Effects of resistance training on physical performance in high-level 800-meter athletes: a comparison between high-speed resistance training and circuit training

! 1! EFFECTS OF RESISTANCE TRAINING ON PHYSICAL PERFORMANCE IN HIGH-LEVEL 800 METERS ATHLETES: A COMPARISON BETWEEN HIGH-SPEED RESISTANCE TRAINING AND CIRCUIT TRAINING ! 2! ABSTRACT This study compared the effects of two resistance training programs during 25 weeks on physical performance and hormonal response in high-level 800 m athletes. Thirteen male athletes (800 m personal best: 1:43-1:58 min:ss) were divided into 2 groups: highspeed resistance training group (RTG) (n = 6) and circuit training group (CTG) (n = 7). Three tests (T1, T2 and T3) including sprint and 800 m running, strength exercises and blood hormones samples were carried out. Both groups showed improvements in 800 m performance (RTG: likely positive, 80/20/0%; CTG: very likely positive, 98/2/0%), however, RTG showed an additional improvement in 200 m (likely positive, 85/15/0%), countermovement jump (very likely positive, 98/2/0%), and squat (likely positive, 91/9/0%), whereas CTG reached likely positive (88/11/1%) effects in CMJ and unclear/possibly negative effects in the rest of the strength variables analyzed. Concerning hormones, RTG resulted in a likely increase (83/15/3%) in testosterone from T1 to T3, and CTG showed a likely increase (79/17/4%) in cortisol from T2 to T3, remaining the rest of the hormones analyzed unclear. These results suggest that a resistance training characterized by high-speed and low-volume produced better improvements in both strength and running performance than a circuit training, accompanied by little changes in the hormonal response. Keywords: Middle-distance Running, Countermovement Jump, Squat, Hormones, Concurrent Training. ! 3! INTRODUCTION The effects of strength training on endurance athletic performance are currently a question of discussion among coaches and sport scientists. Eight hundred-meter running is a middle-distance event where usually both strength and endurance training are performed concurrently. Traditionally, athletes involved in endurance running events have performed little or no strength training to improve specific running performance; in addition, this training had little or no control of the load or intensity. However, the demonstrated benefits of strength training in long and middle endurance performance have led to a growing interest in assessing muscle strength in middle and long-distance athletes (1, 6, 30, 43, 44). Strength training includes both explosive strength and maximal strength training that produce different neuromuscular adaptations. Maximal strength training targets maximal force development through high-load and low-velocity movements, and includes training with loads between 80-100% of 1 repetition maximum (1RM). On the other hand, explosive strength training can be defined as exercises with medium- to high external loading (30–60% of 1RM) and high-velocity movements in the concentric phase (6). Performance in most endurance events is mainly determined by the maximal sustained speed for a given competition distance, and the energy cost of maintaining a given competition speed. In shorter endurance events and during accelerations and sprint situations, anaerobic capacity and maximal speed may also contribute to performance (36). It has been suggested that strength training contributes to enhance endurance performance by improving the economy of movement, delaying fatigue, improving anaerobic capacity, and enhancing maximal speed through neuromuscular adaptations such as: musculotendinous stiffness, motor unit recruitment and ! 4! synchronization, rate coding, intra- and intermuscular coordination and neural inhibition (34, 36). In the last years, several studies have analyzed the effects of different types of strength training on performance in endurance events from medium to long duration (1, 29, 30, 43, 44). Most of these studies tend to underline the importance of carrying out strength training with light-medium or explosive loads, and high or maximum loads achieved with a medium-low number of repetitions, contrary to traditional strength training performed by the endurance athletes, known as strength-endurance, which consisted in circuit type training with low loads, high number of repetitions (>15) and low movement velocity (30, 32, 40, 43, 44). Plyometrics training is another specific form of strength training aimed at enhancing the ability of muscles to generate force by increasing the stiffness of the muscle-tendon system, which decreases ground contact time and reduces energy expenditure (3, 41, 45). Thus, considering that most of sports movements involve the stretch-shorten cycle, which means the ability of muscles to store and return elastic energy effectively when an eccentric muscle contraction is followed immediately by a concentric contraction (23, 45), we are aware of the importance of this type of work in running performance. Similarly, Rate of Force Development (RFD) is another performance indicator of relevance that can be defined as the rate of rise in contractile force at the onset of contraction (2). The RFD is of major importance for athletes engaged not only in explosive sports but also in endurance since in these events it is of relevance the ability of the neuromuscular system to rapidly produce force following a sustained period of high-intensity exercise (32). This ability may be the differentiating factor for elite ! 5! endurance performance as successful athletes at world level can produce high velocities and power outputs to win a race following a sustained period of high-intensity exercise (i.e. sprint finish) (6). In this line, in a recent study, Bachero-Mena et al. (4) reported significant correlations between running performance in 800 m event and sprint times (20 and 200 m), squat strength and, loaded and unloaded jumps height in 800 m highlevel athletes, suggesting the importance of strength levels in 800 m athletes. Exercise is a primary stimulus disrupting homeostasis potentially leading to improvements in various facets of performance (26). Concurrent training is a potent stimulus resulting in acute muscle fatigue. Previous research has reported that concurrent training may be responsible for increased cortisol levels (7, 22, 24). This may implicate elevated endocrine responses and catabolism as a contributing factor to interference (22). Thus recovery from concurrent training requires an integrated response from several physiological systems. The inflammatory process involves the immune system, which is highly influenced by the endocrine system (26). Hormone signals play a variety of roles in anabolism (tissue growth, substrate restoration, and recovery) and catabolism (tissue breakdown and metabolic regulation). Hormonal mechanisms are part of a complex integrated signaling system that mediates changes in the metabolic and cellular processes of skeletal muscle and neural and connective tissue as a function of training. Testosterone induces anabolic and anticatabolic mechanisms involved in muscle tissue growth, recovery and remodeling, and performance enhancement (26). Research has indicated that chronic hormonal adaptations in endurance athletes involve depressed testosterone levels and increased levels of cortisol (17), which indicate a catabolic state in the body that could negatively influence the performance. This fact might reflect a temporal status of fatigue, sometimes termed ! 6! overreaching (18). It is well known that an imbalance between training and recovery can lead to overreaching and, eventually, to a more serious and dreaded condition known as the overtraning syndrome. Because of the need of both strength and endurance capacities in 800 m event and the importance of avoiding a prolonged catabolic state for maintaining fitness and health, we would suggest the necessity of analyzing the effects of resistance training and the hormonal response to concurrent long-term training in 800 m athletes for optimizing training programs. Therefore, this study aimed to analyze the effects of two resistance training programs: a high-speed and low volume resistance training program (RTG) and a circuit training program (CTG) during 25 weeks on physical performance and hormonal response in 800 m high-level athletes. METHODS Experimental Approach to the Problem A quasi-experimental design was undertaken to examine the effects of two different resistance training programs during 25 weeks on performance in high-level 800 m athletes. One group (n = 6) followed a 25-week resistance training program (RTG) divided into 2-cycles, whereas other group (n = 7) followed a circuit training (CTG), in addition to their specific 800 m running training. The length of a 25-week training program was chosen since it took the whole athletics season. The resistance training program of the RTG consisted of low load (40-55% 1RM) and low volume (2-3 sets and 4-6 repetitions) in full squat (SQ), combined with plyometrics and sprint exercises. The CTG followed a circuit training, traditionally considered as a strength-endurance training used by most endurance runners, consisting in a circuit-type training of low ! 7! load (0-40% 1RM) and high volume (>10 repetitions). Three tests including sprint running, strength exercises and blood hormones samples were carried out during the 25- weeks. The 1st test was performed in late October (T1), the 2nd test was performed in February (T2) during the indoor competition period, and the 3rd test (T3) was performed at the beginning of June during the outdoor competition period. Testing sessions were always carried out after a full day of rest, at the same time of the day. Subjects Thirteen male athletes of national and international level in 800 m (with personal best ranging from 1:43 to 1:58 min:ss) participated in this study. Two of them were classified 1st and 2nd in the national championship and national ranking; they also had participated in the latest Olympic Games. The sample was initially formed by sixteen athletes; however, due to injuries unrelated to the training protocol three athletes had to abandon. The subjects were divided into 2 groups: a resistance training group (RTG, n = 6, age: 22.0 ± 5.5 years; height: 178.7 ± 3.9 cm; body mass: 64.8 ± 5.5 kg) and a circuit training group (CTG, n = 7, age: 22.6 ± 5.9 years; height: 172.0 ± 5.9 cm; body mass: 62.2 ± 4.8 kg). This study was not designed as a randomized controlled trial, but rather as a controlled study that may have induced selection bias since the RTG included the athletes whose coaches accepted the resistance intervention program. All athletes had completed resistance training programs in the past and were familiarized with the testing exercises. No physical limitations or musculoskeletal injuries that could affect testing were reported. All participants were fully informed about procedures, potential risks and benefits of the study and they all signed written informed consent prior to the tests. The study was conducted in accordance with the Declaration of Helsinki II and ! 8! approved by the Ethics Committee of the local Institution. Procedures Test Preparation. The first testing day, the athletes performed the tests in the following order after a standardized warm-up protocol: blood sampling, 20 m sprint test (T20m), countermovement jump (CMJ), jump squat (JS), progressive loading test in SQ, and rate of force development (RFD) in plantar flexion. The second testing day the athletes performed the 200 m test (T200m). Blood samples for the hormones analysis were taken before the tests. Sprint testing. Two 0–20 m trials were performed on a synthetic running track in an indoor hall using 2 couples of photocells (Polifemo Radio Light; Microgate, Bolzano, Italy). Runs were performed from static biped start position with the start line located 1 m behind the start photocell. The other pair of photocells were placed at 20 m. The best time of the two 0–20 m trials was recorded (T20m). The rest period between sets lasted 3 min. 200 m running test. One 200 m trial was performed on a synthetic running outdoor track (Mondo). Wind conditions were monitored constantly by an Oregon Scientific WMR- 918 (Oregon Scientific, Tigard, USA) meteorological station. A mathematical model (35) was used in order to adjust the potential influence of wind in the time performances. This mathematical model suggests that a head wind of -2.0 or -1.0 m·s-1 causes a time lost of 0.121 and 0.059 s respectively, and that a tail wind of +2.0 or +1.0 causes a time gain of 0.112 and 0.056 s respectively. The procedures were the same as the above mentioned for the 20 m test. Two pairs of photocells (Polifemo Radio Light; Microgate, Bolzano, Italy) were used for timing recordings. The 200 m test was only performed in ! 9! T2 and T3. 800 m performance. The best 800 m time (T800m) obtained by the athletes in the nearest competition (within 2 weeks) to the tests T2 (winter competition season, February) and T3 (summer competition season, June) was recorded for the analysis. A competition was chosen where the athletes tried to attain their best possible result (time performance). Jump measurements (CMJ and JS). The CMJ and JS were performed on an infrared platform (Optojump, Microgate, Bolzano, Italy) that calculated jump height (h) through flight time (t) and the acceleration due to gravity (g) as follows: h = t2×g/8. The CMJ was performed with both hands on the waist, while performing a downward movement approximately to 90º of knee flexion followed by a vertical jump of maximum effort. The participants were required to do 3 trials separated by 1 min rest, mean height being recorded. The JS test was performed with progressive loads (10 or 5 kg increments) ranging from 20 kg up to the load allowing the subject to jump up no more than 20 cm high. This load of 20 cm has been suggested to correspond approximately to the optimum mean propulsive power load (28). The JS test was performed using a Smith machine (Multipower Fitness Line, Peroga, Murcia, Spain), which allows a smooth vertical displacement of the bar along a fixed pathway. The athletes performed two JS separated by 2 min rest with each load. The load corresponding to the JS height of 20 cm was used for the subsequent statistical analysis. Full Squat test. An incremental loading SQ test was performed on a Smith machine (Multipower Fitness Line, Peroga, Murcia, Spain) using a linear velocity transducer (T- Force System, Ergotech, Murcia Spain). Instantaneous velocity was sampled at a ! 16! stiffness and decrease in the contact phase times (27, 33) or an improved biomechanical efficiency (21), allowing a runner to operate at a lower relative intensity for a given running speed. Previous studies reporting improvements in both strength and endurance running performance have conducted heavy or explosive resistance training programs (30, 43, 44). Taipale et al. (43) suggested that maximal (80-85% 1RM), or explosive (30-40% 1RM) strength training performed concurrently with endurance training were more effective in improving strength (1RM and CMJ) and endurance performance than circuit-strength training (3x40-50s, relative intensity not specified) in recreational runners. Something similar occurred in our study since RTG training (40-55% 1RM) resulted in a better improvement of strength (CMJ, V1load, T200m) than CTG (low load and high number of repetitions) did. In this line, previous studies conducting the typical circuit-strength training characterized by a high number of repetitions per set performed at no maximal velocity did not find significant improvements in strength parameters during an athletics season in middle and long-distance athletes (5). However, it has been shown that a resistance-training program characterized by low-moderate loads and low volume always performed at maximal intended velocity combined with jumps and resisted sprint training (13, 15), as the one conducted by the RTG group in our study, could favor the neuromuscular adaptations related with performance in strength and sprint. Moreover, in a previous study (15), a high-speed and low volume resistance training program did not produce any potential negative effect on aerobic endurance, in fact, it even could have entailed a substantial improvement. A possible mechanism explaining improvement in endurance-related measurements after concurrent (maximal or explosive) training is increased maximum force and increased RFD (1, 42). Research has demonstrated that both explosive and maximal strength training have positive ! 17! influences on endurance running performance, running economy, and increased neural activation in endurance athletes (30, 31, 41, 42). However, future investigations should include appropriate resistance programs (exercise, load and velocity-based prescription) over long-term intervention periods (>6 months) by comparing different training intensities (e.g. heavy vs medium loads). With regard to the hormonal response, no long-term changes were observed. Previous studies have indicated that concurrent training may be responsible for increased cortisol levels (7, 24) and either no (7) or minimal changes (24) in anabolic hormones. CTG showed a likely increase in cortisol from T2 to T3. It appears that the acute cortisol response may reflect metabolic stress whereas chronic changes (or lack of change) may be involved with tissue homeostasis and protein metabolism (25). Thus, a robust rise in resting cortisol has been reported as a complementary approach to assess excessive fatigue at the onset of overreaching (46). The likely increase in testosterone concentration in RTG from T1 to T3 and from T2 to T3 may indicate the development of a more anabolic environment in the body. Guilhem et al. (16) found a significant increase of salivary testosterone in track and field athletes of different disciplines during the pre-competitive period, which could be explained by the load diminution during the competition season. Similarly, Taipale et al. (44) found a significant increase in testosterone in recreational endurance runners after 12 weeks of mixed maximal and explosive strength training; however, cortisol remained statistically unaltered. Hormones with anabolic or catabolic properties like testosterone show quantitative changes, reflective of a catabolic state, in relation to the intensity and duration of training load (14). GH values showed a large variance, which do not allow us to make any conclusion about the training effects. Likewise, studies of the chronic effects of exercise on the circulating IGF-I have yielded inconsistent results. Some studies have ! 18! shown reductions in immunoreactive IGF-I levels after several weeks of exercise despite an improved physical performance (muscle strength and/or VO2max) (11, 12). In our study, the hormonal response to long-term training does not seem to be affected by the training program performed. In general, the resistance training followed by both RTG and CTG did not alter the basal concentrations of the different hormones analyzed. In conclusion, a high-speed and low volume resistance training program performed during 25 weeks resulted in meaningful gains in running and strength-related variables than a circuit training. The hormonal response remained in general unaltered in both RTG and CTG. These results suggest the positive effects that a resistance training program with these characteristics could have in 800 m performance, especially in 800 m races where the final result is decided in the final sprint, which reflects the importance of strength levels in middle-distance athletes. PRACTICAL APPLICATIONS The results of the present study contribute to improve our knowledge about the effects of resistance training in high-level middle distance athletes. According to our results, a high-speed and low volume resistance training can increase the lower limb strength and sprinting speed and this could be positive for the 800 m specific performance while avoiding the interference with endurance and the specific 800 m training sessions. Future investigations should include appropriate resistance programs (exercise, load and velocity-based prescription) over long-term intervention periods (>6 months). REFERENCES ! 19! 1. Aagaard, P, and Andersen, JL. Effects of strength training on endurance capacity in top level endurance athletes. Scand J Med Sci Sports 20: 39-47, 2010. 2. Aagaard, P, Simonsen, EB, Andersen, JL, Magnusson, P, and Dyhre-Poulsen, P. Increased rate of force development and neural drive of human skeletal muscle following resistance training. J Appl Physiol 93: 1318-1326, 2002. 3. Anderson, T. Biomechanics and running economy. Sports Med 22: 76–89, 1996. 4. Bachero-Mena, B, Pareja-Blanco, F, Rodríguez-Rosell, D, Yáñez-García, JM, Mora-Custodio, R, and González-Badillo, JJ. Relationships between sprint, jumping and strength abilities, and 800 m performance in male athletes of national and international levels. J Hum Kinet 58: 187-195, 2017. 5. Balsalobre-Fernández, C, Tejero-González, CM, del Campo-Vecino, J. Seasonal strength performance and its relationship with training load on elite runners. J Sports Sci Med 14: 9-15, 2015. 6. Beattie, K, Kenny, IC, Lyons, M, and Carson, BP. The Effect of Strength Training on Performance in Endurance Athletes. Sports Med 44: 845-865, 2014. 7. Bell, G, Syrotuik, D, Socha, T, Maclean, I, Quinney, HA. Effect of strength training and concurrent strength and endurance training on strength, testosterone, and cortisol. J Strength Cond Res 11: 57–64, 1997. 8. Cohen, J. Statistical Power Analysis for the Behavioral Sciences. Hillsdale, MI: Lawrence Erlbaum, 1988. 9. Consitt, LA, Copeland, JL, Tremblay, MS. Endogenous anabolic hormone responses to endurance versus resistance exercise and training in women. Sports Med 32: 1–22, 2002. 10. Crewther, BT, Keogh, J, Cronin, J, Cook, C. Possible stimuli for strength and power adaptation: acute hormonal responses. Sports Med 36: 215-238, 2006. ! 20! 11. Eliakim, A, Brasel, JA, Mohan, S, Barstow, TJ, Berman, N, and Cooper, DM. Physical fitness, endurance training, and the growth hormone-insulin-like growth factor I system in adolescent females. J Clin Endocrinol Metab 81: 3986–3992, 1996. 12. Eliakim, A, Brasel, JA, Mohan, S, Wong, WL, and Cooper, DM. Increased physical activity and the growth hormone-IGF-I axis in adolescent males. Am J Physiol 275: R308–314, 1998. 13. Franco-Márquez, F, Rodríguez-Rosell, D, González-Suárez, JM, Pareja-Blanco, F, Mora-Custodio, R, Yáñez-García, JM, and González-Badillo, JJ. Effects of combined resistance training and plyometrics on physical performance in young soccer players. Int J Sports Med 36: 906-914, 2015. 14. Gatti, R, and De Palo, EF. An update: salivary hormones and physical exercise. Scand J Med Sci Sports 21: 157–169, 2011. 15. González-Badillo, JJ, Pareja-Blanco, F, Rodríguez-Rosell, D, Abad-Herencia, JL, Del Ojo-López, JJ, and Sánchez-Medina, L. Effects of velocity-based resistance training on young soccer players of different ages. J Strength Cond Res 29: 1329–1338, 2015. 16. Guilhem, G, Hanon, C, Gendreau, N, Bonneau, D, Guével, A, and Chennaoui, M. Salivary hormones response to preparation and pre-competitive training of world-class level athletes. Front Physiol 6: 333, 2015. 17. Hackney, AC, Szczepanowska, E, and Viru, AM. Basal testicular testosterone production in endurance-trained males is suppressed. Eur J Appl Physiol 89: 198–201, 2003. 18. Halson, SL, Jeukendrup, AE. Does overtraining exist? An analysis of overreaching and overtraining research. Sports Med 34: 967-981, 2004. ! 21! 19. Hopkins, WG, Marshall, SW, Batterham, AM, and Hanin, J. Progressive statistics for studies in sports medicine and exercise science. Med Sci Sports Exerc 41: 3-13, 2009. 20. Hopkins, W. G. (2006). Spread sheets for analysis of controlled trials, with adjustment for a subject characteristic. Sportscience 10, 46-50. 21. Johnson, R, Quinn T, Kertzer R, and Vroman NB. Strength training in female distance runners: impact on running economy. J Strength Cond Res 11: 224– 229, 1997. 22. Jones, TW, Howatson, G, Russell, M, and French, DN. Performance and endocrine responses to differing ratios of concurrent strength and endurance training. J Strength Cond Res 30: 693–702, 2016. 23. Komi, PV. Stretch-shortening cycle: A powerful model to study normal and fatigued muscle. J Biomech 33: 1197–1206, 2000. 24. Kraemer, WJ, Patton, JF, Gordon, SE, Harman, EA, Deschenes, MR, Reynolds, K, Newton, RU, Tripplett, NT, and Dziados, JE. Compatibility of high-intensity strength and endurance training on hormonal and skeletal muscle adaptations. J Appl Physiol 78: 976–989, 1995. 25. Kraemer, WJ, and Ratamess, NA. Hormonal responses and adaptations to resistance exercise and training. Sports Med, 35: 339-361, 2005. 26. Kraemer, WJ, Ratamess, NA, and Nindl, BC. Recovery responses of testosterone, growth hormone, and IGF-1 after resistance exercise. J Appl Physiol 122: 549-558, 2017. 27. Kyrolainen, H, Belli, A, and Komi, PV. Biomechanical factors affecting running economy. Med Sci Sports Exerc 33: 1330–1337, 2001. 28. Loturco I, Nakamura FY, Tricoli V, Kobal R, Cal Abad CC, Kitamura K, et al. ! 22! Determining the optimum power load in jump squat using the mean propulsive velocity. PLoS ONE 10(10): e0140102, 2015. 29. Mikkola, J, Rusko, H, Nummela, A, Pollari, T, and Hakkinen, K. Concurrent endurance and explosive type strength training improves neuromuscular and anaerobic characteristics in young distance runners. Int J Sports Med 28: 602- 611, 2007. 30. Mikkola, J, Vesterinen, V, Taipale, R, Capostagno, B, Häkkinen, K, and Nummela, A. Effect of resistance training regimens on treadmill running and neuromuscular performance in recreational endurance runners. J Sports Sci 29: 1359-1371, 2011. 31. Millet, GP, Jaouen, B, Borrani, F, and Candau, R. Effects of concurrent endurance and strength training on running economy and VO2 kinetics. Med Sci Sports Exerc 34: 1351–1359, 2002. 32. Paavolainen, LM, Nummela, AT, and Rusko, HK. Neuromuscular characteristics and muscle power as determinants of 5-km running performance. Med Sci Sports Exerc 31: 124–130, 1999. 33. Paavolainen, LM, Nummela, A, Rusko, H, and Hakkinen, K. Neuromuscular characteristics and fatigue during 10 km running. Int J Sports Med 20: 516–521, 1996. 34. Paavolainen L, Nummela A, Rusko H. Muscle power factors and VO2max as determinants of horizontal and uphill running performance. Scand J Med Sci Sports 10: 286–91, 2000. 35. Quinn, MD. The effects of wind and altitude in the 200-m sprint. J Appl Biomech 19: 49-59, 2003. 36. Rønnestad, BR, and Mujika, I. Optimizing strength training for running and ! 23! cycling endurance performance: A review. Scand J Med Sci Sports 24: 603-612, 2014. 37. Sánchez-Medina, L, Pérez, C, and González-Badillo, JJ. Importance of the propulsive phase in strength assessment. Int J Sports Med 31: 123-129, 2010. 38. Sánchez-Medina, L, Pallarés, JG, Pérez, CE., Morán-Navarro, R, and González- Badillo, JJ. Estimation of relative load from bar velocity in the full back squat exercise. Sports Med Int Open 01: E80-E88, 2017. 39. Schmidtbleicher, D. Training for power events. In: Strength and power in sport. Edited by P. Komi. Blackwell Scientific Publications. London, 381-395, 1992. 40. Sedano, S, Marin, PJ, Cuadrado, G, and Redondo, JC. Concurrent training in elite male runners: the influence of strength versus muscular endurance training on performance outcomes. J Strength Cond Res 27: 2433–2443, 2013. 41. Spurrs, R, Murphy, A, and Watsford, M. The effect of plyometric training on distance running performance. Eur J Appl Physiol 89: 1–7, 2003. 42. Storen, O, Helgerud, J, Stoa, EM, and Hoff, J. Maximal strength training improves running economy in distance runners. Med Sci Sports Exerc 40: 1087– 1092, 2008. 43. Taipale, R, Mikkola, J, Nummela, A, Vesterinen, V, Capostagno, B, Walker, S, Gitonga, D, Kraemer, W, and Häkkinen, K. Strength training in endurance runners. Int J Sports Med 31: 468-476, 2010. 44. Taipale, RS, Mikkola, J, Salo, T, Hokka, L, Vesterinen, V, Kraemer, WJ, Nummela, A, and Häkkinen, K. Mixed maximal and explosive strength training in recreational endurance runners. J Strength Cond Res 28: 689-699, 2014. 45. Turner, AM, Owings, M, and Schwane, JA. Improvement in running economy after 6 weeks of plyometric training. J Strength Cond Res 17: 60–67, 2003. ! 24! 46. Urhausen, A, Gabriel, HH, and Kindermann, W. Impaired pituitary hormonal response to exhaustive exercise in overtrained endurance athletes. Med Sci Sports Exerc 30: 407–414, 1998. ACKNOWLEDGMENTS The authors would like to thank the coaches and athletes who have participated in the study. This study was supported in part by a grant from the FPU Program of the Education, Culture and Sport Ministry of Spain. ! 25! FIGURE LEGENDS Figure 1. Intra-group changes in T20m, T200m, T800m, CMJ, JS, V1load, and RFD in resistance training group (RTG) and circuit training group (CTG). A) T1 vs. T2; B) T1 vs. T3; C) T2 vs. T3. Bars indicate uncertainty in the true mean changes with 90% confidence intervals. Trivial (shaded) areas were calculated from the smallest worthwhile change. A reduction of time in the running variables was interpreted as a positive effect. Figure 2. Between-group changes in T20m, T200m, T800m, CMJ, JS, V1load, and RFD in resistance training group (RTG) and circuit training group (CTG). A) T1 vs. T2; B) T1 vs. T3; C) T2 vs. T3. Bars indicate uncertainty in the true mean changes with 90% confidence intervals. Trivial (shaded) areas were calculated from the smallest worthwhile change.