessed the effects of a 6-week maximal strength intervention on 25 m and 50 m swimming performance in 10 male adolescent competitive swimmers (9 participants in the control group). Strength training was focused on the arm extensor muscles using barbells, with an intensity between 90% and 100% of 1 maximum repetition (1RM). While the control group failed to illustrate any positive effects, the intervention group displayed an improvement of 4.4% and 2.1% in 25 m and 50 m, respectively. Conversely, Tanaka et al. 77 evaluated the effects of an 8-week upper-body DLS training on 25 and 400 yards performance (22.9 m and 365.8 m, respectively) in 24 male collegiate swimmers. The training program was intended to simulate the actions employed during freestyle swimming through weight-stacks machines and free weights. Despite a 30% improvement in land strength, there were no significant differences on swimming velocity between the control and the intervention group. The authors stated that the strength gained on land did not transfer to the propulsive forces used in the water. Muniz-Pardos, B. International Doctoral Thesis - 94 - However, caution must be taken when evaluating these results since this intervention was applied during the competitive phase of the season, including 7 competitions and a high volume of training. As reported by the authors, it is possible that the high physical demand of the training may have masked any minor performance improvements following the strength training intervention 77. Coaches and sports specialists should therefore consider that the ideal moment to implement NSRT and benefit from its effects may be out of the competitive phase of the season. Following the study by Tanaka et al. (42), Trappe and Pearson 43 showed similar results after a 12-week training intervention (6-week DLS training and 6-week swim-only training). Ten highly-trained male collegiate swimmers were divided into 2 intervention groups. During the first 6 weeks, 1 group used a weight-assisted dip and pull-up training device whereas the other group engaged in free-weight training (dips and pull-ups exercises). Despite a significant reduction in time observed in the weight-assisted group, no differences were observed between groups in 25 and 300 yards time (22.9 m and 265.8 m, respectively) or in the biokinetic swim bench variables. These results suggested that weight-assisted training might be as beneficial as free-weight training, although the lack of a control group make it difficult to draw meaningful conclusions. Similarly, Sawdon-Bea and Benson 78 observed positive improvements in core strength in the intervention group after a 6-week core training program in comparison to a control group. However, this enhancement did not result in an improvement in swimming times. This is consistent with Girold et al., who reported positive effects of a 6-week upper-body strength training on strength development, with no changes in swimming performance 75. These authors suggest that 12 weeks of strength training might be the minimum duration of training to improve swimming times 75. Results and discussion - 95 - Only 1 study examined the acute effect of DLS training on swimming performance through the analysis of a PAP stimulus. The protocol of Sarramian et al. 79 incorporated an upper-body (3RM pull-ups), lower-body (5 loaded jumps to box) or a combined (both upperand lower-body stimulus) PAP, 8 min before 50 m freestyle swimming. The PAP stimulus did not affect performance and therefore the authors concluded that a combined PAP was not more effective than a traditional swimming warm-up. Notably, an isolated upper-body PAP stimulus produced negative effects on swimming performance, most likely due to the pull-up exercise differing from the actual kinematic characteristics of the front crawl stroke. After several studies reporting no effects 43,75,77–79, 3 experiments have emphasized the importance of DLS training on swimming velocity, supporting the positive effects of DLS found by the pioneering study of Strass 76. Girold et al. 80 performed a 4-week DLS training, dividing the participants into either an upper-body DLS group (pull-ups and draws) or an upper-body electrical stimulation training group (applied on both latissimi dorsi). Both training protocols enhanced 50 m swimming performance by 1.7% and 2%, respectively. Accordingly, Pichon et al. 81 implemented a 3-week electrical stimulation training similar to that implemented by Girold et al. 80, resulting in comparable improvements in 50 m performance (1.4%). Finally, a 2% improvement in 50 m swimming performance was found by Weston et al. 73 after a 12-week isolated coretraining program. Numerous cross-sectional studies support these positive results, finding significant associations between swimming velocity and isometric 41,45,46,51,71, dynamic 50 and isokinetic strength 72. Interestingly, most of the significant associations found in these studies involve upper-body strength assessments 41,46,50,51,71,72, whereas only 1 study found a significant correlation between swimming velocity and lower-body strength 45. This Muniz-Pardos, B. International Doctoral Thesis - 96 - finding can be explained due to the greater role that upper-body strength plays in the application of force in water, considering that the majority of the investigations studying strength and swimming velocity only analyze performance in freestyle swimming. This is supported by the findings of Czabański et al. 13, who reported that 70% of the propulsive force in freestyle swimming comes from the upper-body muscles. The results from the existing non-specific DLS training intervention studies are unclear (4 out of 9 studies reported positive effects on swimming performance), with upper-body electrical stimulation training being potentially more effective than other methods aiming to improve swimming performance 80,81. Consequently, it would be reasonable to suggest that there is no up-to-date evidence supporting the use of DLS interventions to enhance swimming performance. However, DLS assessments have shown to provide valid indicators to test the swimmers’ level of strength, with upperbody isometric evaluations being the most widely used. Non-specific dry-land power and swimming performance A wide variety of training and assessment methods can be used to determine nonspecific DLP. Plyometric exercises (e.g., jumping and throwing) are among the most common methods and have shown to increase explosive-reactive power maximizing the stretch-shortening cycle, improving power through increased neural drive, changes in muscle coordination, muscle-tendon complex and both muscle size and architecture 82. To the best of our knowledge, there is only 1 study examining the chronic effect of plyometric training on swimming performance. Potdevin et al. 83 implemented a 6week lower-body plyometric training program in adolescent swimmers, finding positive effects on 50 m and 400 m times (3.2% and 4.4% enhancements, respectively). Results and discussion - 97 - Sarramian et al. 79 studied whether a PAP stimulus (5 loaded jumps to box) yielded to an improvement in 50 m swimming performance in 18 national swimmers. No differences were found between traditional swimming warm-up and jumping PAP with the authors concluding that this may be due to a learning effect and the simplicity of the study design (i.e., experimental protocol in a fixed order, without randomization). Additionally, several cross-sectional studies showed positive relationships between horizontal jump 41,47, squat jump 84 and swimming performance. Positive associations have also been reported between upper-body power (3 kg ball throwing test) and swimming performance in pre-pubescent swimmers 85, although there is no evidence in adolescents nor in young adults. Although positive associations between jumping and swimming performance are well documented 41,47,84, there is limited evidence to support the use of plyometric training to enhance swimming performance. Further research in this area is required considering there are only 2 studies investigating the use of plyometrics, with contrasting results. Combination of training methods One of the first researchers to apply a combined training (DLS and DLP) was Romney et al. 86, who assessed the acute effect of 3 different warm-ups on 100 yards (91.44 m) performance. These authors hypothesized that dry-land warm-up using free weights and jumps would be less effective than a swimming-only warm up, but more effective than no warm-up. The results were in accordance with the hierarchy of warm-up theorized by the authors, observing a deterioration in swimming performance of 0.17% after dry-land warm-up (p= 0.054) and 1.24% after no warm-up, in comparison to the swimming warm-up. However, the short recovery duration used in this study (3 min) Muniz-Pardos, B. International Doctoral Thesis - 98 - would fail to allow for an adequate return to homeostasis negating any ergogenic effect of the dry-land PAP. Sarramian et al. 79 investigated the optimum rest time needed to maximize the efficacy of PAP, studying different recovery times to observe increased muscle performance after a PAP stimulus (combined DLS-DLP: pull-ups and jumps). The authors concluded that an 8-min recovery was the adequate time to alleviate fatigue and to obtain an optimal potentiation. This study also showed that the combined stimulus (pull-ups and jumps) was more effective than DLS or DLP warm-up alone (pull-ups or jumps, respectively) in enhancing 50 m swimming performance. However, there were no significant differences between combined PAP stimulus and traditional swimming warmup. Recently, McGowan et al. 87 studied the acute effect of 3 different warm-ups on swimming performance. Sixteen national level swimmers warmed up for 25 min, followed by a 30-min transition phase and a 100 m maximal bout. The same 16 swimmers carried out this regimen in 4 different testing days (over a 2-week period), varying the transition phase. During the first experimental trial, swimmers remained seated throughout the transition phase (control condition). The second experimental trial included the same protocol as the control condition, but swimmers had to wear an additional heated jacket to increase the temperature of the core muscles. During the third experimental day, the swimmers performed a combined DLS-DLP circuit and, finally, during the fourth experimental day, swimmers performed the combined DLS-DLP circuit while wearing a heated jacket. Compared to the control condition, 100 m performance was significantly faster for both combined DLS-DLP circuit conditions, especially with the additional heated jacket (0.68% and 1.5% improvements, respectively). A maintenance of core temperature during the transition phase could improve swim performance, probably through an increased muscle fiber velocity conduction 88. The Results and discussion - 99 - same authors have recently replicated this experimental protocol by recruiting 25 elite level swimmers 89, observing an improvement of 0.8% in 100 m performance. While only a small improvement, an additional ~ 0.3 - 0.4% improvement is relevant to increase the swimmer’ s chances of winning a medal at the elite level 89. Girold et al. 75 designed a 12-week combined DLS (press, pull-up, drawn and squat) and DLP (jumps) program in 21 competitive adolescent swimmers. The control group performed an additional running and cycling session to counterbalance the extra training volume. The intervention group significantly improved 50 m swimming velocity by 2.8%, whereas the control group only improved by 0.9%. In conclusion, the effects (especially acute) of a combined DLS-DLP training remain unclear, probably due to the low number of studies and the small sample sizes used. Among the 4 articles studying acute effects, only those using a heated jacket to maintain a high core temperature showed positive results on swimming performance 87,89. We therefore hypothesize that the PAP effect might be ineffective when the core temperature gained during the PAP stimulus is not maintained in well trained swimmers. Lastly, further evidence to support combined training is required since there is only 1 chronic intervention studying the effect of this practice on swimming performance (with positive effects on swimming performance) 75. Strength and power in start performance Cossor and Mason 44 demonstrated that the swimming start is an important component of the swimming race, having a higher contribution to the success in sprinting events compared to longer distance events. As previously stated 90, the starting phase accounts for 7.7% to 15% of total race time in elite men 50 m freestyle races, highlighting the importance of the force exerted from lower extremities during the swimming block Muniz-Pardos, B. International Doctoral Thesis - 100 - start (SBS) phase. Consequently, over the last decade, several studies have focused on the influence of the lower extremities strength on start performance (distances ranging from 5 m to 15 m). Bishop et al. 91 studied the chronic effect of an 8-week lower-body plyometric training on 5.5 m SBS performance in 11 adolescent swimmers. The participants assigned to the intervention group followed the same aquatic training as the control group but with an additional 2-h plyometric training per week. SBS assessments revealed a greater change after the 8-week training for the plyometric training group (15%) compared with the control group (3%), concluding that an exposure to 2 h of supplementary plyometric training per week had a significant impact on SBS performance. Eight years later, Rejman et al. 92 found beneficial effects of a 6-week plyometric training on SBS performance (flight time to water contact) in 9 national-level male swimmers. This research reported a 7.5% improvement in SBS performance, although methodological limitations (i.e., reaction times to the starting signal were not taken under consideration and the lack of a control group) could distort these results. After these long-term training interventions, other authors have studied the acute effect of a PAP stimulus on SBS performance. Kilduff et al. 93 compared a traditional swimming warm-up (volume of 1,500 m) to a PAP stimulus (1 set of 3 squats at 87% 1RM) 8 min before SBS, observing no differences between warm-ups. Despite not detecting an improvement after PAP, it is interesting to note that a lower volume warmup (i.e., PAP) produced similar effects than a longer traditional swimming warm-up on SBS performance. In contrast, Cuenca-Fernandez et al. 94 found positive effects of 2 different PAP protocols (3 repetitions of lunge exercises and 3 repetitions in a Yo-Yo squat flywheel device, modelling the starting position on the block) on time to 5 m SBS. The Yo-Yo squat stimulus also produced an enhancement on 15 m SBS, agreeing with Results and discussion - 101 - the initial hypothesis that the Yo-Yo squat protocol would be the most appropriate warmup to enhance SBS performance due to comparable movement patterns. Similarly, a recent study 95 has examined the acute effect of a trunk stabilization exercise on SBS performance in 9 national-level swimmers, finding a 2.3% decrease in time to 5 m. However, the lack of a control group limits the applicability of these results. García-Ramos et al. 69 implemented a 17-day DLS training (half squat and lunge exercises) at a moderate altitude of 2,320 m above sea level to find whether any acute changes occur in both SBS performance (time to 5 m, 10 m or 15 m) and vertical jump ability. The results revealed significant improvements in both assessments after the training period. This is in agreement with previous research 96, which also found muscle power improvements after an acute exposure to high altitude, despite the deterioration of the aerobic capacity. In spite of these potential benefits, there are no longitudinal studies to support these results. When examining cross-sectional studies, DLP assessments (horizontal and vertical jump tests) have been positively associated with SBS performance 47–49,69,70,73. In addition, Beretic et al. 45 found a positive relationship between DLS measurements (lower-body isometric strength) and 10 m SBS performance. In conclusion, DLP interventions (especially lower-body plyometrics) seem to enhance SBS and therefore sprint swimming performance. However, the number of training interventions is scarce (2 studies showing chronic effects and 4 studies showing acute effects). It should be noted that the longest and most important part of the start is the underwater phase which should be a focus for future research. Even though the present systematic review focused on analyzing freestyle swimming performance, the literature search identified an important lack of studies using breaststrokers, backstrokers and butterfly swimmers. Finally, in light of several poor Muniz-Pardos, B. International Doctoral Thesis - 108 - OR "Athletic Performance"[Mesh]) AND "Swimming"[Mesh]) with the additional filter of “Humans”. For SPORTDiscus the search was ((DE "MUSCLE strength") OR (DE "PERFORMANCE")) AND (DE "SWIMMING") and, finally, the search strategy for Cochrane Plus was ((Muscle Strength) OR (Athletic Performance)) AND (Swimming). Inclusion criteria The types of studies included in the present systematic review were longitudinal, randomized or non-randomized controlled trials, studying the effects of swim-specific resistance training programs on swimming performance. Cross-sectional studies evaluating the relationship between swim-specific strength/power parameters and swimming performance were also included. Additionally, the type of subjects (following PubMed criteria) recruited were adolescents or young adults, all of which were competitive swimmers. Exclusion criteria Studies in languages other than English, unpublished data, or studies involving triathletes, divers or other that are not competitive swimmers were excluded from the present systematic review. Studies focusing on parameters other than swim-specific strength/power-related interventions or assessments (swimming technique, rehabilitation, physiological parameters or respiratory muscle training) and studies of which power assessments or interventions used non-specific ergometers (e.g., Wingate test) were also excluded. Cross-sectional studies evaluating strength values without considering any kind of swimming performance or vice versa, and training interventions not related to either strength or power training were not included. Finally, training programs only assessing Results and discussion - 109 - strength changes, without considering swimming performance enhancements, or vice versa, were excluded. Quality assessment The manuscripts included in this systematic review were assessed using 2 different tools. For cross-sectional studies, the Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies proposed by the National Heart, lung and blood Institute 67 was used, grading articles on a scale of 14 points. For experimental studies, the Physiotherapy Evidence Database (PEDro) scale 68 was used, classifying articles on a checklist composed of 11 items. Two separate researchers evaluated the quality of the studies independently. Type of studies The articles selected for this review were distributed into 2 categories: swim-specific dry-land resistance training and specific in-water swimming power training. 1. Swim-specific dry-land resistance training (SDLRT). The studies included in this category were those including SDLRT through specific exercises, similar to the movement pattern used during swimming actions (e.g., training on the biokinetic swim bench or weights training from a swim-specific position). 2. Specific in-water swimming power training (SSWPT). The articles included in this category utilized either tethered swimming, active drag swimming, or velocity through a perturbation method to improve swimming performance. Muniz-Pardos, B. International Doctoral Thesis - 110 - RESULTS Included studies Searches identified 1844 potentially relevant articles. Following the review of titles and abstracts and excluding the duplicates, the total was reduced to 77 relevant manuscripts. Of these articles, 25 met the selection criteria and were included in this systematic review (Figure 8). Regarding the 25 manuscripts that met the inclusion criteria, 13 were crosssectional studies evaluating the relationship between swimming performance and swimFigure 8. Flow chart of the studies including swim-specific tests or protocols Results and discussion - 111 - specific dry-land variables (strength/power during these actions) and SSWPT (tethered swimming, active drag, passive drag) variables (Table 5). Twelve out of the 25 included articles were intervention studies assessing the effect (acute or chronic) of different swimspecific protocols on swimming performance (Table 6), such as swim-specific postactivation potentiation (PAP), training on the biokinetic bench or swim-specific inertial training. Muniz-Pardos, B. International Doctoral Thesis - 112 - Table 5. Cross-sectional studies included in the systematic review (n=13) Article Age (years) Level N (Males) Variables Outcomes Kalva-Filho et al., 2017 (126) 18.0±2.0 Regional to National 9(5) S(50), S(100), S(200), SWP (3 min all-out tethered SW) Significant correlation between mean tether F and all distances’ performance. Higher correlations with longer distances. Santos et al., 2016 (125) 21.6±4.8 Competitive SW 21(NS) S(200), SWP (Tethered SW). Positive relationship between PPO (tethered SW) and S(200). Loturco et al., 2016 (84) 17.0±0.7 National 10(10) S(50), S(100), S(200), SWP (tethered SW). Positive relationship between tether F and S(50), S(100). Morouço et al., 2014 (123) 17.2±2.7 National and International 34(34) S(50), SWP (Tethered SW). Positive relationship between SWP and S(50). Papoti et al., 2013 (124) Ms(12.5±0.8) Fs(16.0±1.0) Trained SW 12(9) S(100), S(200), S(400). SWP (Tethered SW). Positive relationship between tether F and S(100), S(200), S(400) (decreasing with distance). Dominguez-Castells et al., 2013 (127) 22.1±4.3 National 18(18) S(25), SWP (Tethered SW). Positive relationship between tether F and S(25). Morouço et al., 2011a (122) Ms(19.0±2.8) Fs(15.3±1.7) International 32(20) S(50), S(100), S(200) in all four strokes. SWP (tethered SW). Positive relationship between absolute values of tether F and S(50),S(100),S(200) in all strokes except for the breaststroke S(200). Results and discussion - 113 - Morouço et al., 2011b (50) 14.9±0.7 National 10(10) S(50), SWP (Tethered SW: whole body, Arms only, legs only). Positive relationship between S(50) and F production (arms only). Arellano et al., 2005 (70) 21.4±2.2 National 11(6) S(5), DLP (CMJ and simulated jump off the block). Positive relationship between horizontal F off the block, S(5) time and S(5) mean velocity. Shimonagata et al., 2003 (111) 21.5±1.0 Competitive SW 11(5) S(100), S(25), SWP (semi-tethered SW), DLP (biokinetic SW bench). Positive relationship between both distance performance and both SWP and DLP. Bradshaw and Hoyle, 1993 (110) NS University students 7(7) S(25) full stroke, arms only, legs only. DLP (biokinetic SW bench). Positive relationship between arm power and S(25) (full stroke and arms only SW). Johnson et al. 1993 (108) 18.0±2.0 Collegiate and high school SW 29(29) S(22.86), SWP (Tethered SW), DLP (biokinetic SW bench). Positive relationship between S(22.86) and PPO (Tethered SW) and DLP (SW bench). Sharp et al. 1982 (102) 15.2±0.3 Competitive SW 40(18) S(22.86)a, DLP (biokinetic SW bench). Positive relationship between DLP and S(22.86)a CMJ= countermovement jump; DLP= dry-land power; F=force; FLX=flexion; Fs=females; Ms=males; NS= not specified; PPO= peak power output; RM=Repetitions Maximum; S(“number”)= sprint (meters); S(“number”)a= yards converted to meters; sess=training sessions; SJ=squat jump; SW=swimmers or swimming; SWP= swimming power WU= warm up. Muniz-Pardos, B. International Doctoral Thesis - 114 - Table 6. Intervention studies included in the systematic review (n=12) Article Age (years) Level N (Males) CG [N] EG [N] Measurements Outcomes Effect Kojima et al., 2017 (116) 13.6±1.1 Regional 24(12) [12] SW T only (10x15 sprints) [12] 10w, 2sess/w. SWP program (tethered 10x10 sprints). S(50), SWP (tethered SW). No differences between groups in the effects on S(50) nor tethered SWP. Chronic Papoti et al., 2017 (115) 16.0±1.5 National 21(12) [11] SW T only [10] 7w, 5sess/w. SWP program (50% of the sets during each sess using tethered SW). S(50), S(100), S(400), SWP (tethered SW). No effects of the SWP program neither on SW times nor tethered SWP. Chronic Naczk et al., 2017 (112) 15.8±0.4 National 14(10) [7] SW T only [7] 4w, 3sess/w. DLS program (Inertial T employing: 4 sets of 15 s) S(50), S(100butterfly), DLP (Inertial training device: max power in a 10-s maximal test) Positive effects on S(50), S(100butterfly) and DLP test. Chronic Cuenca-Fernandez et al., 2015 (94) 17-23 National 14(10) Traditional SW WU - 3RM lunge PAP WU. - 4 reps Yo-Yo squat WU (position used in the block in a SW start). S(15) and S(5) 8 min after each WU/PAP. Positive effects of PAP on S(5) (for both PAP protocols) and S(15) (only for the Yo-Yo squat PAP). Acute Results and discussion - 115 - Hancock et al., 2015 (119) 19-22 Collegiate SW 30(15) [30] Traditional SW WU [30] PAP (4x10m tethered SW). S(100) 6-min after WU/PAP. Positive effects of PAP on S(100). Acute Sadowski et al., 2012 (106) EG (14.0±0.5) CG (14.1±0.5) Young SW 26(26) [12] SW T only [14] 6w, 3sess/w: DLP T (simulated SW on an ergometer; 6x50´´) before SW T. DLS (ISOM shoulder flexion), S(25) driven by upper extremities, F during tethered SW. Positive effect of DLP T on tethered SW F. Chronic Dragunas et al., 2012 (117) EG (19.3±0.9) CG (19.0±1.8) RegionalNational 18(10) [9] 5w, 3sess/w. Interval T: 3x45.72ma + 4x (4x 22.86ma + 16x22.86ma [9] 5w, 3sess/w. Same T than CG but wearing a drag suit. S(50), 6x50 m all-out times with and without drag suit on 2 separate days (r= 10 min). No differences between groups in S(50) after the T period. Chronic Aspenes et al., 2009 (74) EG (17.5±2.9) CG (15.9±1.1) Collegiate 20(8) [9] SW T only [11] 11w, 2sess/w. 4x4 min SW intervals + 3x5RM (cable crossover device) S(50), S(100), S(400), SWP (tethered SW). Positive effects of combined strength and interval SW T on S(400) and tethered SW F. Chronic Girold et al., 2007 (75) 16.5±3.5 National 21(10) [7] 12w, 6 sess/w. SW T only 12w, 6sess/w. - [7] Dry-land T with barbells. 1.5h/w extra. S(50) before, at w6 and after intervention. Positive effects of both dry-land and RAS T on S(50) only at w12. Chronic Muniz-Pardos, B. International Doctoral Thesis - 116 - - [7] RAS T with elastic tubes. 1.5h/w extra. Girold et al., 2006 (114) 16.5±3.0 Regional to national 37(16) [11] SW T only + 6x50m sprints. 3w, 3sess/w. - [11] Assisted T with elastic tubes. - [15] Resisted T with elastic tubes. S(100). Positive effects of both RAS T methods on S(100) (Greater in Resisted G). Chronic Roberts et al., 1991 (105) 19.1±2.1 National 16(16) [NS] SW T only [NS] 10w, 3 sess/w. Biokinetic resistance T on SW bench. S(91.44)a , PPO and fatigue test (biokinetic SW bench). No positive effects of biokinetic resistance T on S(91.44) a nor PPO on the biokinetic swim bench. Chronic Toussaint et al., 1990 (39) NS National 22(16) [11] SW T only (8sess/w; 4500m/sess) [11] 10w, 3sess/w of sprints using POP system (16 POP, mounted below the water surface). S(50), S(100), S(200). Max force, V and power from the POP system. Positive effect of intervention on max power and velocity in the POP system, S(50), S(100) and S(200). Chronic CG=control group; DLP=dry-land power; EG=experimental Group; F=force; G=group; ISOM=isometric; NS=not specified; PAP=post-activation potentiation; POP=push off points; r=recovery; RAS=resisted and assisted sprint; RM=repetition maximum; S(“number”)=sprint (meters); S(“number”)a=yards converted to meters; Sess=sessions; SW=swimming or swimmers; SWP=swimming power; T=Training; V=velocity; w=week; WU=warm up. Results and discussion - 117 - Quality Assessment Scores of the Quality Assessment Tool for Observational Cohort and CrossSectional Studies ranged from 3 to 5 of a maximum of 14 points (Supplementary Table 3), except 1 study that only reached 1 point 70. Some of the criteria assessed were not applicable due to the type of variables measured (exposures that cannot vary in amount or level, exposures measured only once over time or blinding participants, therapists or assessors). Furthermore, some points were not reported in most of the studies, such as the participation rate of eligible persons or loss to follow-up after baseline. Scores of the PEDro scale ranged from 4 to 6 of a maximum of 11 points (Supplementary Table 4). These scores are relatively good, taking into account that some exercise protocols such as tethered swimming or biokinetic power training do not allow for blinding participants or blinding therapists. Furthermore, blinding of the assessors and concealed allocation were 2 variables that were poorly reported in most of the selected studies. Muniz-Pardos, B. International Doctoral Thesis - 124 - (swimming block start after traditional swimming warm-up). The Yo-Yo squat flywheel device uses a wheel to generate a moment of inertia at the end of the concentric phase of the movement. When this point is reached, there is a strong eccentric contraction, which has been demonstrated to evoke greater improvements in muscle peak power than traditional weight training 112. The authors stated that the Yo-Yo squat protocol was the most effective PAP stimulus to enhance both 5 m and 15 m swimming block start performance (5.7% and 2.4%, respectively) due to the similarity in the movement pattern. Similarly, a recent study 112 examined the efficacy of an inertial training method in 14 national-level swimmers. The authors carried out a 4-week upper-body inertial training using the Inertial Training Measurement System (ITMS; a novel device which allows the performance of specific movements 112). Both muscle force and power were determined using the ITMS. The authors reported positive effects in muscle force, power and 100 m swimming performance (improvements of 12.8%, 14.2% and 1.8%, respectively), in comparison to the control group (traditional swimming training). Subsequently, these authors concluded that specific inertial training may provide greater benefits than traditional strength training. Specific in-water swimming power and swimming performance Toussaint and Vervoorn 39 were the first researchers to study the effects of a SSWPT on freestyle swimming performance. They implemented a new training device derived from the MAD-system (system to Measure Active Drag; 113), providing the swimmer with 16 submerged fixed push-off points along the length of a swimming pool. The force applied on these fixed points was measured through a force transducer placed at one end of the swimming pool, measuring maximal force, velocity and power output. The authors evaluated the effects of a 10-week training program with this device on 50 m, Results and discussion - 125 - 100 m and 200 m swimming performance, in 30 competitive swimmers. The intervention group performed sprints using the aforementioned apparatus, with the control group performing traditional sprints. The results demonstrated a positive effect on force, power and swimming velocity on the MAD-system, with additional improvements in 50 m and 200 m performance in the intervention group. The high specificity of this method and the greater force applied on every push-off point in comparison to normal swimming seemed to favour a positive transfer to swimming performance 39. After this pioneering study exploring the active drag training paradigm, others studied different active drag training techniques. Girold et al. 114 analyzed the effects of a 3-week tethered swimming intervention (swimming while being held by a flexible restraining device; e.g., tubes or ropes) on 100 m swimming performance in 37 competitive swimmers. They compared 3 different interventions: resisted tethered swimming with elastic bands (resistance against direction of motion; 6 x 30 s sprints), assisted tethered swimming with elastic bands (pull force in the direction of motion; 12 x 25 m sprints), and traditional swimming (control group: 6 x 50 m sprints without elastic bands). Although the authors witnessed swimming improvements in the resisted group, this improvement was not accompanied with strength enhancements but a higher stroke rate. These authors carried out a longer and more rigorous research 75, examining the effects of a 12-week resistedand assistedsprint (RAS) training on 50 m performance in 21 adolescent swimmers. The participants involved in the RAS group showed an improvement of 2.3% in swimming performance whereas the control group only showed minimal changes (0.9%). This study showed that the stroke depth and the stroke rate were the best predictors of the 50 m performance in the RAS group, confirming their previous findings. Muniz-Pardos, B. International Doctoral Thesis - 126 - In contrast, a recent investigation 115 showed no effects of a 7-week tethered swimming program in 21 adolescent swimmers. The differences between the control and intervention group in 100 m, 200 m, 400 m times, and tethered swimming force did not differ after the training period. However, the intervention group increased their lactate production capacity, speculating that the inclusion of tethered swimming in the training routine may increase the anaerobic glycolysis contribution during exercise, despite the lack of improvement observed in swimming force. Kojima et al. 116 showed no significant differences between the intervention and the control group after a 10-week resisted training intervention on 50 m swimming performances in adolescent swimmers. These authors suggested that the level of maturation of the athletes might be a determinant confounding factor in the ability of an adolescent swimmer to respond to any specific training load. Dragunas et al. 117 studied the effects of training with a drag suit on 50 m performance. Eighteen regional and national level young swimmers were equally divided into the control group and drag suit-trained (DST) group. For 5 weeks, the swimmers involved in the DST group performed the same training as the control group, but while wearing the drag suit (a total training volume of 950 m/week). The authors showed no significant changes in swimming performance, although the DST group was more effective at maintaining technique than the traditional training method. The limitations of this study however, make it difficult to interpret these results. For example, 30 swimmers were initially recruited, but the high dropout (n=18) reduced the statistical power. Furthermore, the use of manual timing 118 and the lack of control over the training regimen may have biased the final results 117. After these studies focusing on the chronic effect of different SSWPT interventions, Hancock et al. 119 investigated the acute effect of a PAP protocol on Results and discussion - 127 - swimming performance, when compared to the control condition in 30 young swimmers. Their protocol consisted of 4 repetitions of 10 m all-out tethered swimming 6 minutes before a 100 m maximal effort. The PAP load was individually prescribed, taking into account the swimmers’ best time in 100 m (t) and their lean body mass (LBM), using the formula: Load= 0.2·LBM/(100·t-1). The results showed a significant improvement in 100 m performance for the PAP condition, compared to the control condition (0.86% enhancement). Although it is well known that PAP increases the rate of force development 120,121, further analyses through muscle biopsies would confirm the true effect of PAP on lower-body explosive power (i.e., confirming an increased phosphorylation of regulatory myosin light chains 119). Additional cross-sectional studies support the use of swimming power assessments through tethered swimming 50,84,108,122–127, semi-tethered swimming 111 and active drag (towing a perturbation buoy) 128 to predict swimming performance in distances ranging from 25 yards (22.86 m) to 400 m. The association between swimming power and swimming performance seems evident since the 11 existing cross-sectional studies reported positive associations. However, further interventions would be necessary to confirm the effectiveness of this specific training, as 4 interventions improved swimming performance 39,75,114,119 whereas 3 studies reported no effects 115–117. Despite the effective MAD-system tested by Toussaint et al. 39, no other studies have used this methodology neither in intervention nor in cross-sectional studies, probably due to the high cost of this specific equipment. In conclusion, the inertial training method seems more beneficial than traditional resistance training to improve swimming performance, although more research is needed to verify this. Although several studies showed cross-sectional associations between the biokinetic bench values and swimming performance, no training interventions have found Muniz-Pardos, B. International Doctoral Thesis - 128 - improvements in swimming performance following biokinetic bench training. However, training on the MAD-system appear to be the most effective method to improve the propulsive forces used in water as well as swimming performance. However, only 1 study investigated this system. Finally, tethered swimming as an effective SSWPT method to improve performance remains under debate due to the contrasting results. The present systematic review has identified limited research utilizing female swimmers, as well as elite level swimmers, finding important methodological limitations in the training protocols susceptible to bias the results (analyzing males and females as a whole, not adjusting by maturity status or level of performance, or the lack of a control group). Finally, the literature search performed in the present systematic review identified an important lack of studies using breaststrokers, backstrokers and butterfly swimmers. PRACTICAL APPLICATIONS Research indicates that swim-specific resistance training is an effective method to improve specific muscle strength and swimming performance. Since this practice allows direct transfer to sports performance, coaches should design training protocols as specific as possible, especially with regard to movement pattern. Based on a critical evaluation of the existing evidence, coaches and practitioners should consider inertial training as a method that potentially offers greater benefits on both strength development and swimming performance, than traditional free weight training. Regarding the different SSWPT methods examined, further high-quality studies are needed to confirm the efficacy of tethered swimming as a method to improve swimming performance. Tethered swimming forces have shown to elicit the greatest relationship with swimming performance. Lastly, training on the MAD-system seems highly effective, although the high cost of this equipment may reduce its availability to most swimmers. Results and discussion - 129 - Muniz-Pardos, B. International Doctoral Thesis - 130 - Results and discussion - 131 - Chapter 4.3 Relationship between strength, power and swimming performance Muniz-Pardos B, Gómez-Bruton A, Matute-Llorente A, González-Agero A, Olmedillas H, Gómez-Cabello A, Sutehall S, Pitsiladis Y, Casajús JA, Vicente-Rodríguez G. Lowerbody strength and power contribution in sprint swimming performance in trained adolescent swimmers. Int J Perf Anal Spor. Submitted. Muniz-Pardos, B. International Doctoral Thesis - 132 - Results and discussion - 133 - ABSTRACT The purpose of this study was to determine the association between lower-body strength (LBS) and lower-body power (LBP) capacities with sprint swimming performance in adolescent competitive swimmers. A total of 44 competitive swimmers (27 males and 17 females) performed LBS tests (maximal isometric strength [MIS] half squat, dynamic half squat with 20%, 30% and 40% of the MIS, and MIS knee extension) and LBP tests (squat jump [SJ], countermovement jump [CMJ] and Abalakov jump [ABA]). Further swimming best times in 50 and 100 m races were recorded from official swimming competitions. Swimming performance was correlated with LBP variables (SJPEAK, CMJRFD, ABAPEAK ABARFD; P≤0.05) and LBS (MIS half squat; P≤0.05) for both 50 and 100 m performance in males but not in females. Additional age, maturity and freestyle technique-adjusted linear regression models showed that LBP capacity partly predicted both 50 m (ABARFD; r2= 0.58; change in r2= 0.18) and 100 m (SJPEAK; r2= 0.66; change in r2= 0.15) performance in trained male swimmers. This study emphasizes the greater importance of LBP on sprint swimming performance in adolescent males compared to females, suggesting that adolescent male swimmers have a higher ability to transfer LBP to swimming performance than females. Muniz-Pardos, B. International Doctoral Thesis - 140 - fitness test. Power calculation and sample size estimations were computed based on the primary outcome which is reported in the corresponding methodological article published elsewhere 59. The present study is based on a secondary analysis and therefore a specific power calculation was not developed for the present calculations. The Statistical Package for the Social Science 24.0 software (SPSS) was used for all analyses and p value was set at p≤0.05. RESULTS Sex differences Personal data and anthropometric characteristics are presented in Table 7. The variables for both LBS and LBP tests, as well as 50 m and 100 m freestyle swimming performances are shown in Table 8. Males presented higher values for maturity, height and weight (Table 7) and also for MIS half squat, MIS KE, MPV of 20 and 30% of the MIS Squat, SJPEAK, CMJPEAK and ABAPEAK (Table 8), than females for both unadjusted and ageand maturity-adjusted data (p≤0.05). In addition, males presented better performance times in both 50 m and 100 m when compared to females. No differences were found in age, swimming experience (y), training volume (hours/week), MPV of 40% of the MIS Squat, SJRFD, CMJRFD, and ABARFD, between sexes (p>0.05). Results and discussion - 141 - Table 7. Descriptive analysis of anthropometrics and personal data. ALL (M±SD) BOYS (M±SD) GIRLS (M±SD) CI (95%) ANOVAa N= 44 N= 27 N= 17 Lower Higher F p p2 Age (y) 14.6±1.3 14.75±1.26 14.4±1.4 14.2 15.0 0.668 0.418 0.016 Tanner Stage (I/II/III/IV/V) 0/3/14/22/5 0/1/7/14/5* 0/2/7/8/0 3.4 3.9 4.505 0.041 0.114 Height (cm) 166.5±10.5 172.10±8.41* 157.6±6.5 163.3 169.7 36.78 <0.001 0.467 Weight (kg) 56.0±11.1 61.34±10.12* 47.5±6.4 52.6 59.4 25.295 <0.001 0.376 BMI (kg·m-2) 20.0±2.4 20.63±2.58* 19.1±1.6 19.3 20.8 4.962 0.031 0.106 Freestyle T (0-10) 6.3±1.7 6.3±1.8 6.3±1.5 5.8 6.8 0.008 0.930 0.000 SW Exp (y) 8.2±2.6 7.9±2.9 8.6±2.1 7.4 9.0 0.770 0.385 0.018 Tr Vol (h/wk) 10.2±2.0 10.3±1.9 10.2±2.2 9.6 10.8 0.012 0.913 0.000 * = Significant differences between sexes (p≤0.05). a = Sex differences p2=partial eta squared; CI=confidence interval; BMI=body mass index; SW Exp=swimming experience; Tr Vol=training volume; Freestyle T=freestyle technique; Muniz-Pardos, B. International Doctoral Thesis - 142 - Table 8. Descriptive analysis the freestyle swimming times, LBS and LBP variables. ALL (M±SD) BOYS (M±SD) GIRLS (M±SD) CI (95%) ANOVAa ANCOVAb N= 44 N= 27 N= 17 Lower Higher F p p2 F p p2 SW Performance T-50 (s) 30.18±2.38 28.89±1.63* 32.22±1.93 29.45 30.90 37.628 <0.001 0.473 26.495 <0.001 0.665 T-100 (s) 65.26±4.77 62.80±3.24* 69.16±4.19 63.81 66.71 31.994 <0.001 0.432 26.537 <0.001 0.666 LBS Variables MIS Squat (N) 1280.3±400.5 1460.4±405.35* 994.1±149.4 1158.5 1402.0 20.587 <0.001 0.329 19.108 <0.001 0.589 MIS KE (N) 913.4±195.8 1005.3±182.86* 767.5±108.6 853.9 973.0 23.416 <0.001 0.358 13.034 <0.001 0.494 MPV20 (m·s-1) 0.63±0.08 0.66±0.08* 0.58±0.05 0.61 0.66 12.934 0.001 0.235 5.854 0.002 0.305 MPV30 (m·s-1) 0.54±0.08 0.57±0.09* 0.50±0.05 0.52 0.57 6.642 0.014 0.137 4.411 0.009 0.249 MPV40 (m·s-1) 0.47±0.09 0.46±0.10 0.45±0.08 0.43 0.49 0.174 0.679 0.004 1.013 0.397 0.071 LBP Variables SJPEAK (N) † 1167.1±286.6 1289.4±278.12* 941.3±114.1 1071.5 1262.6 18.475 <0.001 0.345 9.703 <0.001 0.469 SJRFD (N·s-1) † 5156.0±2618.2 5649.0±3055.96 4245.9±1132.2 4283.1 6029 2.524 0.121 0.067 0.970 0.418 0.081 CMJPEAK (N) 1194.6±273.6 1316.2±250.37* 1001.4±186.0 1111.4 1277.8 19.892 <0.001 0.321 11.989 <0.001 0.473 CMJRFD (N·s-1) 8653.3±5648.4 8787.1±5728.51 8440.6±5686.6 6936.0 10370.5 0.038 0.846 0.001 0.508 0.679 0.037 ABAPEAK (N) 1158.6±287.9 1285.9±260.20* 956.5±204.8 1071.1 1246.1 19.553 <0.001 0.318 13.268 <0.001 0.499 ABARFD (N·s-1) 6893.0±4178.6 7826.6±4677.57 5410.1±2750.3 5622.5 8163.4 3.709 0.061 0.081 1.372 0.265 0.093 Results and discussion - 143 - * = Significant differences between sexes (p≤0.05). † = The original sample was reduced to 37 swimmers (24 males and 13 females) due to incorrect jumping execution during the SJ. a = Sex differences. b = Sex differences following age and maturity adjustments. p2=partial eta squared; CI=confidence interval; T-50=best performance time in 50 m; T-100=best performance time in 100 m; LBS=lower-body strength; LBP=lowerbody power; MIS squat=maximal isometric strength from half-squat position; MIS KE=maximal isometric strength of the knee extensors; MPV20=mean propulsive velocity in half squat at 20%; MPV30=mean propulsive velocity in half squat at 30%; MPV40=mean propulsive velocity in half squat at 40%; SJPEAK=squat jump peak force; SJRFD=squat jump rate of force development; CMJPEAK=countermovement jump peak force; CMJRFD=countermovement jump rate of force development; ABAPEAK=abalakov jump peak force; ABARFD=abalakov jump rate of force development. Muniz-Pardos, B. International Doctoral Thesis - 144 - Bivariate Pearson correlations For the correlation of the LBP variables, three males and four females had to be excluded due to incorrect jumping execution during SJ (i.e., countermovement prior the concentric phase). Thus, subsequent correlation and linear regression analyses were performed with a sample of 37 swimmers when including SJ (24 males and 13 females). In males, bivariate correlations exhibited significant values between swimming performance and LBP variables (SJPEAK, CMJPEAK, ABAPEAK and ABARFD for both 50 m and 100 m performance; p≤0.05, Table 9). Only one LBS variable correlated with swimming performance in males (MIS Squat; p≤0.05, Table 9). However, in females, bivariate correlations did not find any significant relationship with swimming performance (p>0.05, Table 9). Results and discussion - 145 - Table 9. Pearson bivariate correlations in males (n=27) and females (n=17) between freestyle swimming times and LBS variables and LBP variables. MALES FEMALES LBS variables and swimming performance LBS variables and swimming performance MIS KE MIS SQ MPV20 MPV30 MPV40 MIS KE MIS SQ MPV20 MPV30 MPV40 T-50 -0.142 -0.430* -0.212 -0.008 -0.037 -0.305 -0.076 -0.246 0.018 -0.298 T-100 -0.161 -0.443* -0.313 -0.120 -0.105 -0.358 -0.104 -0.124 -0.045 -0.343 LBP variables and swimming performance LBP variables and swimming performance †SJPEAK †SJRFD CMJPEAK CMJRFD ABAPEAK ABARFD †SJPEAK †SJRFD CMJPEAK CMJRFD ABAPEAK ABARFD T-50 -0.573** -0.252 -0.497** -0.262 -0.503** -0.452** -0.082 0.037 -0.150 0.099 -0.274 -0.101 T-100 -0.642** -0.322 -0.544** -0.296 -0.488** -0.415* -0.208 0.063 -0.222 -0.062 -0.308 -0.111 * p≤0.05; ** p≤0.01. † = The sample size was reduced to 24 males and 13 females due to incorrect jumping execution during SJ. ABAPEAK=ABA peak force; ABARFD=ABA rate of force development; CMJPEAK=CMJ peak force; CMJRFD=CMJ rate of force development; LBS=lower-body strength; LBP=lower-body power; MIS KE=maximal isometric strength of the knee extensors; MIS SQ=maximal isometric strength from half-squat position; MPV20=mean propulsive velocity in half squat at 20%; MPV30=mean propulsive velocity in half squat at 30%; MPV40=mean propulsive velocity in half squat at 40%; SJPEAK=SJ peak force; SJRFD=SJ rate of force development; T-50=best performance time in 50 m; T-100=best performance time in 100 m. Muniz-Pardos, B. International Doctoral Thesis - 146 - Multiple linear regressions Table 10 displays the results of the linear regression analyses performed between performance times and the variables that had shown a significant correlation in the aforementioned correlation analysis, being solely for male swimmers. For 50 m performance, age, maturity and freestyle technique adjustments were firstly performed, indicating a significant predictive value of r2=0.40 (p≤0.01; Model 1, Table 10). When LBP variables were introduced in the regression analyses, only ABARFD significantly increased the predictive value (r2 = 0.58, change in r2= 0.18, p≤0.05; Model 2, Table 10). Similarly for 100 m performance, age, maturity and freestyle technique adjustments indicated a significant predictive value of r2=0.51 (p≤0.01; Model 1, Table 10). When adding the LBP variables that previously showed a significant correlation with 100 m performance, SJPEAK was the only included in the regression model (Model 2, r2= 0.66, change in r2=0.15, p≤0.01; Table 10). Results and discussion - 147 - Table 10. Linear Regression models to predict 50 m and 100 m swimming performance in male swimmers (n= 24). SEE r r2 Change in r2 Adjusted r2 β Semip corre p T-50 Model 1 1.30 0.635 0.404 0.404 0.314 0.014* Age -0.491 -0.513 0.015* Tanner -0.096 -0.117 0.605 Freestyle T -0.249 -0.296 0.180 Model 2 1.11 0.764 0.584 0.180 0.496 0.010* ABARFD -0.426 -0.550 0.010* Age -0.459 -0.555 0.009* Tanner -0.087 -0.126 0.586 Freestyle T -0.253 -0.354 0.116 T-100 Model 1 2.34 0.714 0.509 0.509 0.436 0.002* Age -0.459 -0.524 0.012* Tanner -0.069 -0.093 0.682 Freestyle T -0.423 -0.504 0.017* Model 2 2.01 0.811 0.657 0.148 0.585 0.010* SJPEAK -0.431 -0.549 0.010* Age -0.280 -0.381 0.088 Tanner -0.090 -0.144 0.534 Freestyle T -0.361 -0.507 0.019* *p≤0.05 Model 1: Linear regression model introducing Age, Tanner and Freestyle technique assessment (Enter method). Model 2: Linear regression model introducing Model 1 + all the LBP variables that previously showed significant correlations (step-wise method). ABARFD=Abalakov jump rate of force development; β =estimated standardized regression coefficient; B=estimated non-standardized regression coefficient; Freestyle T=freestyle technique; SEE=standard error of estimation of the model; Semip corre=semi-partial correlation; SJPEAK=squat jump peak force; T-50=best performance time in 50 m; T100=best performance time in 100 m. Muniz-Pardos, B. International Doctoral Thesis - 148 - Reliability of the tests Table 11 displays the ICC results for all the fitness variables. All of them presented excellent ICC (range: 0.912 - 0.977), indicating that the performed tests were highly reliable methods to test strength and power in this group of adolescent swimmers. Table 11. Intraclass correlation coefficient for the strength and power tests. ICC MIS KE left 0.950 MIS KE right 0.924 SJ 0.954 ABA 0.977 CMJ 0.972 MIS Squat 0.923 MPV20 0.941 MPV30 0.912 MPV40 0.944 ICC=intraclass correlation coefficient; MIS squat=maximal isometric strength from half-squat position; MIS KE=maximal isometric strength of the knee extensors; MPV20=mean propulsive velocity in half squat at 20%; MPV30=mean propulsive velocity in half squat at 30%; MPV40=mean propulsive velocity in half squat at 40%; SJ=squat jump; CMJ=countermovement; ABA=abalakov jump. DISCUSSION The main findings of the present study revealed that LBP through vertical jumps has a superior influence in sprint swimming performance than LBS, in adolescent male swimmers. This study also highlights the importance of LBP depending on the distance swam, with arm coordination during the jump being remarkably more important during shorter events. Our findings showed that vertical jumps explained 18% and 15% of the Results and discussion - 149 - variance for 50 m and 100 m swimming performance, respectively, after age, maturity status and freestyle technique adjustments. None of the LBS variables explained swimming performance in this group of swimmers neither for 50 m nor 100 m performance. In males, data exhibited a significant relationship between LBP tests and swimming performance and only one LBS test correlated with swimming performance (MIS half squat). When LBS and LBP variables were analysed through linear regression models, no LBS variables were included in any regression model. The importance of LBP showed in the present study is in agreement with Garcia-Ramos et al. 49, who only found associations between LBP (SJ and CMJ) and swimming start performance (time to 5, 10 and 15m), but not with LBS (MIS KE and flexion). Several cross-sectional studies 41,47,84 reinforce the relationship between LBP (jumping ability) and swimming performance although this association appears arguable when the including adolescent females 41. The difference between sexes observed in the present study agrees with previous longitudinal data which examined the LBP capacity over different maturation stages among males and females 140. This longitudinal study observed that adolescent males experienced an enhancement in LBP (vertical jumping performance) from prepubescence (Tanner stages 1-3) to postpubescence (Tanner stages 4-5), whereas adolescent girls did not. The authors suggested that the neuromuscular increase that takes place in prepubertal males naturally, may be artificially induced through neuromuscular training in females across puberty. According to this hypothesis, strong evidence supports that lower extremity power can be modified in adolescent females with neuromuscular training 141– 143, and this could explain why LBP did not correlate with swimming performance in this group of female swimmers. Other factors are likely to predict swimming performance in female swimmers, such as a reduced energy expenditure for staying afloat because of