Effects of different protocols of postactivation performance enhancement on the resistance training performance of physically active men

1 Department of Physical Education and Sport, University of the Basque Country UPV/EHU EFFECTS OF DIFFERENT PROTOCOLS OF POSTACTIVATION PERFORMANCE ENHANCEMENT ON THE RESISTANCE TRAINING PERFORMANCE OF PHYSICALLY ACTIVE MEN Presented by Arkaitz Garbisu Hualde Vitoria-Gasteiz, 2023 2 3 Department of Physical Education and Sport, University of the Basque Country UPV/EHU EFFECTS OF DIFFERENT PROTOCOLS OF POSTACTIVATION PERFORMANCE ENHANCEMENT ON THE RESISTANCE TRAINING PERFORMANCE OF PHYSICALLY ACTIVE MEN Presented by Arkaitz Garbisu Hualde Supervised by: Jordan Santos Concejero Universidad del País Vasco/Euskal Herriko Unibertsitatea, UPV/EHU Vitoria-Gasteiz, 2023 (cc)2023 ARKAITZ GARBISU HUALDE (cc by 4.0) 5 ACKNOWLEDGMENTS I have never been brilliant studying, but I have never stood not understanding why things happen or how things work. So, when I was taught how to search and filter quality information, I discovered a whole world of knowledge available. Furthermore, when I was proposed to do a thesis, I was excited. The very idea that I could contribute to the knowledge people looks for all around the globe was incredible. I also had to become familiar with the other side of researching, rejection. It was tough at the beginning but clarifying. One of the most important things I have learned during these years is that no matter how convinced you are about something, you can still be completely wrong. These four years have been insane, but I have been surrounded by my family and friends, and I would like to dedicate them some lines before I start my dissertation. My mom, Idoia Hualde, taught me that patience and being cold-minded are two of the greatest virtues a person can have. Thank you for teaching me that not even the greatest botanist can grow a tree in three days. My dad, Mikel Garbisu, taught me the importance of seeking perfection through discipline and professionality. Thank you for teaching me that just because perfection is impossible, it doesn´t mean we shouldn´t look for it. My brother, Oier Garbisu, taught me that staying focused on something is crucial when tough times arrive. Thank you for reminding me why I always walked behind you when we used to go trekking and climbing. 6 My grandmother, Sara Ituarte, who has lost the most during these years. Even if life stroke you hard, you have always been lovely and supportive. Thank you for your unconditional support, even when you did not understand what was going on. My uncle Dr. Javier Ordóñez and my aunt Natalia Perez-Galdós have never discouraged me from learning and studying, even when this last year I called them to ask about a completely insane idea. Thank you for teaching me that education is not something you can finish. My good friend and writer Iker Samper reminded me the importance of taking things easy. Thank you for teaching me that we live in an overly accelerated culture where productivity relies more on doing something fast than doing the right thing in time. My girlfriend, Laura Gutierrez, who has been with me every day since I started this journey, who has made me laugh when I most needed it and who has forced me to rest when I didn´t want to. Thank you for teaching me that no one can pass over a destroyed bridge. Finally, my supervisor Dr. Jordan Santos Concejero, if it was not for you, I would have dropped out of college during the 3rd year. Thank you for teaching me that being a scientist is an attitude, not a job. 7 We´ve arranged a global civilization in which most crucial elements profoundly depend on science and technology. We have also arranged things so that almost no one understands science and technology. This is a prescription for disaster. We might get away with it for a while, but sooner or later this combustible mixture of ignorance and power is going to blow up in our faces. Carl Sagan – The demon-haunted world: science as a candle in the dark 8 9 SCIENTIFIC CONTRIBUTIONS Peer-reviewed publications Study 1: Garbisu-Hualde, A., & Santos-Concejero, J. (2021). Post-Activation Potentiation in Strength Training: A Systematic Review of the Scientific Literature. Journal of Human Kinetics, 78(1), 141-150. Quality indicators: ISI-JCR Impact factor: 2.923. 42/87 (Q2) SPORT SCIENCES 2021 Study 2: Garbisu-Hualde, A., Gutierrez, L., Fernández-Peña, E., & Santos-Concejero, J. (2023). Intermittent Voluntary Isometric Contractions Effects on Performance Enhancement and Sticking Region Kinematics in the Bench Press. Journal of Human Kinetics, 87(1), 105117. Quality indicators: ISI-JCR Impact factor: 2.923. 42/88 (Q2) SPORT SCIENCES 2022 Study 3: Garbisu-Hualde, A., Gutierrez, L., & Santos-Concejero, J. (2023). Post-Activation Performance Enhancement as a strategy to improve bench press performance to volitional failure. Journal of Human Kinetics, 88(1), epub ahead of print. Quality indicators: ISI-JCR Impact factor: 2.923. 42/88 (Q2) SPORT SCIENCES 2022 10 Congress communications Oral presentation: Efectos de las contracciones isométricas voluntarias intermitentes en el rendimiento y la cinemática de la región de estancamiento en el press de banca X Jornadas Internacionales de la Sociedad Española de Medicina del Deporte (Badajoz, Spain); From 25/11/2022 to 26/11/2022 Others External reviewer for a study on the topic for the Research Quarterly for Exercise and Sport External reviewer for a study on the topic for the Journal of Human Kinetics 17 RESUMEN El entrenamiento de fuerza ha ganado una notable popularidad en los últimos años. Este creciente interés por los deportes relacionados con la fuerza, como el powerlifting, la halterofilia o el culturismo, ha aumentado el número de competidores y entrenadores. Dado que la función principal de los entrenadores es mejorar el rendimiento de sus atletas, se han estudiado varias estrategias, como la mejora del rendimiento post-activación (PAPE). La PAPE se ha utilizado comúnmente como una estrategia para mejorar la producción de fuerza voluntaria de manera aguda. No deben confundirse los términos PAPE y potenciación post-activación (PAP), ya que existen ciertas diferencias: (i) la necesidad de una confirmación inducida eléctricamente en el caso de PAP y (ii) el perfil de tiempo-potenciación. En este sentido, la PAP tiene una vida más corta (~4-5 minutos), mientras que la PAPE puede durar hasta 20 minutos. La PAPE ha ganado una notoria popularidad en deportes que se basan en la potencia debido a su capacidad para mejorar la tasa de desarrollo de fuerza de manera aguda mediante protocolos que siguen diferentes estrategias. Entre esas estrategias, podemos encontrar protocolos con cargas muy variadas, como cargas óptimas para la potencia (optimal power load), cargas medias e incluso protocolos basados en contracciones pliométricas. Estos protocolos de PAPE son especialmente útiles cuando se trata de mejorar el rendimiento en tareas deportivas de carga submáxima, como pueden ser el sprint, el salto o los lanzamientos. Otro factor importante que tienen en cuenta los entrenadores es la región de estancamiento. Este término hace referencia a la región del levantamiento donde se puede fallar durante el entrenamiento o la competición. Se define como la región entre el primer pico máximo de velocidad y el pico mínimo de velocidad durante la fase ascendente de 18 un levantamiento. La causa del fenómeno parece ser una desventaja mecánica del grupo muscular más débil involucrado. Esta región es altamente individual ya que depende de la cantidad de masa muscular del participante y cómo está distribuida, el grado de experiencia del individuo en el patrón motor que se testea y la forma articular y longitud ósea del participante. Se han adoptado diferentes estrategias para combatir la región de estancamiento, que podemos clasificar en estrategias a largo plazo (como el entrenamiento específico) o a corto plazo (donde la PAPE podría ser útil). Por tanto, los propósitos principales de esta tesis fueron: (i) observar si una estrategia para mejorar el rendimiento de forma aguda (es decir, PAPE) es efectiva sobre la cinemática y las características de la región de estancamiento, (ii) comparar diferentes protocolos de PAPE basados en diferentes regímenes de contracción y (iii) estudiar si un protocolo de PAPE puede mejorar el volumen de entrenamiento de fuerza. La tesis comienza con una revisión sistemática de la literatura científica, cuyo objetivo es determinar la combinación ideal para un protocolo de Potenciación Post-Activación cara a mejorar el rendimiento. Realizamos una búsqueda bibliográfica en octubre de 2020 (PubMed y Scopus) de artículos de investigación originales. Después de analizar 202 resultados, solo se incluyeron los estudios que cumplieron con los siguientes criterios de inclusión: a) la edad de los sujetos era entre 18 y 30 años; b) los estudios analizaron levantadores experimentados; c) la Potenciación Post-Activación se estudió en deportes con altos requisitos de la tasa de desarrollo de la fuerza; d) el protocolo de potenciación se realizó con ejercicios con barra; e) la evaluación previa y posterior se realizó con un ejercicio de fuerza, salto vertical o similar (es decir, un squat jump, salto en contramovimiento o drop jump). Solo se seleccionaron estudios revisados por pares 19 escritos en inglés. Diecisiete estudios cumplieron con los criterios de inclusión, diez tenían un nivel de evidencia 1b (ensayos controlados aleatorios de buena calidad) y los 7 restantes tenían un nivel de evidencia 2b según la escala de nivel de evidencia de Oxford. De acuerdo con los resultados, incluso si se pueden usar diferentes protocolos para lograr la PAP, parece que las intensidades más altas inducen una mayor mejora del rendimiento. Más precisamente, parecía que (i) los levantadores experimentados se benefician más que sus compañeros sin experiencia, (ii) las intensidades altas brindan mejores resultados, y (iii) los intervalos de descanso de 7-8 minutos parecen apropiados. Basándonos en esos resultados, intentamos comparar un protocolo de PAPE basado en un régimen de contracción diferente con un protocolo de PAPE tradicional de alta intensidad. El estudio tuvo como objetivo analizar el papel de un protocolo de PAPE de contracción isométrica máxima en el rendimiento y sus efectos en la cinemática de la región de estancamiento en comparación con un protocolo de PAPE tradicional. Para ello, se reclutaron veintiuna personas (edad 26.4 ± 5.4 años) que realizaron dos sesiones experimentales: una consistía en realizar una serie de una única repetición en press de banca plano con el 93% del 1RM (considerado un protocolo tradicional para inducir PAPE) y otra sesión que consistía en realizar 15 contracciones isométricas voluntarias máximas en la región de estancamiento de 1 segundo de duración, con 1 segundo de descanso entre contracciones. Los datos analizados incluían la velocidad de propulsión media de la región pre-estancamiento, el primer pico máximo de velocidad y el primer pico mínimo de velocidad. Solo el protocolo isométrico de PAPE mejoró el rendimiento en la región de pre-estancamiento (velocidad media desde el inicio del levantamiento hasta el primer pico máximo de velocidad) (p < 0.001; ES = 0.67, efecto moderado), velocidad en el primer pico máximo (p = 0.005; ES = 0.71, efecto moderado) y en el 20 primer pico mínimo (p = 0.025; ES = 0.38, efecto pequeño). Los hallazgos de este estudio sugieren que un protocolo de PAPE isométrico de alta intensidad mejora la cinemática de la región de estancamiento y de la región previa al estancamiento, lo que ayuda al levantador a superar el levantamiento. Esta mejora es especialmente evidente en el primer periodo de la subida, previo a la región de estancamiento, mejorando el primer pico máximo de velocidad después del protocolo isométrico. Esto tiene como resultado la obtención de mayor impulso, facilitando así el paso por la región de estancamiento y cambiando el perfil velocidad-tiempo. Como los protocolos de PAPE suelen usarse en tareas con carga submáxima o tareas explosivas, tratamos de probar si un protocolo de PAPE es útil cuando se trata de mejorar el volumen de entrenamiento. Este último estudio tuvo como objetivo analizar la influencia de un protocolo de PAPE en el rendimiento de press de banca en una serie de entrenamiento al fallo muscular en individuos entrenados en comparación con un grupo control sin protocolo de PAPE. Reclutamos catorce participantes con al menos 2 años de experiencia en entrenamiento de fuerza (edad 24.57 ± 2.7 años; masa corporal 77.47 ± 12.2 kg; altura 174.21 ± 7.4 cm 1 repetición máxima en press de banca con agarre medio (1RM): 101.6 ± 25.8 kg), de los cuales 14 completaron el protocolo de control y 12 el experimental. En el grupo que realizó un protocolo de PAPE, los participantes realizaron más repeticiones que en la condición control (p=0.008; ES=0.5, efecto pequeño), su última repetición fue más lenta (p=0.02; ES=0.52, efecto pequeño) y tuvieron una mayor pérdida de velocidad (p=0.004; ES=0.75, efecto moderado). Los resultados de este estudio sugieren que realizar un protocolo de PAPE tradicional antes de una serie al fallo podría hacer que el atleta sea más resistente a la fatiga en tareas de larga duración, lo que podría resultar en una potencial forma de mejorar el volumen de entrenamiento. La 21 cantidad de trabajo realizado por sesión, entendido como número de series o como tonelaje (series x repeticiones x kilogramos) está relacionado con la cantidad de masa muscular ganada. Por lo tanto, si la PAPE mejora el número de repeticiones hasta el fallo concéntrico, el tonelaje por sesión aumentaría, y lo mismo podría ocurrir con la hipertrofia muscular. Puede asumirse que esa hipotética mejora en la hipertrofia muscular se debe a una tensión mecánica mayor en la última repetición y al mayor número de repeticiones realizadas. Sin embargo, el estudio se realizó con una sola serie, pero no se probó el efecto de esta serie y su fatiga en posteriores series. La principal conclusión de esta tesis fue que existe un distanciamiento entre la ciencia y la práctica, ya que los protocolos que se utilizan habitualmente no son efectivos en los contextos comúnmente usados (es decir, sesiones de fuerza máxima o competiciones de powerlifting). Los protocolos alternativos (como los protocolos isométricos) son más adecuados para esas situaciones, ya que las ligeras mejoras en la cinemática de la región de estancamiento pueden ser cruciales cuando se tiene éxito en un intento de 1RM. En cuanto a los protocolos de PAPE tradicionales de alta intensidad, deben usarse principalmente en tareas de carga submáxima, donde combinados con un intervalo de descanso adecuado pueden brindar claros beneficios. Palabras clave: Potenciación Post-Activación; mejora del rendimiento post-activación; Región de estancamiento; powerlifting; AMRAP; fallo muscular; entrenamiento de fuerza; volumen de entrenamiento; fuerza isométrica 22 23 LIST OF SYMBOLS AND ABBREVIATIONS PAPE: Post-Activation Performance Enhancement ATP: Adenosine Triphosphate PAP: Post-Activation Potentiation CA: Conditioning Activity 1RM: 1 Repetition Maximum ROM: Range of Motion min: minutes Ca2+: Calcium ions MLCK: Myosin Light Chain Kinase S1: Subfragment 1 S2: Subfragment 2 mM: millimolar KCl: Potassium chloride CAD: Catalytic Domain CD: Converter Domain LD: Lever Domain nm: nanometre! A1: Low energy state A2: High energy state " G: Basic free energy change " H: Change in enthalpy T " S: T stands for temperature; " S stands for entropy change MVC: Maximal Voluntary Contraction SERCA: Sarco/Endoplasmic Reticulum Ca2+-ATPase 24 # : Pi " P: Pressure change r4: Radius to the fourth power η: Viscosity l: length F: Blood flow Fthickness: Compressive force upon the muscle Fradial: Force in the radial direction caused by Fthickness CMJ: Counter Movement jump CPX: Complex CPX – CNT: Contrast complex CPX – DT: Descending complex CPX – AT: Ascending complex CPX – FCNT: French complex RFD: Rate of Force Development Vs.: Versus PPA: Potenciación-Post Activación PEDro: Physiotherapy Evidence Database scale RCT: Randomised Controlled Trials BPM: Beats Per Minute RPE: Rate of Perceived Exertion Vmax: Maximum velocity Vmin: Minimum velocity Vmax peak: Maximum peak velocity Vmin peak: Minimum peak velocity 25 ISO: Isometric conditioning protocol TRAD: Traditional conditioning protocol MVIC: Maximal Voluntary Isometric Contraction ANOVA: Analysis of Variance ES: Effect Size CI: Confidence Interval s 2: variance of the response variable 1b : statistical power a : alpha level CON: Control condition SD: Standard deviation η2: Eta-squared, used in statistics as an index of the proportion of variance RPD: Rate of Power Development p: Significance level. Statistical measure used to validate a hypothesis. EMG: Electromyography F: Force m: Mass a: acceleration Vmax1: First maximum peak in velocity m·s-1: meter per second 26 33 4.1.3 Characteristics of the Participants ............................................................................................ 97 4.1.4 Studies matching volume load .................................................................................................. 99 4.1.5 Studies not matching volume load ......................................................................................... 101 4.2 Publication 2: Intermittent Voluntary Isometric Contractions Effects on Performance Enhancement and Sticking Region Kinematics in the Bench Press ............................... 104 4.2.1 Kinematics of the sticking region ........................................................................................... 104 4.2.2 Mean velocity changes and RPE ............................................................................................ 107 4.3 Publication 3: Post-Activation Performance Enhancement as a strategy to improve bench press performance to volitional failure .................................................................. 109 5. DISCUSSION .......................................................................................................... 113 5.1 Publication 1: Post-activation potentiation in strength training: A systematic review of the scientific literature .................................................................................................... 113 5.2 Publication 2: Intermittent Voluntary Isometric Contractions Effects on Performance Enhancement and Sticking Region Kinematics in the Bench Press ............................... 115 5.3 Publication 3: Post-Activation Performance Enhancement as a strategy to improve bench press performance to volitional failure .................................................................. 119 5.4 General Discussion ........................................................................................................ 121 5.4.1 General concepts .................................................................................................................... 121 5.4.2 PAPE protocols for heavy lifting ........................................................................................... 126 5.4.3 Isometric PAPE and heavy lifting .......................................................................................... 129 6. LIMITATIONS & FUTURE RESEARCH ............................................................ 137 6.1 Limitations ..................................................................................................................... 137 6.2 Future research ............................................................................................................. 137 7. CONCLUSIONS ...................................................................................................... 141 7.1 Conclusions .................................................................................................................... 141 34 7.2 Conclusiones .................................................................................................................. 143 8. REFERENCES ........................................................................................................ 149 9. ADDENDUMS ......................................................................................................... 163 35 1 INTRODUCTION E´er to a truth that hath a falsehood´s face Ought one to close his lips as best as he can, For, though one faultless be, it brings him shame Dante Alighieri – Inferno XVI (124-126) 36 37 1. INTRODUCTION 1.1 General Introduction to the Topic This thesis studies the effect of a novel and different protocol of post-activation performance enhancement (PAPE) in the performance and kinematics of the sticking region of the bench press and the effects of the PAPE on training volume. The 3 published studies1–3 start with a review of the scientific literature to establish what is recommended by experts and coaches in the field1. The purpose for that previous literature search was based on the broad spectrum of available protocols and rest time recommendations. The second study2 then examined the effectiveness of an isometric PAPE protocol in performance based on previous literature4,5. As new protocols with different contraction regimes have been proposed6 and previous literature suggested the effectiveness of isometric contractions as potentiation stimuli 4,5, the intention of this study was to prove the efficacy of those isometric protocols in field conditions. Thus, a common exercise was tested, the barbell bench press7, with an isometric contraction protocol, 15 maximal voluntary contractions of 1 second with 1 second rest interspersed. Shortly, we discovered that 15 maximal contractions interspersed with 1 second rest resulted in an effective potentiation stimulus. Finally, the third study3 analysed the effect of a traditional PAPE protocol on a task to failure in the bench press, as a possible mechanism to enhance training volume and maximise muscle mass gains. In this within-subject study, participants performed the control and experimental conditions in a randomised order. In both conditions 38 participants performed a task to volitional failure with the 80% of their 1RM in the bench press, but in the experimental condition they performed a traditional PAPE protocol prior to the task to failure. Results indicate that PAPE could be a useful tool to improve performance in submaximal intensity tasks, improving performed volume. But, before starting into much of what is described above, some important and recurrent terms are defined in the following section. 1.2 Background and Definitions Resistance training has gained popularity over the years as its benefits 8–10 and muscle tissue´s benefits11–13 for health have been unmasked. Also, the popularity of strength competitions (such as strongman, weightlifting, and powerlifting competitions) and physique shows (such as bodybuilding) have increased exponentially. In an eagerness for rapid muscle mass gain, some basic concepts of resistance training are devaluated, such as the importance of a proper warm-up. Warming up prepares the whole system for what is coming during the training session raising body temperature (increasing Adenosine Triphosphate -ATPturnover, muscle fibre functionality, cross-bridge cycling, and conduction velocity) and thus, being able to achieve the best performance possible14. There are plenty of protocols available, such as passive body temperature raises, plyometrics, low volume, and heavy resistance training bouts, mobility drills, postactivation potentiation and so on14. Briefly said, warming up prepares the body for the stress bout that training supposes. Once the warm-up is performed, we are ready to train. It is known that in order to adapt, we must put our bodies under a controlled amount of stress15–17, which has been 39 postulated both in the general adaptation syndrome 15 and in the subsequent fitnessfatigue model17. To standardise training bouts and fatigue management, and consequently the applied stress, several quantification methods have been proposed. One of those methods is the measurement of the mean propulsive velocity (in m·s-1) and the velocity loss (expressed in m·s-1 or by a percentage) of the lift18. When a set is performed really close to muscle failure, the velocity-time relationship of the lift changes, and a zone known as sticking region appears19,20. It is well known that training to failure leads to greater stress than not training to failure21. Nevertheless, whenever the objective is to improve performance per se or to improve performance to gain muscle mass, training near to muscle failure is needed to optimise gains22,23 , but should be wisely periodised21,24. The closer to failure, the higher the produced fatigue 25,26. As higher volumes seem to bring more gains27,28 , it seems intelligent approaching failure but trying not to detriment the total amount of volume that we are capable to tolerate. In this way, resistance training should be executed always under the supervision of a professional, or at least, following some good, evidence-based, and structured guidelines. As evidence seems solid against training systematically to failure, whether you are an experimented or novice lifter, it seems reasonable to optimise the warming up strategy to avoid failure during lifting. As mentioned earlier, several strategies are available, but this thesis focuses on the effects of the post-activation performance enhancement on the sticking region kinematics. 40 1.2.1. Post-Activation Potentiation or Post-Activation Performance Enhancement? 1.2.1.1 Brief definition Both Post-Activation Potentiation (PAP) and PAPE refer to an improved contractile capacity of a muscle after an intense bout of exercise, known as conditioning activity (CA). This is, after an intense bout of exercise the muscle tissue has an improved capacity to exert force voluntarily for a given stimulus or against a given resistance. The main difference between PAP and PAPE is that to confirm the presence of the first one, electric stimulation is used, while the term PAPE is used when confirmation via electric stimulation is not carried out29 . The conditioning activity must accomplish some requirements to produce an improvement in performance: a) There must be a biomechanical similarity between conditioning activity and the tested movement1 b) A minimum intensity is needed, achieved using either high-load and low-velocity exercises or light-load and high-velocity exercises30. When high-load and lowvelocity protocols are used, excessive proximity to muscle failure must be avoided, as high fatigue is produced21,24. c) Recovery between conditioning activity and the tested movement is of utmost importance. In this way, stronger individuals can benefit from shorter rest intervals (5-7 mins) than non-trained or weaker individuals (>8 mins)31. In addition, it seems that self-selected rest intervals adjust well to each one´s needs32. 41 The mechanisms behind PAPE are not fully elucidated. In the beginning, the phosphorylation of the regulatory light chain of myosin was proposed as the underlying mechanism (as in the case of PAP), but discrepancies in the occurring time were found. This led to the creation of new terms and the proposal of new mechanisms. 1.2.1.2. History of Post-Activation Potentiation First investigations talking about a topic related to post-activation potentiation (PAP) go back to the early XX century, where Lee33 studied in more detail what described Bowditch in 1867. In his study, Lee described what the treppe was, or, in English, the staircase potentiation. This staircase potentiation is based on the achievement of better muscle contraction after the application of repeated low-frequency stimulations as conditioning activity or stimulus. Later in that century, in 1937, the term post-tetanic potentiation34 was born, which refers to an observed augmentation of the tension produced by a fibre after tetanic contraction. Finally, in 1976 Burke et al. (1976)35 introduced the term post-activation potentiation (PAP), where they used frequencies and number of pulses more similar to what our body can achieve physiologically. This established a well-marked difference between post-tetanic potentiation and PAP. Nowadays, is commonly accepted that PAP is confirmed when an amplitude in twitch contraction is measured (a twitch is the contractile response to a single electrical stimulus applied directly to the muscle or the motor nerve). Thus, PAP needs of electrically evoked verification, but this seldom happens. This is one of the reasons why the term PostActivation Performance Enhancement (PAPE) was proposed, which refers to a performance enhancement produced by a voluntary action (like PAP), but with no twitch 42 verification29 . This problematic will be further discussed in “the confusing taxonomy problem” section. 1.2.1.3 The diversity of PAPE protocols Numerous PAPE protocols have seen the light over the last years with the intention of improving acute performance in resistance training. The most common protocol that can be seen in almost every gym is a high-intensity and low-velocity protocol, where a near maximal lift is performed. For this type of protocols, a heavy set of one repetition is performed before the effective work or the training session itself, looking for improved performance in that exercise. This “heavy set” is usually performed between 93-100% of the one repetition maximum (1RM), and frequently, this repetition reaches failure and relies on the help that a spotter (the person taking care of the participant not getting hurt during the training session) gives to that trainee. This usual protocol of PAPE reminds us the “forced repetitions” advanced training method for hypertrophy36, which relies on reaching failure and getting help to perform some more repetitions. While the forced repetitions method could be interesting to achieve muscle hypertrophy when prescribed wisely, they are not to achieve PAPE, as acute fatigue is more than needed and could underpin the benefits. Anyway, the use of this protocol is derived from a misinterpretation of scientific work behind. When applied correctly, this high-intensity and low-velocity protocol should be relatively near to, but not reach failure1. This is based on the fitness-fatigue model17, which usually is applied in the long term, but nothing seems to suggest that it cannot be used for shortterm or acute adaptations as PAPE. According to this model, the higher the applied 49 I. Two or more conditions must be compared II. Subjects must have marked experience in the performed task. If they do not have experience, familiarisation is crucial, as the learning effect could lead to misinterpretations of results. III. Conditioning protocols should be applied in random order IV. If possible, a double-blind approach should be considered. As the researcher cannot be blinded, it is important that subjects do not know what protocol they are going to run, and is of uttermost importance that they do not know what researchers expect of the protocol, as results could be biased Is worth mentioning that MacIntosh et al. (2012)29 highlight the importance of monitoring body temperature whenever possible to be able to distinguish effects caused by PAP/PAPE or by the temperature raise itself14,70. Blazevich and Babault59 mention some possible mechanisms to explain PAPE´s time course, such as increased water flow into the muscle cell, which could lead to improved Ca2+ sensitivity, which ultimately leads to an augment of rate of force development. As a matter of fact, Sugi et al. (2013)71 stated that in conditions of low ionic strength (low concentration of ions in a solution), muscle fibres could exert higher forces than compared to those conditions of higher ionic strength (50mM and 170mM KCl respectively), even if maximum unloaded shortening velocity remained unchanged. They attributed the improvement of force to an amplification of the force exerted by every myosin head (increasing twofold) rather than to the quantity of formed crossed bridges. In a later study by the same group72 an electron microscope combined with an environmental gas chamber was used to assess more accurately myosin´s power stroke mobility. Shortly, the 50 myosin head can be divided into the following parts: (i) The catalytic domain (CAD) that has a proximal (respect to the myosin backbone) and a distal part, (ii) the converter domain (CD) and (iii) the lever domain (LD), which is bound to the Subfragment 2 (S2) of the myosin (Figure 1). Figure 1. Representation of the myosin head and its parts. CAD: Catalytic domain; 1: Binding site for antibody 1, CAD distal part respect to the myosin backbone; 2: Binding site for antibody 2, CAD proximal part respect to the myosin backbone; CD: Converter domain; LD: Lever domain; S2: myosin subfragment 2. Adapted from Sugi et al. (2015)72 In this study72, researchers showed that under normal conditions, the distal portion of CAD performed a power stroke of 3.3 ± 0.2 nm (n = 732) and the proximal region of CAD 2.5 ± 0.1 nm (n = 613), while in low ionic strength condition, the distal portion of CAD performed a power stroke of 4.4 ± 0.1 nm (n = 361) and the proximal portion 4.3 ± 0.2 nm (n = 305). This increment in the performed power stroke is in line with the hypothesis that in low ionic strength conditions the improved force is due to the 51 augmentation of force per myosin head rather than to the number of cross-bridges formed. Nevertheless, the accretion of water content inside muscle cells (also known as cell swelling) is produced after an intense bout of resistance training23 or by augmented blood perfusion, so further research is needed to know if the minimal cell swelling that a warmup produces could be effective to improve the force exerted by each myosin head. Another promising mechanism to explain the elusive mechanism behind PAPE is muscle temperature. Decostre et al. (2005)73 studied the effect of different temperatures over the working stroke of actin-myosin. In this study, the working stroke was divided into four phases: I. Phase 1 is a change in length due to the elastic components of the sarcomere II. Phase 2 is the shortening directly produced by the working stroke III. Phase 3 is the reduction of shortening velocity due to detachment and reattachment of the myosin head along the actin filament IV. Phase 4 Final shortening at constant velocity due to steady-state detachment/attachment of myosin heads In their study, Decostre et al. (2005)73 found that with increasing temperature phases 2 and 4 were faster, while phase 3 was shorter. Additionally, they found that both passive and active elements of the sarcomere were strained in proportion to force, pointing to a rise in force per myosin head as the reason why the increase in temperature produces more force. This augmented force per myosin head could be due to a higher quantity of myosin heads in a high energy state. Authors propose that the increase in temperature provokes a lowering of basic free energy, facilitating the transition of myosin heads from 52 a low energy state (A1) to a high energy state (A2). The lowering in basic free energy makes the reaction happen easier or even spontaneously (DG = DH - TDS; this results in DG > 0), making it easier for myosin heads to achieve the high energy state. But the principal limitation of this study is that the temperatures they use are very low (between 2ºC – 17ºC), which makes the results difficult to extrapolate to physiological conditions, even if it shows that temperature could play an important role. In addition, the role of temperature in muscle contraction was recently addressed by Rodrigues et al. (2022)74 in their review. In this study are mentioned two further studies, one by Racinais et al. (2017)75 where participants improved their maximal voluntary contraction (MVC) and another study by Goto et al. (2011)76 where participants improved their muscle mass. I would like to highlight some points here. First, in Racinais et al. (2017)75 there is no description of participants´ experience level in strength training, so possibly they could be novices. In Goto et al. (2011)76 participants do not practice serious or competitive level sport, only recreationally. Also, in this last study, muscle mass is augmented, but muscle mass augmentation and improvement of functional hypertrophy are not the same, as the main objective of any resistance training plan is to seek an augmentation of contractile protein quantity52. Finally, is worth mentioning that both studies talk about passive heating protocols, which helps us understand the potential interests of studying temperature effect on performance, but results should be taken carefully, as passive heating protocols are long and not applicable in field conditions (1h / day over 11 days75 and 8h /day, 4 days /week over 10 weeks76 ). Leaving aside long-term temperature raise´s potential benefits, Rodrigues et al. (2022)74 focus the rest of the review on the acute effects of temperature raises. They defend that 53 temperature raises improves half-relaxation time of the contractile apparatus, which would be caused by the improved kinematics of Ca2+, as suggested by the improved activity in the Sarco/Endoplasmic reticulum Ca2+-ATPase (SERCA), that has as the main function to transport Ca2+ from cytosol back to the sarcoplasmic reticulum61. Water and temperature seem two promising mechanisms for PAPE, but could they be related somehow? Could they be two sides of the same coin? The review by Rodrigues et al. (2022)74 follows exposing that temperature raises can cause blood perfusion by an increased liberation of adenosine triphosphate (ATP), which acts as a vasodilator. When vasodilation occurs, blood flow increases following Poiseuille´s law61, which states that: $=#·"'·(! 8·*·+ Here we can see that if pressure difference ( " P), viscosity (η), and length (l) are kept constant, blood flow (F) will increase proportionally to the fourth power of the radius of the blood vessel (r4). This augmented flow is due to the laminar flow of the blood when the radius increases. In small vessels, all the blood inside causes friction against the vessel´s walls, while increasing the blood vessel´s radius increases the amount of blood that can get inside. If more blood gets inside the vessel, only the outermost layer of blood will cause friction against the blood vessel´s wall and the centermost layer will circulate faster (which is known as laminar flow parabolic velocity profile)61,77,78. 54 This augmented blood flow due to vasodilation increases intramuscular water content, which leads us again to Sugi et al. (2013; 2015)71,72 , but still we do not know why it happens. The answer could be in the studies by Eng et al. (2018)79 and Eng & Roberts (2018)80. In their studies, Eng et al. (2018)79 and Eng & Roberts (2018)80 state that due to the isovolumetric property of the muscle fibre, when it contracts, it can be deformed in any direction orthogonal to the line of action. When McKibben actuators were disposed close to each other to mimic real muscle fibre conditions during muscle contractions, they saw that each actuator limited the upwards and downwards expansion of the adjacent actuators, producing a radial expansion to the sides, and even rotations, if needed. One of the most important things highlighted in this model is that those forces that are not aligned with fibres´ line of action (those off-axis forces), load intramuscular connective tissue and fluid. These off-axis forces are caused by compression of intramuscular fluid, requiring fibres to expand radially as they shorten. The second cause of the off-axis forces are the thickness forces generated in pennate muscles, the component of the force that compresses muscle in the thickness direction. Thus, there are two forces deforming muscle fibres: (i) the thickness force that crushes muscle “from upside down” and (ii) the radial force that expands the fibre. Authors hypothesise that to counteract the thickness force, intramuscular fluid expands causing the radial expansion. This radial expansion loads muscular elastic elements such as endomysium, perimysium, and epimysium (Figure 2). 55 Figure 2. Link among temperature, water content and force transmission. Filled blue circles represent water molecules; Fthickness: Force in the upside-down direction that crashes or compresses the muscle; Fradial: Radial expansion produced by fluid in response to Fthickness . Adapted from Rodrigues et al. (2022)74 In summary, intramuscular fluid redirects the thickness compressive force to radial stretching force, for which muscles are prepared due to their elastic components. Considering this, increments in intramuscular water may enhance muscle compressive force tolerance, acting like a spring, and improving force transmission74, which could explain the link between water and muscle temperature in enhanced produced forces. Even if this seems promising as an explanation of PAPE, what difference would be between a temperature raise caused by a classic warm-up and a temperature raise caused by PAPE? Could this temperature raise be significant? Weigert et al. (2018)81 found small and non-significant temperature changes after 10 repetitions of biceps curl at 70% 1RM, suggesting that some PAP/PAPE protocols (the traditional heavy protocols consisting of 1 set of 1 repetition for example) are unlikely to rely on this mechanism. Boullosa et al (2018)82 mention that a possible Ca2+ raise due to low-frequency twitches that could enhance performance in those protocols. This would produce an effect similar to MLCK activation, but without myosin phosphorylation. 56 In any case, existing PAP/PAPE protocols are diverse, some could rely in a mechanism and others in other mechanism. Could it be that PAPE is an extension of PAP over time? It is reasonable to think that myosin regulatory light chain phosphorylation occurs and that its effect is lengthened due to temperature raises, intracellular water content, improved Ca2+ sensitivity or another mechanism. As scientists, we tend to compartmentalise effects and its causes, but the human body is complex and nothing seems to suggest that PAP and PAPE happen independently one of each other59,83. This is, PAP occurs before PAPE because of the rapid action of MLCK, and PAPE occurs after for some mechanism not fully elucidated yet. But, even if PAPE has another mechanism behind, this does not mean that PAP could not contribute to PAPE´s effects. PAPE could perfectly be an extension of PAP over time by another synergistic mechanism. 1.2.1.6 The confusing taxonomy problem Considering all that has been said in the previous section and remembering that PAP lasts ~4-5 min, why are so many studies talking about PAP and reporting performance improvements 8,10 or even 20 min after conditioning activity? There are more terms than PAP, and the unification of those terms seems harder than the elucidation of the mechanism behind PAPE. Originally, PAP and PAPE were created with the intention of simplifying things to researchers: PAP was used to talk about potentiation verified with electric twitch, and PAPE when that verification was not carried out84 even if in both cases the conditioning activity relies on a voluntary contraction. 57 However, a variety of new terms have been proposed in addition to those two original terms. One of the attempts to establish a more precise terminology was carried out by Boullosa et al. (2020)85. They defend that a more accurate terminology is needed to include some influencing factors like training experience1, sex differences86 and training background87 . Following this line of reasoning, they propose the following scheme to name any potentiation protocol: Post-(conditioning activity) (verification test) Potentiation in (Population) Some examples of that scheme are provided in their paper: • Post-high intensity long intervals, sprint potentiation in well-trained runners • Post-eccentric flywheel squat swim start potentiation in varsity trained male swimmers This is a really complete proposal, where all details are specified in the name of the potentiation protocol, but it does not seem really applicable in real life where more straight forward names are easier to remember. Based on the extreme complexity of this proposal, Smith and MacIntosh (2021)83 replied, defending their old and simpler two term taxonomy (PAP & PAPE, irrespective of protocols, sex, or population). Recently, Cormier et al. (2022)30 tried a new taxonomy proposal. In this case, researchers summarise existing protocols to three possible types: A. One or several sets of high-load exercise (back quat) before a low-load exercise (CMJ) 58 B. One or several sets of low-load and high-velocity exercise (CMJ) before a highload exercise (squat) C. Alternating high-load and low-load exercises in a set-by-set fashion They propose a broader terminology than PAP and PAPE at the time that they notoriously simplify the terminology proposed by Boullosa et al. (2020)85. They start proposing “Complex” as an umbrella term which is going to be combined with four different subterms or implementations, namely: I. Contrast: An exercise sequence alternating high-load and low-load exercises in a set-by-set fashion (e.g., Back Squat 85% 1RM – CMJ - Back Squat 85% 1RM – CMJ) II. Ascending: Several sets of low-load exercise before a high-load one (e.g., CMJ – CMJ – Back Squat 85% 1RM – Back Squat 85% 1RM) III. Descending: Several sets of high-load exercise before a low-load one (e.g., Back Squat 85% 1RM – Back Squat 85% 1RM – CMJ - CMJ) IV. French Contrast: A heavy compound exercise – A plyometric exercise – Light or moderate compound exercise – Assisted plyometric exercise Thus, combining the umbrella term (Complex – CPX) with its four sub-terms, depending on the characteristics of the protocol that is going to be used, the new terminology would be: (i) Contrast complex (CPX – CNT), (ii) Descending complex (CPX – DT), (iii) Ascending complex (CPX – AT), or (iv) French Contrast Complex (CPX – FCNT). This last proposal seems the more suitable one, as it remains concise at the time it is specific. I would like to add that a new term should be created specifically for eccentric6 65 1.3 Theoretical Framework of the Thesis 1.3.1 Ideological Basis As a new strength coach and Sport Science student, I wanted to learn as much as possible to apply all that knowledge into my work. When I started reading and learning on my own and realising how things work, how I could improve my training, I started noticing how people around me train in the gym. Of course, not everyone had a coach (either online or presential), but even some of those people under the supervision of an online coach, used to train following old guidelines or scientifically discredited methods, exercises or technical standards. Among those discredited terms and techniques, PAPE (what in gyms is still known as PAP) and Sticking Region (what, again, is still known as Sticking Point) were really popular. When I realised about this situation, interesting question came to me: 1. Why do so many coaches use the term PAP instead of PAPE 2. Why do so many powerlifting coaches say Sticking Point, and not Region? 3. Why do coaches use a PAP/PAPE strategy prior to a heavy set when most of studies are oriented to submaximal intensity work? 4. Why do coaches assume that better performance produced by PAP/PAPE equals more muscle mass gain? 5. Why don´t coaches use more than a traditional PAPE protocol? This disconnection between scientific literature and coaches´ practice sparked the idea for this thesis. Why was this happening? Is science available and understandable for graduated students and coaches? 66 Strength training has gained notorious popularity. The quantity of publications, whether it is for health or performance, has raised notably (437 publications indexed in PubMed in 2000 vs. 5,886 publications indexed in PubMed in 2021). This is not unusual considering that in science, what is true today, can be refuted tomorrow by another study. But are those publications read by someone? When studying the Sport Science degree, we are taught how to look for information and how to filter that information, what in my own opinion, is the most important competence acquired during the 4-year journey. Those who really enjoy studying or who really appreciate the potential source of knowledge they are presented when learning these skills, are later known as scientists. Usually, whether they undergo a PhD or dedicate their life to divulgation in personal blogs or social networks, it does not matter, they are responsible for creating, interpretating and teaching what is new and what needs to be known. But what about trainers and coaches? Is science the privilege of a small population wearing white lab coats, the strange language of people that has never trained? Is one on one coaching reserved to those people that are only based on experience, to those foot soldiers? Nowadays this division is happening and is something we should think about. If scientists are making contributions and advances in sport science knowledge, but this knowledge is not being read by coaches, what use does it have? Of course, researching about a topic for the mere purpose of knowing, even if it does not have real world applications is legit, and must be done, as this can derive in different research lines in the future that will have application. 67 1.3.2 From Theory to Reality: When did science and practice take different paths? In a study based on a transcription of a roundtable of the inaugural congress of the Australian Association for Exercise and Sports Science by Bishop et al. (2006)105, participants were asked about what sport-science for them was, about the necessity of instant applicability of research and if the problem was due to the lack of communication abilities of the scientists. Shortly, there was a clear differentiation on base sport-science and applied sport-science. The first was made for publication, and the applicability in field was secondary, while the last one was science made for application in coaching, and it may or may not be published. This difference was still present in the second question, where it was stated that the broad majority of coaches asked for rapid solutions, while less coaches were patient enough to wait for longitudinal studies or series of studies. These two questions stand out something: coaches think that studies should be more appliable and short-term oriented. But, as Dr. Robert Newton stated during that roundtable, important results cannot come out of nowhere, they need time and a series of preceding studies; asking only for short-term oriented studies does not make sense. Instead, coaches need patience and ability to interpret results and seek for application. Even if coaches are patient, it is still the doubt if the problem relies on the lack of interpretation ability of coaches or poor communicating ability of scientists. As their name indicate, sport-scientists research around sport, that is, to improve performance, reduce injury risks, improve technologies… Thus, a good communication among the 68 principal stakeholders is crucial, and researchers need to improve their communicating abilities as much as their research abilities105. This is why there are scientists that are not dedicated to research, but to interpretation of research for the less specialised public, as undergraduate students. This is of paramount importance, as nowadays social networks are used by everyone to obtain information. Based on www.statista.com , Facebook, Youtube and Instagram are 3 of the 4 most used social networks, with 2,910 million, 2,562 million and 1,478 million users in 2022. Everyone follows people or subjects in which are interested, and there is where scientists should come in. They should be there to fight misinformation and misinterpretation; they should be there to provide audience with quality information. Science is useless if it reaches nobody. Dr. Carl Sagan believed that there was an urge to stop considering science as an elitist language, that science needed to be understood by everyone. Dr. Richard Feynman once said that if you cannot explain something in simple terms, you don´t understand it. These two great scientists believed in the necessity of communication skills, to make science available to anyone who needed it. Of course, there are scientific subjects and discussions that cannot be simplified enough to be understood by people without previous studies, and those subjects are inevitably reserved for the scientific community. Anyways, I still think that most of the people perceive science as an inaccessible field. One of the main reasons why I decided to do my PhD was promoting the “scientific coach” figure. I strongly believe that scientists can be coaches or advisors if they are really interested, and coaches can (and should) be scientists, as they need to keep up with 69 science to do their job the best they can. There is no need to publish papers by coaches if they do not want to, but at least they should be able to search, filter and read scientific studies. I am not pretending to disdain the importance of experience when coaching. Experience can be critical, especially when the sport involves difficult technical or tactical aspects. But what is clear is that there is a need for a bridge between science and practice, and promoting general public oriented divulgation could help. Specialised texts are needed and have their purpose; but as scientists, we should not forget that science, even if made by a small, specialised population, is meant for everyone, and should be as accessible as possible. Thus, the starting point of this thesis was to try to remove as many biases as possible, as many misinterpretations as possible, and creating a document to which any strength, powerlifting or bodybuilding coach thinking to implement PAPE in their athletes´ training could go to. I have tried separating mere curiosity and non-applicable content (as physiological mechanisms) from the more applicable and field oriented (as different existing protocols and their possible uses), with the intention of easing the reading and facilitate finding what the reader is looking for. 70 71 2 HYPOTHESES & AIMS The culture we have does not make people feel good about themselves. And you have to be strong enough to say if the culture doesn´t work, don´t buy it. Mitch Albom – Tuesdays with Morrie 72 73 2. HYPOTHESES & AIMS 2.1. Hypothesis In order to make this case 3 papers were planned. The first study was a systematic review of the scientific literature available on PAP and PAPE for strength sports, and it was performed to serve as an introduction. Following this we analysed the possible effect of an isometric PAPE protocol compared to a traditional PAPE protocol in high-loaded (85% 1RM) strength training, and its effects on sticking region kinematics. Finally, we analysed the effects of a high-loaded (93% 1RM) traditional PAPE protocol compared to a control group (no PAPE) on performed repetitions until volitional failure in a set of bench press, as a potential way to improve muscle mass adaptations. In essence, we analysed the possible application of a different contraction regime PAPE protocol, to increase the number of available useful protocols and the effect of a PAPE protocol in training volume. This was performed with the intention of expanding the number of available tools for coaches and strength training specialists. 2.2. Summary of Aims • Examine the literature regarding Post-Activation Potentiation and Post-Activation Performance Enhancement (Publication 1) • Examine the importance of volume and intensity in Post-Activation Potentiation and Post-Activation Performance Enhancement (Publication 1) • Examine the effects of an isometric PAPE protocol compared to a traditional PAPE protocol (Publication 2) 74 • Examine the effects of an isometric and traditional protocol over the sticking region (Publication 2) • Compare the performed volume in a training bout with and without a high-loaded traditional PAPE protocol (Publication 3) • Compare the velocity loss during a training bout to volitional failure with and without a high-loaded traditional PAPE protocol (Publication 3) • Explore why any potential discrepancies occurred (All Publications) 81 3. METHODOLOGY 3.1 Publication 1: Post-activation potentiation in strength training: A systematic review of the scientific literature 3.1.1 Experimental Approach to the Problem A literature search was conducted on October 23, 2020. The following databases were searched: PubMed and Scopus. The previously named databases were searched from inception to October 2020, with language limitations: only peer reviewed articles in English were selected. Citations from scientific conferences were excluded. 3.1.2 Literature Search In the database, the title and abstracts were searched. The following MeSH terms and key words, combined with the Boolean operators (AND, OR), were used: “athletic performance”, “resistance training”, “post activation potentiation”, “PPA”, “PAP”, “postactivation potentiation”, “potentiation post activation”, “potentiation post-activation”, “performance”, “strength performance”, “strength training”, “strength” and “powerlifting”. No additional filters or search limitations were used. 3.1.3 Inclusion Criteria Studies were eligible for further analysis if the following inclusion criteria were met; a) subjects´ age ranged between 18-30 years; b) studies analysed experienced lifters; c) post- 82 activation potentiation was studied in sports with high requirements of the rate of force development; d) the potentiation protocol was conducted with barbell exercises; e) preand post-evaluation was done with a resistance exercise, vertical jump or similar (i.e. squat jump, counter movement jump or drop jump). In the studies where volume was not directly reported, it was calculated as follows: volume = sets x repetitions x kilograms. Figure 4. Flow chart of search strategy and selection of articles 3.1.4 Quality assessment Oxford’s level of evidence108 and the Physiotherapy Evidence Database (PEDro) scale109,110 were used in order to assess the methodological quality of the studies included in the review. Oxford’s level of evidence ranges from 1a to 5, with 1a being systematic 83 reviews of high-quality randomized controlled trials (RCT) and 5 being expert opinions. The PEDro scale consists of 11 different items related to the scientific rigor. Given that assessors are rarely blinded and that blinding participants is almost impossible, items 57 (which are specific to blinding) were removed from the scale28. With the removal of these items, the maximum result on the modified PEDro scale was 7 (the first item is not included in the final score) and the lowest, 0. Zero points are awarded to a study that fails to satisfy any of the included items and 7 points to a study that satisfies all the included items. 84 Table 1. Physiotherapy Evidence Database (PEDro) ratings and Oxford evidence levels of the included studies. Study 1 2 3 4 5 6 7 8 Total Evidence level Andrews et al. (2016) Yes 1 1 1 1 1 1 1 7 1b Comyns et al. (2007) Yes 1 1 1 1 1 1 1 7 1b Dello Iacono et al. (2019) Yes 1 1 1 1 1 1 1 7 1b Do Carmo et al. (2018) Yes 1 1 1 1 1 1 1 7 2b Gilbert & Lees (2007) Yes 1 1 1 1 1 1 1 7 1b Golas et al. (2017) Yes 1 1 1 1 1 1 1 5 2b Kilduff et al. (2008) Yes 0 0 1 1 1 1 1 5 2b Kobal et al. (2019) Yes 0 0 1 1 1 1 1 5 2b Krzysztofik et al. (2020a) Yes 1 1 1 1 1 1 1 7 1b Krzysztofik et al. (2020b) Yes 1 1 1 1 1 1 1 7 1b Krzysztofik et al. (2020c) Yes 1 1 1 1 1 1 1 7 1b Krzysztofik and Wilk (2020) Yes 1 1 1 1 1 1 1 7 1b Lowery et al. (2012) Yes 1 1 1 1 1 1 1 7 2b Mina et al. (2019) Yes 1 1 1 0 1 1 1 6 1b Poulos et al. (2018) Yes 1 1 1 1 1 1 1 7 1b Reardon et al. (2014) Yes 1 1 1 1 1 1 1 7 2b Thomas et al. (2015) Yes 1 0 0 1 1 1 1 5 2b Items in the PEDro scale: 1 = eligibility criteria were specified; 2 = subjects were randomly allocated to groups; 3 = allocation was concealed; 4 = the groups were similar at baseline regarding the most important prognostic indicators; 5 = measures of 1 key outcome were obtained from 85% of subjects initially allocated to groups; 6 = 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 1 key outcome were analysed by “intention to treat”; 7 = the results of between-group statistical comparisons are reported for at least 1 key outcome; 8= the study provides both point measures and measures of variability for at least 1 key outcome 85 3.2 Publication 2: Intermittent Voluntary Isometric Contractions Effects on Performance Enhancement and Sticking Region Kinematics in the Bench Press 3.2.1 Participants Twenty-one participants (age 26.4 ± 5.4 years; body mass 79.4 ± 9.7 kg; body height 176.2 ± 6.9 cm; medium grip bench press 1 repetition maximum (1RM) 97.4 ± 19.8 kg; relative strength (1RM/body mass) 1.22 ± 1.9) with at least two years of resistance training experience voluntarily took part in this study. Participants were required to meet the following inclusion criteria: 1) men between the age of 18–40 years; 2) lack of musculoskeletal disorders or injury in the previous 6 months; 3) experienced in resistance training, defined as consistently lifting weights at least 3 times per week for a minimum of 2 years. A total of 20 participants completed the study: one participant dropped out prior to completion due to personal reasons. We did not control for nutrition nor hydration levels, but participants were told not to make any changes in the above during the testing period. Participants were asked to refrain from training 48 h before each testing session and not to take caffeine. All participants performed the three sessions at the same time of the day with at least 48 h of rest between sessions. Written informed consent was obtained from each participant after a thorough explanation of the testing protocol, the possible risks involved, and the right to terminate participation at will. The study was conducted according to the Declaration of Helsinki111, and the Institutional Review Board of the University of the Basque Country (UPV/EHU) approved the experimental protocol. 86 3.2.2 Procedures Participants visited the laboratory on three separated occasions. Prior to every experimental session and the 1RM test, participants performed a standardised warm-up protocol, consisting of 5 min of cycling and bench press warm-up sets consisting of 1 set of 12 repetitions with the barbell only, followed by 3 sets of 8, 6 and 3 repetitions with 40%, 60% and 75% 1RM, respectively. The rest interval between warm-up sets was to 2 min. In every session, during the bench press, participants performed the descent with a 2 s tempo, followed by a 1 s pause in the chest with help of a metronome at 60 beats per minute (BPM) to standardise repetitions. Participants were instructed to perform the concentric phase as fast as possible. Bench press grip width was set at 1.4 times biacromial distance as described elsewhere 89. 3.2.2.1 1RM calculation During the first visit, participants underwent a direct 1RM test for the bench press. The 1RM was defined as the highest load lifted by participants without any compensatory movement and only if they completed the pause on the chest properly. When an attempt was successful, the next attempt was decided asking the participant and evaluating the reported mean propulsive velocity by the velocity linear (Speed4Lifts, Spain) 112. Participants rested for 3 min between attempts. The test finished when participants reported a rate of perceived exertion (RPE) of 10 in the repetitions in the reserve based RPE scale 54. If participants failed an attempt, the weight was reduced by 2.5 kg and another attempt was performed after a 3 min rest interval. 87 3.2.2.2 Sticking region identification In addition, each lift on familiarization was recorded from a side view at 300 Hz using an active LED marker on the barbell’s edge and a high-speed video camera (Casio ExilimEX-F1). Video recordings were analysed using kinematic analysis software Kinovea (version 0.8.15), which is valid, precise, and reliable 113. Data exported from Kinovea to Excel (version 16.16.27) were filtered (Butterworth low pass filter at 5Hz) and then used to determine where the sticking region was, defined as the region of the lift between the first peak (Vmax peak) in velocity and the first minimum after the peak (Vmin peak) 114. This region differs inter-individually due to differences in anatomical crosssectional area of the muscle, force-length relationship, force-velocity relationship, fatigue, motor unit recruitment, fiber type and biomechanical factors that affect torque development 20. Once the sticking region was detected, the height of the barbell at this region was calculated. Since the sticking region is not a specific point, but a range of motion of the lift, to ensure that the isometric contraction affected the sticking region, the protocol was performed in the middle of this region, as isometric contractions had been demonstrated to strengthen 20º-50º away from the adopted joint angles 4. 3.2.2.3 Conditioning Activities Measurements of the experimental conditions lasted 45 minutes and were scheduled one week after the first visit to the laboratory. The study followed a within-participant design, where each participant was his own control. In this way, in both experimental sessions 88 participants performed a pre-conditioning lift (control lift), a conditioning activity and several post-conditioning lifts. Thus, the second day, participants were randomly assigned to one of the following two experimental conditions: an isometric contraction conditioning protocol (ISO) or a traditional conditioning protocol (TRAD). Volume was not matched between conditioning activities. On the third session, participants changed experimental conditions. After completing the conditioning protocol (ISO or TRAD), participants were asked for RPE. Figure 5. Scheme of followed investigation procedure Each experimental session consisted of the standardised warm-up protocol followed by a 3-min rest interval and a pre-conditioning lift (control lift), which consisted of 1 set of 1 repetition of the bench press at 85% 1RM. After the pre-conditioning lift, participants rested 3 min and then they proceeded with the conditioning activity to which they were randomly assigned. 89 The ISO conditioning activity consisted of 15 maximal voluntary isometric contractions (MVIC) of 1 s with 1-s rest interval between contractions5 at their sticking region as previously described. Participants were encouraged to exert force as fast as possible. Isometric contractions were performed by fixing the barbell of a Smith machine at the appropriate height (using a 11mm diameter rock climbing rope) to match the middle of the sticking region height. The TRAD conditioning activity consisted of 1 set of 1 repetition with 93% of their estimated 1RM of the familiarisation day1. Post-conditioning measurements (1 set of 1 repetition of the bench press at 85% 1RM) were recorded 0, 4, 8, 12 and 16 minutes later (post0, post4, post8, post12 and post16)103,115 using the same velocity linear transducer. Participants were instructed to lift the barbell as fast as possible during the ascending phase of the movement. If participants improved performance from pre-conditioning to any of the postconditioning lifts, they were chosen for further analysis of the sticking region kinematics. This distinction between responders and non-responders to conditioning activity was based on the calculated smallest meaningful difference. When the difference in the best post-conditioning lift and the pre-conditioning lift was higher than the smallest meaningful difference, participants were considered responders and chosen for analysis of the sticking region. 90 3.2.3 Statistical Analyses Data were screened for normality of distribution using the Shapiro-Wilk test. Two-way ANOVA with repeated measures (lift x time) was used to determine if any of the postconditioning lifts improved performance under each experimental condition and to compare same time point across experimental conditions (ISO vs. TRAD). The magnitude of differences of effect sizes (ES) were calculated using Cohen’s d 116 and interpreted as small (>0.2 and <0.6), moderate (≥0.6 and <1.2) and large (≥1.2 and <2) or very large (≥2) according to Hopkins et al. (2009) 117. All statistical analyses were performed using Prism 9 for Mac. Significance for all analyses was set at p < 0.05. 95% confidence intervals are reported as 95% [Lower limit, Upper limit]. Additionally, for those participants for whom the conditioning activity improved performance (responders), the velocity until the first peak in velocity was measured and compared using one-way ANOVA with repeated measures (lift x time), comparing pre-conditioning velocity, post0 velocity and velocity of the fastest time point. To select those responders, we calculated the smallest meaningful difference following the formula below: , 2·." /·!(1#$% +1#$& ") The reported results by the formula were smaller (e.g., measured smallest meaningful difference = 0.005 m·s-1) than values reported by the used velocity linear transducer (e.g., 0.23 m·s-1). Thus, if any participant improved performance in any post-conditioning lift (e.g., from 0.23 m·s-1 to 0.24 m·s-1), the difference must have been higher than the calculated smallest meaningful difference. We also performed a Fisher´s exact test to 97 4. RESULTS 4.1 Publication 1: Post-activation potentiation in strength training: A systematic review of the scientific literature 4.1.1 Studies Selected The search strategy yielded 202 total citations as presented in Figure 1. From those 202 articles, 17 met the inclusion criteria. Excluded studies had at least one of the following characteristics: the potentiation protocol included strategies different from resistance training (e.g., electrostimulation or vibration), participants were not experienced lifters (had less than 2 years of resistance training experience or less than 2 x bodyweight squat 1-RM) or the evaluation protocol was done with sprinting bouts (Table 1). 4.1.2 Level of Evidence and Quality of the Studies Ten of the seventeen included studies had a level of evidence 1b (good quality randomized control trials). The 7 remaining studies had a level of evidence of 2b (individual cohort studies). Also, the mean score in the PEDro scale was 6.47 ± 0.87, with values ranging from 5 to 7 (Table 1). 4.1.3 Characteristics of the Participants Participants were characterized as experienced or well-trained athletes due to their training experience or their one repetition maximum (the maximum amount of weight that a person is able to lift for one repetition). A summary of participants´ characteristics 98 is presented in Table 2. The total number of participants was 279 (253 men, 6 women and 20 unknown). Table 2. Included studies Study Number (M/F) Age (years) RT experience (years) Main Outcome Andrews et al. (2016) 14 (8/6) M 21.3 ± 1.8 / F 21.2 ± 0.4 ≥ 2 Unilateral PAP and fatigue Comyns et al. (2007) 12 (12/0) 23.3 ± 2.5 1RM ≥ 2x bodyweight Optimal resistive load and PAP Dello Iacono et al. (2019) 26 (26/0) 23.2 ± 5.1 ≥ 2 Traditional sets PAP vs cluster sets PAP Do Carmo et al. (2018) 12 (12/0) 25.4 ± 3.6 ≥ 3 PAP rest interval Gilbert and Lees (2007) 15 (15/0) 24.3 ±3.3 unknown Changes in force development Golas et al. (2017) 16 (16/0) 18-35 ≥ 5 Used PAP load magnitude Kilduff et al. (2008) 20 (Unknown) 25.4 ± 4.8 3.1 ± 1.6 Recovery time and PAP Kobal et al. (2019) 18 (18/0) 25.42 ± 3.58 3 Different volume and PAP Krzysztofik et al. (2020a) 12 (12/0) 25.2 ± 2.1 3 PAPE and training volume Krzysztofik et al. (2020b) 32 (32/0) 28.4 ± 4.5 3 Eccentric and concentric PAP Krzysztofik et al. (2020c) 13 (13/0) 25.7 ±1.9 6.5 ± 2.2 Eccentric PAP Krzysztofik and Wilk (2020) 24 (24/0) 24.5 ± 2.6 6.3 ± 2.5 Plyometric PAP protocol Lowery et al. (2012) 13 (13/0) 21 ± 3 3 PAP stimuli and recovery time Mina et al. (2019) 15 (15/0) 21.7 ± 1.1 ≥ 5 PAP: free weight vs variable resistance Poulos et al. (2018) 15 (15/0) 24.3 ± 2.6 ≥ 2 Back Squat intensity and PAP Reardon (2014) 11 (11/0) 25.18 ± 3.60 1RM ≥ 2x bodyweight Muscle architecture and PAP Thomas et al. (2015) 11 (11/0) 23 ± 4 ≥ 2 PAP and neuromuscular function PAP = post activation potentiation; M = male; F = female; RT = resistance training 99 4.1.4 Studies matching volume load Five of the included 17 studies matched the volume load in the protocols used. From these five studies, three compared different intensity protocols103,115,119 and two the optimal rest interval32,40. Mina et al. (2019)103 performed a study comparing free weight back squats and variable resistance back squats (elastic bands were used to generate the 35% of the total load at the upper part of the squat). Under the free weight condition, no significant changes were found in jump height, peak power or a normalized (to body weight) rate of force development (RFD) compared to pre-intervention performance. On the other hand, under the variable resistance condition, statistically significant increases (p < 0.05) in CMJ height were observed at 30s (5.9 ± 1.2%), 4 min (5.6 ± 1.8%), 8 min (6.5 ± 2.6%) and 12 min (5.3 ± 2.5%) compared to pre-intervention. In addition, statistically significant increases (p < 0.05) were evident in peak power at 30s (4.7 ± 1.2%), 4 min (5.9 ± 1.3%), 8 min (4.4 ± 1.7%) and 12 min (4.8 ± 1.7%) time points. These changes in CMJ height and peak power were also significantly different from the free weight condition group (p < 0.05). Dello Iacono et al. (2019)119 compared the effect of two protocols using the individualized optimal power load with traditional and cluster-set configuration in a randomized crossover design. Although both protocols increased jump height 4 and 8 min postintervention, the cluster set configuration reached significantly better results by 1.33 cm (95% CI, 1.02 to 1.65 cm) and 1.64 cm (95% CI, 1.41 to 1.88 cm), respectively. 100 Additionally, cluster set configuration was able to maintain 10% higher power output (95% CI, 8 to 12%) relative to their relative mean propulsive power. Lowery et al. (2012)115 studied the effects of three different loads (light, 56% 1RM; medium, 70% 1RM; and heavy, 93% 1RM) on vertical jump height. Vertical jumps after the light load protocol did not reach statistically significant differences. Moderate and high load protocols decreased vertical jump performance right after the conditioning activity (p < 0.05; ESmedium loaded = -2.45, large; ESheavy loaded = -2.87, large). Additionally, a medium loaded protocol reached a significant performance increase at 4 min in the post activation training protocol (p < 0.05; ES = 1.46, large) and a high loaded protocol reached statistically significant improvements at both 4 and 8 min post protocol (p < 0.05; ES4min = 1.34, large; ES8min = 1.48, large). Kilduff et al. (2008)40 attempted to set the optimal recovery time for a complex training session. Participants performed 3 sets of 3 repetitions at 87% 1RM back squats before an explosive activity. They reported a statistically significant (p < 0.05) decrease at 15s post conditioning activity and a statistically significant (p < 0.05) increase at 8 min post conditioning activity for power output and for jump height. A statistically significant (p < 0.05) increase in the RFD 8 min post conditioning activity was also reported. Additionally, Do Carmo et al. (2018)32 suggested that self-selected rest intervals were better than a fixed rest interval in order to dissipate the fatigue created by the conditioning activity. They conducted a study, and no significant changes were observed after the conditioning activity in the fixed rest interval group (38.0 ± 5 cm vs. 37.7 ± 5.1 cm; p = 0.4; ES = 0.04) nor in the self-selected rest interval group from preto post-test (38.2 ± 4.6 cm vs. 40.5 ± 4.4 cm). 101 4.1.5 Studies not matching volume load The remaining twelve of the included 17 studies did not match the volume load in the protocols used. Four of these studies6,38,41,120 support the relationship between a higher volume load and potentiation stimuli. Of the remaining 8 studies, one analysed the neuromuscular function121, compared PAP in exercised and contralateral legs122, compared the relationship between PAP and time under tension50 and another studied the effects of plyometric PAP in bench press throw123. The remaining 4 reported contradictory results39,106,124,125. Four studies6,38,41,120 support the notion of higher volume loads as better potentiation stimuli. These four studies compared different intensities and volumes ranging from 65% 1 RM to 130% 1 RM. Gilbert and Lees (2005)38 found statistically significant increases in the isometric RFD in the 1RM group at 15 min (p = 0.021) and 20 min (p = 0.006), with a peak increase of 11.8%. In the optimal power load group, a statistically significant increase (p = 0.038) in the isometric RFD was found at 2 min, with a peak increase of 6.7%. Comyns et al. (2007)120 found that contact time showed a statistically significant reduction (p < 0.05) and vertical leg spring stiffness indicated a significant increase (p < 0.05) for the heavy loaded protocol (93% 1RM). However, there were significantly (p < 0.01) shorter flight times for all the protocols. Krzysztofik et al. (2020)6 compared the differences between a classic PAP protocol (2 sets of 2 repetitions of concentric bench press at 90% 1-RM) and eccentric protocols (2 sets of 2 repetitions of either only eccentric 90% 1-RM, only eccentric 110% 1-RM or only eccentric 130% 1-RM bench press). The study reported better potentiation results with eccentric only protocols, achieving greater peak velocity (η2 = 0.441; p = 0.019) and greater mean velocity (η2 = 0.011; p = 0.041) 102 after the 110% 1-RM eccentric only protocol and greater peak velocity after the 130% 1RM eccentric only protocol (η2 = 0.323; p = 0.037). In another study by Krzysztofik et al. (2020)41 with the same eccentric protocols, the bench press throw with a load of 30% 1-RM improved peak power by 10.5 ± 6.0% (effect size = 0.34) and by 9.9 ± 8.1% (effect size = 0.33) for the 110 and 130% 1-RM conditions, respectively. Peak velocity increased by 5.9 ± 5.5% (effect size = 0.4) and by 6.1 ± 6.1% (effect size 0.43) for the 100 and 130% 1-RM protocols, respectively. Since sets and repetitions remained the same through protocols, the differences in volume load were a result of the different intensities. Four studies39,106,124,125 showed conflicting results. In the study by Poulos et al. (2018)106 both protocols (10 sets of 3 or 5 repetitions with 87% 1RM vs. 65% 1RM respectively) enhanced jump height (65% 1RM: +3.3 ± 2.2% [CI: 1.0 to 5.6]; 87% 1RM +2.6% ± 1.9% [CI: 0.7 to 4.5]) after 10 sets. Nevertheless, there was a larger chance of jump height improvement when CMJs were performed across the 10 sets of squats in the protocol of 87% 1RM. Golas et al. (2017)39 compared five different protocols and they observed statistically significant (p = 0.01) differences in the RFD and the rate of power development (RPD) (p = 0.02) in the medium volume load group (80% 1RM) compared to the other conditions. Additionally, Kobal et al. (2019)124 found that a lower volume load with a higher intensity (100% 1RM) protocol induced similar results to a higher volume load and lighter load protocol (93% 1RM and 87% 1RM). Reardon et al. (2014)125 found no performance improvement in any of their protocols (3 sets of either 10 or 3 repetitions with 75% 1RM vs. 90% 1RM). Thomas et al. (2017)121 analysed neuromuscular function using EMG during a PAP protocol. Countermovement jump height increased significantly (p = 0.008) from preto 103 post-potentiation (from 41.0 ± 4.3 cm to 44.7 ± 4.1 cm). Neuromuscular function was measured before the first CMJ and after the last CMJ. A small and statistically nonsignificant decrease in the maximum voluntary contraction (MVC) (p = 0.142) and in voluntary activation (p = 0.06) was observed, but potentiated twitch force was significantly (p < 0.001) reduced after strength training (235 ± 65 N to 185 ± 51 N) in comparison to the control group. Andrews et al. (2016)122 studied the effect of unilateral squats potentiation in the exercised leg and in the contralateral leg using a low fatigue protocol. The results showed no statistically significant differences at 1, 5 and 10 min in comparison to pre-test values for the drop jump contact time or the drop jump reactive strength index. Regarding the CMJ, a condition x time interaction indicated that the exercised leg exhibited significant but small to trivial magnitude jump height increases of 4.0% (p = 0.02; d = 0.36), 0.9% (p = 0.06; d = 0.08) and 1.6% (p = 0.04; d = 0.15) at 1, 5 and 10min post-intervention, respectively. The contralateral leg, on the other hand, had trivial CMJ deficits post intervention: 1.3% (p = 0.23; d = 0.12), 0.9% (p = 0.09; d = 0.10) and 1.7% (p = 0.03; d = 0.19) at 1, 5 and 10min post-intervention, respectively. Krzysztofik and Wilk (2020)123 showed that 3 sets of 5 repetitions of plyometric push ups with 1 min rest interval improved bench press peak velocity (p < 0.01) and mean velocity (p < 0.01) compared to a control group. In addition, Krzysztofik et al. (2020)50 also found that a PAP protocol consisting of 3 sets of 3 repetitions at 85% 1-RM achieved higher training volume based on time under tension at the end of the training session (p < 0.01) when compared to a control group, despite completing the same number of repetitions. 104 4.2 Publication 2: Intermittent Voluntary Isometric Contractions Effects on Performance Enhancement and Sticking Region Kinematics in the Bench Press 4.2.1 Kinematics of the sticking region To analyse the changes in the kinematics of the sticking region, subjects who improved performance in any time point (i.e., responders) were analysed, even if statistical significance was not reached. The Fisher´s exact test reported a significant difference between the number of responders and non-responders (p = 0.001). Thus, seven subjects were analysed for the TRAD condition and 17 for the ISO condition. When comparing mean velocity from the start of the ascending phase until the first maximum peak in velocity (pre-sticking region), no differences were found in the TRAD experimental condition for the responders (n = 7; 33.33% of participants) (p = 0.229; 95% CI [−0.1545, 0.035]) (Figure 6A). 105 Figure 6 Lifting velocities pre-conditioning and in the slowestand fastest post-conditioning activity time points. (A) TRAD post-activation performance enhancement experimental session. (B) ISO post-activation performance enhancement experimental session. * p < 0.05; ** p < 0.001 In contrast, when comparing the velocity from the start of the ascending phase until the first maximum peak in velocity under the ISO experimental condition in the same time points, we found that for responders (n = 17; 85% of participants), the first maximum peak in velocity was higher in the fastest time point (i.e., higher velocities were achieved 106 prior to the initiation of the sticking region) (p < 0.001; ES = 0.67; 95% CI [0.07, 0.02]). However, when comparing the first maximum peak in velocity from prewith the first maximum peak in velocity in the slowest time point (post0), we found that in the slowest time point, the first maximum peak was smaller (lower velocities were recorded prior to the initiation of the sticking region) (p = 0.004; ES = 0.64; 95% CI [-0.012, -0.063]). Additionally, the ascending phase velocity until the first maximum peak in the fastest time point was higher than in the post0 time point (p < 0.0001; ES = 1.22; 95% CI [0.112, 0.053]) (Figure 6B). We compared Vmax peak and Vmin peak from preto the fastest post-conditioning lift of each participant for whom either the TRAD or the ISO experimental condition was effective. The TRAD experimental condition showed no improvements in Vmax peak (p = 0.457; ES = 0.37; 95% CI [-0.074, 0.146]) nor in Vmin peak (p = 0.125; ES = 0.85; 95% CI [-0.0271, 0.173]), while the ISO experimental condition showed improvements in both Vmax peak (p = 0.005; ES = 0.71; 95% CI [0.02, 0.093]) and Vmin peak (p = 0.025; ES = 0.38; 95% CI [0.006, 0.072]) (Figures 7A–D). 113 5. DISCUSSION 5.1 Publication 1: Post-activation potentiation in strength training: A systematic review of the scientific literature The main finding of this systematic review is that the volume load plays an important role in performance enhancement after a conditioning activity. Four studies firmly support that the volume load is the main conditioning factor to achieve an optimal potentiation effect6,38,41,120, while four showed contradictory results39,106,124,125. This systematic review also shows that when the total volume is low, intensity seems to be decisive106,122. Recruitment of type II fibers is needed to achieve potentiation, which is the result of combining volume and intensity126–129. As stated by Schoenfeld (2010)22, in order to recruit high order motor units, light loads are not as effective as heavy loads. In the four studies6,38,41,120 firmly supporting our hypothesis, high intensities were used (up to 130% 1 RM) to achieve higher volume loads. However, potentiation can be achieved using lower volume loads as well39,124. Gołas et al. (2017)39 and Kobal et al. (2019)124 performed between 3 and 5 sets with different loads ranging from 60% 1 RM to 100% 1RM with a fixed rest interval. Considering that fatigue is especially evident when training is performed close to 1-RM or to failure26,130, the better potentiation achieved in these studies with lower volume loads may rely on the rest-time between the conditioning activity and the re-test. Although according to Do Carmo et al. (2018)32 a self-selected rest may be sufficient to improve performance, other studies suggest that potentiation values peak after 8 min or longer resting periods38,40. 114 The second finding is that a minimum effective intensity is needed to achieve potentiation. However, intensity should be understood as the number of repetitions in reserve and not as the percentage of 1-RM. In order to achieve potentiation, we can either use light loads with high volumes or high intensities with low volumes127. Thus, when leaving at least 2 repetitions in reserve, performing multiple sets leads to potentiation without accumulating excessive fatigue106,122. However, although lowering intensity during the conditioning activity may lead to lesser fatigue103, leaving too many repetitions in reserve may not provide enough stimuli to elicit potentiation54,125. On the other hand, leaving too few repetitions in reserve (between 0 and 1) may lead to excessive fatigue and impaired performance after the conditioning activity 54,125. In this way, the higher the intensity, the longer the rest interval the athlete needs to dissipate fatigue32,38. We also found different time-potentiation profiles for highand medium-load protocols. In the study by Lowery et al. (2012)115, heavy and medium protocols peaked at the same time point, but potentiation achieved with the heavy loaded protocol was maintained for a longer time. These findings are in line with those of Gilbert and Lees (2005)38, who reported different time-potentiation profiles; while the optimal power load group peaked earlier, the heavy loaded protocol group peaked later but with a higher potentiation effect (6.7% vs. 11.8%, respectively). These findings are in line with those by Krzysztofik and Wilk (2020)123, who observed the greater increase in peak velocity and mean velocity of the bench press in the first set after the plyometric push ups protocol. Thus, the timepotentiation profiles seem to be determined by the intensity of the stimuli and the resting time (fatigue-potentiation relationship). Fatigue in resistance training, as suggested by Zajac et al. (2015)130, is produced by post-exercise intramuscular perturbations (i.e., decrease in phosphocreatine, glycogen, ATP stores and augmentation of phosphate and 115 hydrogen ions) and modulation of central motor drive during exercise by nociceptive afferent input (III and IV muscle afferents). These changes are especially evident when training sessions are close to 1-RM. During submaximal contractions, the closer to failure, the more motor units are recruited, but also the higher the metabolite accumulation, which contributes to fatigue26. This may partially explain the differences in the potentiation protocols leaving too many122 or too little125 repetitions in reserve during submaximal efforts. We have to acknowledge several limitations. These include the lack of raw data for a deeper analysis. The main purpose of the review was to summarize the evidence so far and, if possible, to analyse differences in used protocols based on the volume load. While the most recent studies included raw data, the oldest ones did not. This limited our intention to compare the volume load of different protocols as we could not calculate it for 2 of the 13 studies. Another important limitation was related to the heterogeneity of the protocols used. Finally, the results of this review cannot be extrapolated to the general population as it only analysed trained subjects and almost all subjects were men. All these limitations imply that the conclusions of this review should be interpreted with caution. 5.2 Publication 2: Intermittent Voluntary Isometric Contractions Effects on Performance Enhancement and Sticking Region Kinematics in the Bench Press The findings of this study support one of our two hypotheses, i.e., the kinematics and the characteristics of the sticking region change considerably (Figure 11 A-C) after the ISO conditioning protocol, however, mean propulsive velocity remains unchanged. 116 The main finding of this study was that an isometric PAPE protocol improved sticking region kinematics in the first part of the range of motion of the medium grip bench press. This improvement is evident in the first period of the ascending phase, prior to the sticking region, which helps the lifter to overcome that sticking region 20. The main enhancement is that the first maximum velocity peak is greater after the ISO conditioning protocol (Figure 7C), which provides the lifter with a greater impulse to overcome the sticking region. Also, the minimum velocity is higher after the ISO conditioning protocol (Figure 7D). The augmented first maximum and minimum velocity peaks result in a change in the velocity-time profile of the lift (Figure 11 A-C). This change in the velocity-time profile of the lift and the sticking region (i.e., less velocity loss from maximum to minimum velocity points) is due to the greater impulse that the lifter has achieved prior to the sticking region. The improvement in the first part of the lift can provide the lifter with enough impulse to avoid excessive velocity loss from Vmax peak to Vmin peak, making the sticking region imperceptible (Figure 11C). 117 Figure 11. (A) Illustration of a typical Sticking Region velocity-time profile, which could be observed in preconditioning lifts. (B) Illustration of a Sticking Region velocity-time profile post-conditioning, with enhanced the first peak in the velocity of the load (Vmax) and its first local minimum peak thereafter (Vmin). (C) Illustration of a postconditioning lift where the impulse prior to the initiation of the Sticking Region is augmented to the point that no velocity loss occurs and, thus, Vmin disappears. The ISO conditioning protocol improved performance in more participants (85%) than the traditional conditioning protocol (33.33%), which could be related to the interaction between stimuli and fatigue 17. Recruitment of type II fibers is needed to achieve PAPE, which is the result of a correct selection of intensity 1. The TRAD conditioning protocol implied a higher RPE than the ISO conditioning protocol (ES = 0.36). Nonetheless, in both conditioning protocols participants reported an RPE 7, which is in line with previous literature 1. The ISO conditioning protocol includes 1-second rest intervals, which via the reduction in inorganic phosphate accumulation could help reduce excessive fatigue 5. Is worth mentioning that the ISO conditioning protocol produced greater decrease in performance immediately post conditioning (post0) compared to the TRAD protocol. This could be due to the total time under tension, which is greater in the ISO conditioning 118 protocol (15-seconds in total). However, in contrast to the TRAD conditioning protocol, improvements in velocity until the initiation of the sticking region were found in the ISO conditioning protocol, which suggests that isometric contractions produce less fatigue than dynamic contractions, or that subjects recover from the produced fatigue faster 4. This lower cumulative fatigue may be due to the lower consumption of ATP in lengthened and isometric contractions compared to shortening contractions131. Regarding the neural factors limiting maximal force production, it is widely accepted that motor unit recruitment strategies play a key role 132. The origin of this central fatigue could be at spinal level, due to inhibitory intramuscular afferents (i.e., group Ia and II muscle afferents) and recurrent inhibition by Renshaw cells 133. We must acknowledge some limitations. Considering that a linear encoder provides the mean propulsive velocity 134 and that the minimum mean propulsive velocity for a successful lift on the bench press has been calculated 18, more velocity implies more distance from that mean minimum propulsive velocity, and thus, furthers subjects from failure. Unfortunately, changes in magnitude (e.g., from 0.27 m·s-1 to 0.29 m·s-1) were so small that statistical significance was not reached. Nevertheless, when comparing instantaneous velocities, magnitude changes were greater (e.g., from 0.26 m·s-1 to 0.34 m·s-1). Also, the selected intensity for the control lift (85% 1RM) was high. Even if notorious mean propulsive velocity changes are hard to see at those intensities, 85% 1RM was chosen for two main reasons: (i) higher similarity to a real strength training or competition and (ii) this is the minimum intensity needed to record a sticking region 96, which was one of the intentions of the study. Finally, it is worth mentioning that this study does not include a classic control group with no conditioning activity carried out. These limitations imply that conclusions of this study should be interpreted with caution. 119 5.3 Publication 3: Post-Activation Performance Enhancement as a strategy to improve bench press performance to volitional failure The main finding of this study was that performing a traditional PAPE protocol consisting of a single set of a single repetition with the 93% 1RM 1 improved bench press performance, measured as number of repetitions to volitional failure. These results are in agreement with previous research 49. Performing a traditional PAPE protocol prior to a set to volitional failure could make the athlete acutely more resistant to fatigue in long-lasting tasks (in our case, a set to volitional failure of various repetitions) 127. The exact rationale behind this improvement in performance is not fully elucidated. Temperature raises are a commonly mentioned mechanism behind performance enhancement, but based on the results of Weigert et al. (2018) 81, where small and non-significant temperature changes were seen after 10 repetitions at 70% in a biceps curl, it seems unlikely. Nevertheless, Boullosa et al. (2018)82 mention a possible mechanism, where the elevation of Ca2+ levels increase due to low-frequency twitches, what can cause the performance enhancement independent of myosin regulatory light chain phosphorylation. Interestingly, performance improvements observed in this study may be related to improved capacity to perform slower repetitions, as under the PAPE condition, participants performed, on average, one more repetition, which was slower than under the CON condition (Figure 10). In consequence, the velocity loss was greater from the first to the last repetition. The attempt to complete one more repetition so close to failure could 120 be due to psychological reasons47. The maximal adaptability theory47 states that hyperstress situations could lead to bad performance. In this way, those last, hard, and slow repetitions would be the stressing situations where participants need to strive to fulfil the lift. Performing the conditioning activity in the PAPE protocol (a heavy repetition prior to the tested task), could improve participants´ confidence when struggling with those last repetitions 47. Another possible explanation for the performance improvement observed could be the training velocity specificity, which means that after performing that specific conditioning activity, participants gain acute fitness or adapt acutely to low velocity lifting 135. The quantity of work performed per session, understood as total number of sets 27, or the volume load (sets x repetitions x kilogram) 136 is related to the quantity of gained muscle mass. Thus, if PAPE leads to an increased number of repetitions performed until failure, volume load per session would be improved, and so could muscle hypertrophy. Following this line of reasoning, it could also be assumed that hypothetic muscle hypertrophy benefits could be due to both the improved mechanical tension of the last repetition (due to obtained lower mean propulsive velocities, Figure 10) and a greater number of performed repetitions (Figure 9). This means that the performed additional repetition may be effective when aiming at muscle hypertrophy. Previous studies support the notion that with a greater velocity loss, muscle hypertrophy gains can be more significant 134, but only to some extent 137,138. Evidence suggests that when velocity loss is excessive (40%), subsequent sets could be affected138. This is in line with findings of Alves et al. (2019)49, where significant differences were found in the number of repetitions performed in the first (PAP = 11.5 ± 3.1; CON = 10.4 ± 2.7; p < 0.05; ES = 0.38) and the second (PAP = 6.5 ± 1.9; CON = 5.5 ± 1.8; p < 0.05; ES = 0.54) set between PAP and CON groups, but 121 not in the third set. This suggests that performance enhancements can increase training volume significantly, but when velocity loss is too pronounced, fatigue may overcome potentiation and impair performance in subsequent sets. Our study is in line with that by Krzysztofik, Wilk, Filip, et al. (2020)50, as increasing the number of repetitions led to higher time under tension. Based on this, future research should address whether a group performing a PAPE protocol combined with a moderate velocity loss (i.e., 20% velocity loss) can complete more repetitions than a control group for several sets (four to six sets). Furthermore, if the PAPE experimental condition group can perform more repetitions, it would be interesting to carry out a long-term intervention to examine whether this protocol would bring more muscle mass gain than a control condition. This study faced several limitations, including relatively small (n=12) sample size, which make it difficult to generalise the obtained results. Our results prove that a PAPE protocol can be useful to improve performance in a task to failure, which makes PAPE a potential strategy to increase muscle hypertrophy gains. However, this was not measured, and therefore, further studies are warranted. Furthermore, our study included only one set until volitional failure, however, the effect of this set on performance in subsequent sets was not evaluated. 5.4 General Discussion 5.4.1 General concepts Considering all the information above, the basis for any PAP or PAPE protocol are both intensity and volume. This is not unexpected, because if manipulation of those two 122 concepts is of paramount importance when creating a training stimulus of any duration (a session, microcycle, mesocycle, macrocycle or any length cycle)16,17, why should it be different when considering shorter time periods? Thus, when trying to achieve a correct PAP or PAPE stimulus during a warm-up, the goal should be focused on properly combining a certain volume load (sets · repetitions · kilograms or pounds)1 with an appropriate rest interval. To correctly achieve a potentiation effect from a PAP or PAPE protocol, recruitment of type II fibers is needed, and this can be done either using high intensities22 or low intensities close to muscle failure26. As type II fibers are more sensitive to calcium, athletes with higher proportion of type II fibers will benefit more from high intensity (PAP or PAPE protocols where high forces are exerted) protocols59. Considering that fiber type percentage is not fully inherited139 and that training background can have a huge impact on it140, interindividual differences in response magnitude of the protocol are common. In any case, proximity to failure (understood as at least an RPE 6 in the repetitions in reserve based RPE scale54) is needed to recruit type II fibers 128,141. As this intensity need can be fulfilled in two different ways, is there any difference in a protocol based on intensity or based on volume? In the study by Lowery et al. (2012)115 the medium load protocol (70% 1RM) improved performance 4 min post conditioning activity (p < 0.05; ES = 1.46, large), while the high load protocol (93% 1RM) achieved peak performance 4 and 8 min post conditioning activity (p < 0.05; ES4min = 1.34; ES8min = 1.48, large). These results are supported by Gilbert and Lees (2005)38, where increases in the isometric RFD were seen at two different time points for each group. The 1RM group (high intensity group) improved 129 5.4.3 Isometric PAPE and heavy lifting If isometric contractions are energetically less demanding than shortening contractions (i.e., a hypothetic isometric PAPE protocol should be energetically less demanding than a traditional high intensity dynamic PAPE protocol) and considering that the sticking region is the weakest point of a lift, performing an isometric PAPE protocol in the sticking region is interesting. In this approach, researchers would be trying to acutely improve performance of the weakest point of the lift. Nevertheless, there is something important to consider, the muscle length. It has been mentioned earlier in “1.2.2.2 Sticking Point or Sticking Region?” that the sticking region seems to be the point where a clear mechanical disadvantage occurs. This disadvantage can be due to the ratio between internal and external moment arms or due to excessive muscle length. Muscles are formed by sarcomeres, and these sarcomeres have their length-force relationship, meaning that they have an optimum length where they can apply force155 . When a sarcomere (and by extension a muscle) is too elongated (or too shortened) its capacity to apply active force decreases102,148. Thus, whenever the sticking region of an individual is due to excessive elongation of the involved muscle, performing isometric contractions there could not be helpful, but experimental data is lacking. For this reason, in our study2 bench press grip width was set at 1.4 times biacromial distance, to ensure that the pectoralis major nor the triceps brachii were not excessively elongated. In that study2 we compared the effects of an isometric (15 maximal isometric contractions of 1 second with 1 second of rest in between -15 MVIC-) PAPE protocol with a traditional dynamic protocol (1 repetition with 93% 1RM) on mean propulsive velocity and sticking region kinematics of a high loaded (85% 1RM) lift. Mean propulsive velocity did not 130 show significant improvements but sticking region kinematics did. The isometric protocol improved velocity in the pre-sticking region (prior to the initiation of the sticking region) (p < 0.001; ES = 0.67, moderate effect; 95% CI [0.07, 0.02]), the 1st maximum velocity peak (p =0.005; ES = 0.71, moderate effect; 95% CI [0.02, 0.093]) and the minimum velocity peak (p = 0.025; ES = 0.38, small effect; 95% CI [0.006, 0.072]). These results could seem of trivial importance, but nothing is further from reality. For experienced lifters, a small change in mean propulsive velocity can suppose the difference between succeeding a lift or failing it (what was confirmed by the smallest meaningful difference calculation carried out in our study´s statistical analyses). Changes in the velocity-time profile of the lift and the sticking region kinematics (i.e., less velocity loss from maximum to minimum velocity points) are due to the greater impulse that the lifter has achieved prior to the sticking region. This augmented capacity to apply force are presumably due to acute strength adaptations by the isometric PAPE protocol, because as stated by Lum & Barbosa (2019)4, isometric strength training enhances force from the adopted joint angles (in our study2 the middle of the sticking region) until 20-50º away. This improvement of the first part of the lift (pre-sticking region) can provide the lifter with enough impulse to avoid excessive velocity loss from Vmax peak to Vmin peak, furthering participant from failure. Until this point, it seems that isometric PAPE protocols applied in the Sticking Region are useful to enhance slightly performance (reducing reported RPEs), but not enough to allow lifting more weight. Shortly, findings up to this point suggest that this protocol is helpful to raise success probability in a heavy lift, but it does not improve maximal strength. Exerted active force is a result of the number of cross bridges formed in the high 131 force state156, and if all motor units are recruited during a maximal strength task128,141, presumably the maximum quantity of cross bridges are being formed. Thus, it should not be possible to improve the quantity of formed cross bridges without creating new sarcomeres, resulting in an impossibility to improve maximal strength acutely. What can be done is improving RFD per cross bridge. PAP and PAPE protocols improve RFD due to myosin light chain phosphorylation, what improves myosin´s mobility and gets myosin heads closer to actin binding sites63,157. The role of PAP or PAPE in strength sports might be improving performance in submaximal loaded tasks, such as performing sets until volitional failure with 60-80% 1RM. As PAP or PAPE are already commonly used for sprinting107, jumping115 or throwing41, it seems reasonable to think that it will be effective in submaximally loaded strength tasks. Following this line of reasoning, Krzysztofik et al. (2020)50 and Alves et al. (2019)49 found conflicting results. Krzysztofik et al. (2020)50 found no statistically significant differences in the number of performed repetitions, but they found an increase in time under tension for the PAPE group. Contrary to this, Alves et al. (2019)49 found differences in the performed number of repetitions in the first and second set. The PAPE group performed more repetitions (p < 0.05) (PAPE 11.5 ± 3.1 and 6.5 ± 1.9, in the first and second sets respectively) than control group (10.4 ± 2.7, 5.5 ± 1.8, in the first and second sets respectively). One key difference is the used load. While Krzysztofik et al. (2020)50 used 60% 1RM, Alves et al. (2019)49 used 75% 1RM. This could suggest that when using excessively light loads, fatigue could underpin results26,130,141. Following Alves et al. (2019)49 results, we conducted a study using only one set until volitional failure with a higher intensity (80% 1RM) in the bench press, where we 132 compared a control group (no PAPE protocol) and a PAPE group (the same warm-up than in control group, but with 1 set of 1 repetition at 93% 1RM prior to the task to failure). Our results supported those by Alves et al. (2019)49, but also those by, Krzysztofik et al. (2020)50. On one hand, participants in the PAPE group performed more repetitions (10.83 ± 2.5 repetitions) than in CON condition (9.76 ± 1.72 repetitions) (p=0.008; ES=0.5, small effect). On the other hand, mean propulsive velocity of the last repetition prior to the concentric volitional failure was lower in the PAPE condition (0.16 ± 0.06 m·s-1) than in the CON condition (0.2 ± 0.09 m·s-1) (p=0.02; ES=0.52, small effect), what increases time under tension as in Krzysztofik et al. (2020)50. Although this improved performance in submaximal load tasks is not directly relevant for strength competitors, who need for maximal strength improvements, it could be indirectly relevant. As muscle mass is the key determinant of strength performance in advanced lifters58, improving the hypertrophic stimulus (i.e., improved mechanical tension) received per unit of training bout (i.e., a training set) could be interesting in the long term. To be stronger, hypertrophy is needed at myofibrillar protein level52, to impulse new sarcomere creation, even though any resistance training programme impulses protein creation also at intracellular level (e.g., ribosome biogenesis158) or extracellular level159. Depending on trained muscle length or contraction regime, sarcomeres will be added in parallel or in series102,160, shifting force-length relationship upwards102 or leftwards101. In any case, the interesting aspect of using PAPE for muscle hypertrophy training is based on the augmentation of performed work. If PAPE improves performed number of repetitions until failure, volume load per session (sets x repetitions x kilogram)136 would improve, and so could muscle hypertrophy. Following this line of reasoning, it could also 133 be assumed that hypothetic muscle hypertrophy benefits could be due to both, improved mechanical tension22,56 of the last repetition and to higher performed repetitions136. In our study3 performing a traditional PAPE protocol prior to volitional failure made participants more fatigue resistant, achieving more repetitions (9.76 ± 1.72 repetitions the control group, vs. 10.83 ± 2.5 repetitions in the PAPE group). This performance improvements in a task to volitional failure in the bench press could be related to an improved capacity to perform slower repetitions, as the PAPE group performed slower the last repetition (0.2 ± 0.09 m·s-1 in the control group vs. 0.16 ± 0.06 m·s-1 in the PAPE group). In consequence, the velocity loss was greater from the first to the last repetition and previous studies support the notion that with a greater velocity loss, muscle hypertrophy gains can be greater 134. This means that the performed extra repetition may be effective when aiming to produce muscle hypertrophy, what means that the efficacy of every set would be higher (i.e., the achieved hypertrophy for every set would be higher). 134 135 6 LIMITATIONS & FUTURE RESEARCH Look to what is within: do not allow the intrinsic quality or the worth of any one fact to escape you Marcus Aurelius – Meditations VI 3 136 137 6. LIMITATIONS & FUTURE RESEARCH 6.1 Limitations Our work faced some limitations: • Our first original research aimed to measure mean propulsive velocity changes with a device that was not sensitive enough (e.g., real magnitude changes were about 0.001 m·s-1 and the used device measures 0.01m·s-1 or higher velocities). This difficulted statistical analysis and result interpretation in mean propulsive velocities. Nevertheless, indirect measurements using Kinovea based digitalisation allowed for instantaneous velocity comparisons, what allowed to compare results more precisely. • Our second original research faced with sample size limitation, making difficult to generalise achieved results. However, obtained sample was formed by trained individuals, what is representative of the competitive population. • In our second original research we talk about hypothetic applications of PAPE for muscle mass gain, but longitudinal data is needed to firmly confirm that hypothesis. 6.2 Future research Future work, based on what has been presented above could focus on the following: • A wide range of available protocols exist already. It would be interesting to test the different time-potentiation profiles of two protocols matched in volume load 138 but with different intensities, to see if the time-potentiation profile is the same in shape but different in time until peak performance. • Isometric PAPE protocol applied in the middle of the sticking region based on 15 MVIC with 1 second rest in between has proved its efficacy improving lifting efficiency. It would be interesting to test the application of the same protocol in larger muscle lengths, to test whether it causes more fatigue due to acute muscle damage or improves performance. • As PAPE protocols seem to improve performed work per session, it would be interesting to design a study between 8 – 16 weeks to test if the potential benefit on hypertrophy is real or only hypothetic. • PAP´s and PAPE´s underlying mechanisms are not mutually exclusive, as PAPE could be a side effect of PAP or an extender PAP. The role of temperature seems interesting but remains to be proven as a mechanism for PAPE. It would be worth researching the real role of temperature in any PAPE protocol. 145 isométricas son energéticamente más exigentes que las contracciones excéntricas, pero técnicamente más simples, los intervalos de descanso podrían ser relativamente similares para ambos tipos de protocolos (es decir, 5-7 min para individuos más fuertes y ≥8 min para individuos más débiles30). Otro escenario en el que los protocolos de PAPE isométricos son útiles, es cuando los entrenadores buscan mejorar el rendimiento en levantamientos pesados con carga casi máxima. Como hemos demostrado en nuestro estudio2, es posible que la velocidad media propulsiva no mejore significativamente, pero sí lo hacen las velocidades máximas de la región de estancamiento, lo que facilita el levantamiento. Esta mejora en los puntos clave de la región de estancamiento (es decir, Vmax1 y Vmin) se logra con un protocolo de PAPE basado en contracciones isométricas realizadas en la zona de desaceleración164 de la región de estancamiento. El tercer y último objetivo de esta tesis fue probar el impacto de un protocolo de PAPE en el volumen de entrenamiento. En este caso, el protocolo de PAPE propuesto era un protocolo dinámico tradicional1, ya que las series al fallo concéntrico se realizan con carga submáxima. La evidencia en esta área es escasa y contradictoria, pero nuestros resultados respaldaron los de Alves et al. (2019) 49, afirmando que un protocolo de PAPE puede mejorar el volumen de entrenamiento de fuerza. Esta mejora puede deberse a razones psicológicas 47 o a razones de especificidad de la velocidad de entrenamiento 135. Como entrenador que soy, al comenzar esta tesis, mi objetivo era tratar de comprender mejor la brecha entre la ciencia y la práctica. Esta tesis trata el tema de la PAPE en 146 diferentes (y frecuentes) situaciones para un entrenador de fuerza, y por lo tanto, me gustaría resumir las conclusiones extraídas: • Conclusión 1: Los entrenadores deben comenzar a diferenciar PAP de PAPE y región de estancamiento de punto de estancamiento • Conclusión 2: Un protocolo de PAPE es un modelo de fitness-fatiga agudo. Si el volumen es alto, la intensidad debe ser baja y viceversa. Si el volumen y la intensidad usados son altos, el uso de series cluster puede ser útil • Conclusión 3: Las personas fuertes se benefician más de los protocolos pesados que las personas novatas. Además, los individuos más fuertes necesitan intervalos de descanso más cortos (5-7 min) que sus compañeros más débiles (≥8 min) • Conclusión 4: Los protocolos de PAPE tradicionales no son útiles cuando se trata de mejorar el rendimiento en levantamientos pesados. 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