Rugby is a very strenuous sport that places an emphasis on jumping, running speed, and ball throwing. In addition, rugby requires substantial maximal strength levels to be able to hit, block, push, and hold during game actions. High levels of maximal strength and muscle power output and a high aerobic capacity are required to perform these actions and to successfully participate in elite rugby leagues (25,28). Given the relevance of power development in rugby, plyometric training is widely performed during the physical training of rugby players, mainly because of the influence of this type of training on power, jump, and sprint performance (16). However, the influence of the amount of jumping during training on neuromuscular recovery has been poorly investigated. Several studies have shown that plyometric training improves strength (23,36,39), power (6,9), jump height (6,8,23,36,39), and sprint performance (6,36). Furthermore, the plyometric training volume has been shown to possibly influence the development of speed, jump height, and power gains normally induced by training (36,38). In a study by Villarreal et al. (36), a low volume (i.e., 420 jumps per week) of plyometric training was found to be a sufficient stimulus to promote speed increases when compared with moderate (i.e., 840 jumps per week) and high plyometric training volumes (i.e., 1,680 jumps per week). However, the same was not observed in the jump height performance, in which only moderate and high volumes promoted improvements after 12 weeks of training (36). However, to the best of our knowledge, no previous studies have investigated the acute effects of different plyometric exercise volumes on strength and power recovery. Understanding the influence of different volumes of plyometric exercise on strength and power output recovery could help coaches to monitor and optimize recovery time between physical and technical training sessions.
Strength training volume has a strong influence on the acute hormonal responses to strength training (4,10,18,34). In addition, several studies suggest that the hormonal response to individual training sessions is related to the magnitude of chronic neuromuscular adaptations to strength training (4,12,19,30). In a study by Hansen et al. (12), young subjects performing a strength training session that stimulated acute elevations in testosterone (i.e., arm + leg exercises = greater muscle mass involved) had greater strength gains in their arms than did another group that performed a strength training session designed to not stimulate an acute testosterone response (i.e., only arm exercises = less muscle mass involved). In a similar study, Ronnestad et al. (30) showed that greater strength gains and hypertrophy occurred in young men who performed strength training composed of sessions that elicited acute hormonal elevations. In contrast, West et al. (40) showed that the exposure of loaded muscle to exercise-induced elevations in endogenous anabolic hormones does not enhance strength training adaptations. However, the order of the exercises performed in that study may explain these results because the exercise that was supposed to acutely elevate the serum hormones (i.e., the leg exercise) was performed after the exercise for the investigated muscle (elbow flexors). In a study by Ronnestad et al. (30), the exercise order was reversed, that is, the legs (greater muscle mass involved and, consequently, greater stimulus to the hormone elevation) were exercised first, and therefore, the elbow flexor exercises were performed during the period when the hormone levels were elevated. With this approach, Ronnestad et al. (30) observed greater strength training adaptations induced by the training protocol that elicited greater acute hormonal elevations. Thus, to determine the main factors related to strength training sessions that greatly influence acute testosterone responses, it is important to create an anabolic hormonal response and optimize the neuromuscular adaptations to training. Although strength training volume has a critical influence on the magnitude and duration of the acute response of testosterone and cortisol (COR [10,34]), to the best of our knowledge, there are no data available regarding the influence of plyometric training volume on the acute responses of testosterone and COR.
A knowledge of the neuromuscular slope recovery after different plyometric exercise volumes is very important for helping coaches monitor and periodize the physical training of elite athletes. Thus, given the lack of information regarding neuromuscular recovery and the acute hormonal responses to different plyometric training volumes, we investigated the effect of different volumes of plyometric training (i.e., 100, 200, or 300 hurdle jumps) on strength and jump performance and on the acute hormonal and lactate responses in rugby players. Our hypothesis was that greater exercise volumes would result in a greater decrease in strength and jump performance after exercise. Second, we hypothesized that greater exercise volumes would result in larger responses of testosterone, COR, and lactate.
Experimental Approach to the Problem
To investigate the effect of exercise volume on the acute neuromuscular, metabolic, and hormonal responses to plyometric exercise, the subjects came to the laboratory on 4 different occasions. On the first day, the subjects signed a written consent form, and their anthropometric characteristics were evaluated. On the last 3 days, the plyometric exercise protocols (i.e., 100, 200, and 300 jumps) were performed in random order, with 1 week of rest between each protocol. All the subjects completed the 3 plyometric exercise protocols, and the subjects performed each session individually. It has been shown that only 100 jumps are necessary to elicit important acute neuromuscular responses, such as maximal voluntary force and evoked force declines (33). However, we chose to investigate exercise volumes >100 jumps per session because these plyometric exercise volumes have been extensively used in this research area and in the training routines of high-level athletes (36–38). Before and after (5 minutes, 8 hours, and 24 hours) each jumping volume, the subjects performed strength and jumps tests. In addition, before and immediately after each jumping volume, the subjects had their blood drawn to measure serum hormone and lactate levels. We had previously tested the stability and reliability of all variables with the same population before the study. All the subjects performed all the plyometric exercise protocols at the same time of the day (between 8 and 9 AM) throughout the study period. The ambient conditions were kept constant during all tests (temperature: 22–24° C).
Eleven young male national level rugby players (Mean ± SD: 23.5 ± 0.9 years), who were engaged in regular and systematic physical and technical training program for at least 3 years (6 times a week), volunteered for the study after completing an ethical consent form and signing an informed consent document. The subjects were carefully informed about the design of the study with special information given regarding the possible risks and discomfort related to the procedures. Ethics Committee of Federal University of Rio Grande do Sul approved the study in accordance with the Helsinki Declaration. Exclusion criteria included any history of neuromuscular, metabolic, and hormonal diseases. The subjects were not taking any medication with influence on hormonal and neuromuscular metabolism and were advised to maintain their normal dietary intake throughout the study. The physical characteristics of the subjects are shown in Table 1. Body mass and height were measured using an Asimed analog scale (resolution of 0.1 kg) and an Asimed stadiometer (resolution of 1 mm), respectively. Body composition was assessed using the skinfold technique. A 7-site skinfold equation was used to estimate body density (15), and body fat was subsequently calculated using the Siri equation (32).
The training routine of the subjects consisted of 3–4 sessions per week of rugby (specific physical work, technical, and tactical actions) and 3 strength training sessions per week. In the period of study, the subjects had been training 5 sets of 100 hurdle jumps, with 5 minutes of rest between sets. The subjects had been performing plyometric training routine at least 2 years before the study. However, during the data collection (∼5 weeks), no plyometric exercise session was performed by the subjects. In addition, these subjects were in the second place in the Brazil Cup immediately before the study and recently had started their period out of season, which avoided the influence of their training sessions in the study protocols.
Maximal Voluntary Contraction and Rate of Force Development
Maximal isometric peak torque (PT) and rate of force development (RFD) were obtained using an isokinetic dynamometer (Humac, CSMI, Stoughton, MA, USA) immediately before (pre), 5 minutes after (post), 8 hours after (8 hours post), and 24 hours after (24 hours post) the exercise protocols. The subjects were positioned seated with their hips and thighs firmly strapped to the seat of the dynamometer, with the hip angle at 85°. After that, the subjects warmed up for 10 knee extension and flexion repetitions at an angular velocity of 90°·s−1, performing a submaximal effort. After having their right leg positioned by the evaluators at an angle of 120° in the knee extension (180° represented the full extension), the subjects were instructed to exert the maximum strength as fast as was possible when extending or flexing the right knee. All the subjects were very much familiarized with the isometric test protocol. Before the plyometric exercise protocols, and after 8 and 24 hours, the subjects had 2 attempts at obtaining the maximum voluntary contraction (MVC) of the knee extensors, each lasting 5 seconds. Immediately after (post) the exercise protocols, the subjects made 1 attempt to perform the MVC. The rest interval between each attempt of the protocol was 2 minutes. During all the maximum tests, the researchers provided verbal encouragement so that the subjects would feel motivated to produce their maximum force. The force-time curve was obtained using Humac software, with an acquisition rate of 2,000 Hz. Maximal PT was defined as the highest value of the torque (newton meter) recorded during the unilateral knee extension. The isometric force-time analysis on the absolute scale included the maximal RFD (newtons per second), defined as the greatest increase in the force in time. The RFD was derived as the average slope of the moment-time curve ([INCREMENT]moment/[INCREMENT]time) over time intervals of 50 milliseconds relative to the onset of contraction, which was considered the point that the torque exceeded 7.5 N·m (1), and were determined using the Excel software. The test-retest reliability coefficients (intraclass correlation coefficient [ICC]) were >0.94 for all the variables in the isometric protocol.
After the MVC assessment (pre, immediately post, 8 hours post, and 24 hours postplyometric exercise protocols), the subjects performed a jump test using an electronic contact mat system (Jumptest, Hidrofit, Belo Horizonte, Brazil). Jump height was determined using an acknowledged flight-time calculation (3) and the software Miltsprint. Each subject was instructed to perform with maximum effort during the double-leg squat jump (SJ) and drop jump (DJ) tests. They were given 3 attempts to obtain their maximum jump height in each test, with 30 seconds of rest between attempts. All the subjects were very familiar with the jump test protocol. During the SJ test, the subjects were instructed to start the jump with their knees at a 90° angle and to avoid any countermovement. During the DJ test, the subjects started the test on a 40-cm block because in the previous trial, all the participants had their best DJ performance from this height (30, 40, and 60 cm were tested). They were instructed to jump for maximal height and minimal contact time. The subjects were again instructed to leave the electronic contact mat system with their knees and ankles fully extended and to land in a similarly extended position to ensure the validity of the test. Four basic techniques were stressed: (a) correct posture (i.e., spine erect, shoulders back) and body alignment (e.g., chest over knees) throughout the jump; (b) jumping straight up with no excessive side-to-side or forward-backward movement; (c) soft landing, including toe-to-toe heel rocking and bent knees; and (d) instant recoil preparation for the next jump (36). When performing the jumps, all the subjects held their hands on their hips. The test-retest reliability coefficients (ICCs) were 0.87 for the SJ test and 0.93 for the DJ test.
Blood Collection and Analysis
Blood was obtained between 8 and 9 AM, after 8 hours of sleep, 12 hours of fasting and 2 days with no physical training session. The time of blood collection was chosen because of its use in many studies conducted with these procedures for the control of the circadian hormonal range (29,36,37). The subjects sat in a slightly reclined position during 15 minutes, and, after that, 10 ml of blood was drawn from the antecubital vein before, and 3 minutes after the plyometric exercise protocols with similar techniques. After collection, the blood was maintained in ambient temperature for 45 minutes and then centrifuged for 10 minutes at 2,000 rpm, and serum was removed and frozen at −20° C for later analysis. With this blood sample, concentrations of total testosterone (TT) (nanograms per milliliter) and COR (milligrams per deciliter; MP BioMedicals, Twinsburg, OH, USA) were determined in duplicate, using radioimmunoassay kits. From these values, it was possible to calculate the TT/COR ratio. To eliminate interassay variance, all the samples were analyzed within the same assay batch, and all intraassay variances were ≤6.3%. Antibody sensitivities were 0.7 nmol·L−1 for TT, and 1.4 nmol·L−1 for COR. The test-retest reliability coefficients (ICC) were 0.85 to COR, 0.94 to TT.
Capillary blood lactate samples were obtained from a hyperemic earlobe. After cleaning and puncturing, a 5-μl sample was drawn. Lactate accumulation was determined before and 3 minutes after the plyometric exercise protocols through the enzymatic reaction technique by using a portable lactimeter (Accutrend Lactate). The test-retest reliability coefficient (ICC) was 0.87.
Plyometric Exercise Protocols
Plyometric exercise protocols (i.e., 100, 200, or 300 jumps) were performed in a random order, with 1 week of rest between each protocol. First, the subjects performed a standardized 3-minute dynamic warm-up consisting of high-knees, lunges, abdominal exercises, side shuffles, and power skips. The warm-up was carefully monitored during each session to ensure that each session was performed in the exact same fashion. The subjects performed sets of 100 hurdle jumps with 10 seconds of active rest between each repetition of 10 jumps (i.e., return to the first hurdle using low-intensity running). During the protocols with greater exercise volumes (i.e., 200 and 300), the rest intervals between sets were passive and lasted for 5 minutes. The hurdles were positioned with an approximately 45-cm distance between each hurdle. All the hurdles had a height of 40 cm because in the previous trial, all the subjects had shown their best DJ performance when jumping from this height (30, 40, 50, and 60 cm were tested).
Results are reported as mean ± SD. Comparisons between different plyometric exercise protocols were assessed using a 2-way analysis of variance with repeated measures (volume × time). When a significant F value was achieved, least significant difference post hoc procedures were used to locate the pair wise differences. The sample size was calculated using the GPOWER program (version 3.0.1), which determined a sample of n = 11 subjects, with a statistical power of >85% in all variables. The retrospective statistical power provided by SPSS after analysis was >0.95 in all variables in which a significant time effect was observed. Significance was accepted when p ≤ 0.05.
At baseline, no significant differences were observed for all the variables analyzed between the different treatments (i.e., 100, 200, or 300 jump interventions).
Regarding SJ performance, there was a significant time effect (p < 0.001), but no protocol effect and time vs. protocol interaction were observed. Post hoc analysis showed that all plyometric exercise volumes (100, 200, and 300 jumps) significantly reduced the SJ performance 24 hours after the protocol (P100: −3.6 ± 5.7%; P200: −2.5 ± 10.2%; P300: −4.2 ± 5.7%), whereas no changes were observed immediately after or 8 hours after the protocols (Figure 1).
Regarding the DJ performance, there was a significant time effect (p < 0.01), but no protocol effect and time vs. protocol interaction were observed. Post hoc analysis showed that all plyometric exercise volumes (100, 200, and 300 jumps) reduced the DJ performance after 24 hours (P100: −9.0 ± 5.8%; P200: −6.3 ± 5.5%; P300: −4.0 ± 3.85%), whereas no changes were observed immediately after or 8 hours after the protocols (Figure 2).
Regarding the isometric PT, there was a significant time effect (p < 0.02), but no protocol effect and time vs. protocol interaction were observed. Post hoc analysis showed that the reductions were observed immediately (P100: −13.8 ± 11.3%; P200: −12.3 ± 8.9%; P300: −10.1 ± 20.9%), 8 hours (P100: −12.9 ± 15.2%; P200: −13.8 ± 15.5%; P300: −9.8 ± 22.3%), and 24 hours (P100: −17.9 ± 11.9%; P200: −15.4 ± 16.1%; P300: −15.0 ± 17.9%) (p < 0.02) after all plyometric exercise volumes (100, 200, and 300 jumps) when compared with the preexercise values (Figure 3).
A significant time effect (p < 0.001) was also observed for the maximal RFD; however, no protocol effect and time vs. protocol interaction were observed for this variable. Post hoc analysis showed reductions immediately (P100: −10.3 ± 8.0%; P200: −12.4 ± 6.8%; P300: −16.3 ± 9.7%), 8 hours (P100: −10.8 ± 17.9%; P200: −10.2 ± 14.8%; P300: −9.2 ± 8.3%), and 24 hours (P100: −14.9 ± 35.1%; P200: −11.4 ± 12.3%; P300: −9.8 ± 13.8%) (p < 0.001) after all plyometric exercise volumes (100, 200, and 300 jumps, respectively) when compared with the preexercise values (Figure 4).
Hormonal and Lactate Responses
Regarding TT values, there were significant time effects (p < 0.001), but no protocol effect and time vs. protocol interaction were observed. The serum TT increased after 100 (10.9 ± 8.2%), 200 (27.6 ± 15.3%), and 300 jumps (10.9 ± 19.1%) (Figure 5). Furthermore, there were significant time effects for the serum COR after all exercise protocols (p < 0.05), but no protocol effect and time vs. protocol interaction were observed. Serum COR increased after 100 (17.4 ± 50.2%), 200 (21.2 ± 69.2%), and 300 jumps (42.4 ± 70.9%) (Figure 6). However, no significant time effect, protocol effect, and time vs. group interaction were observed for the TT:COR ratio. There was a significant time effect on the lactate concentrations (p < 0.001), but no protocol effect and time vs. protocol interaction were observed. Increases in the lactate concentrations were observed after 100 (177.0 ± 114.0%), 200 (195.0 ± 202.3%), and 300 jumps (178.0 ± 65.1%) (Figure 7).
The primary finding of this study was that neuromuscular performance was impaired 24 hours after all 3 plyometric exercise volumes (i.e., 100, 200, and 300 jumps). Furthermore, an interesting finding was the similar acute lactate and hormonal responses observed independent of the exercise volume performed. Considering the present results, we conclude that the 100-jump session is an optimal stimulus to produce marked acute physiological responses even in highly trained athletes. Within the volume range tested (100—300 jumps), a greater exercise volume (300 jumps) appears to result in the same neuromuscular impairment as the lower exercise volumes 24 hours after exercise in rugby athletes.
Both traditional strength training (i.e., heavy load and slow contraction velocity) and explosive strength training (i.e., light to moderate loads and high contraction velocity) acutely impair neuromuscular function, resulting in decreased force, power, and RFD (14,20,21). This decreased performance may be explained by central (20,21) and peripheral (7) mechanisms of fatigue. Regarding stretch-shortening cycle (SSC) exercises, it has been demonstrated that submaximal, long duration, and exhaustive SSC exercise, such as marathon running, has an acute deleterious effect on strength performance (26,35). However, data on the effect of explosive SSC exercises, such as plyometric training bouts, on neuromuscular function are scarce. In a study by Drinkwater et al. (7), 212 ground contacts performed during a plyometric exercise session acutely impaired strength and RFD, which were recovered after 2 hours, in physically active subjects. In another study, Skurvydas et al. (33) showed that only 100 jumps, continuous or divided into 5 sets of 20 jumps, resulted in marked declines in maximal voluntary contraction and in the evoked force. To the best of our knowledge, no study has compared the effect of performing different amounts of plyometric exercise on the neuromuscular function of athletes. Our results are consistent with those of Drinkwater et al. (7) because significant decreases were observed in the PT and RFD immediately after all volumes performed. A unique finding of this study was that the PT and the RFD were not recovered at 8 or 24 hours after plyometric exercise, even when a lower exercise volume (i.e., 100 jumps) was performed. It should be noted that despite the highly trained status of the rugby players, their strength performance was impaired until 24 hours after exercise. From a practical point of view, the results of this research indicate that coaches should carefully monitor the volume of plyometric exercise sessions during microcycles because athletes might not be able to perform strength or power activities at their best within at least that first 24 hours after plyometric training.
In this study, no differences in the jump height values were observed between the preexercise, immediately postexercise, and 8 hours postexercises. Nevertheless, all plyometric training volumes resulted in an impaired jump performance 24 hours after exercise. A possible explanation is that the performance immediately postexercise had the benefits of the postactivation potentiation (PAP) phenomenon (5,13,16,31). Indeed, it has been shown that the performance of strength exercises before plyometric tests improves jump performance, resulting in greater jump height, power, and velocity (31). This response appears to be greater in type 2 muscle fibers, which are preferentially recruited during explosive contractions (20,21). Whether the PAP phenomenon prevented impairments in the jump performance immediately after the plyometric exercise bouts remains speculative. If true, the PAP phenomenon only prevented a decline in performance for tests in which the SSC was involved because marked reduction was observed in the MVC performance immediately after exercise.
Although jump performance was not reduced immediately after exercise, all strength and power parameters (i.e., PT, RFD, SJ, and DJ) were impaired 24 hours after plyometric exercise. This impairment might be explained by the fact that plyometric exercise can impart eccentric loads over 5 times the subjects' body weight on the active muscle groups (13), resulting in a force production beyond what could be voluntarily produced. This eccentric overload might impair the skeletal muscle power and strength for several days after exercise because of damage of the contractile mechanisms (27). Indeed, it has been shown that recovery after eccentric exercise might consist of a full recovery immediately after exercise with subsequent strength reductions in the subsequent days, which can be described as a bimodal recovery of SSC exercises (26).
Another interesting finding of this study was the similar strength and power output recovery pattern after all amounts of plyometric exercise. A possible explanation for these results is the period of recovery used between sets in the longer protocols (i.e., 5 minutes), which might have allowed the subjects to start each subsequent set in a similar neuromuscular condition. These results may have an important practical implication because a recovery of 5 minutes might be an optimal rest period for performing multiple sets of 100 ground impacts. Notwithstanding, we did not compare the 5-minute period of recovery with longer or shorter periods of recovery between sets, and this hypothesis remains untested. In addition, the subjects of this study were rugby athletes who often perform plyometric training sessions of approximately 500 ground impacts, and it is possible that the same pattern of neuromuscular responses would not be observed in untrained subjects. Indeed, it has been shown that fatigue after resistance exercise depends on the subject's athletic background (11). Thus, caution has to be exercised when interpreting our results because it is possible that these acute neuromuscular responses would occur only in elite rugby players who often perform high-volume plyometric training.
Regarding the hormonal responses, the acute response to traditional strength training has been shown to be volume dependent because greater responses are observed as the number of sets increases (34). However, there are few data regarding the hormonal response to plyometric exercise. In a study by Beaven et al. (2), small increases in salivary testosterone (∼13%) and COR (∼29%) were observed in response to the same volume of different jump exercises (i.e., 3 sets of 3 repetitions). To the best of our knowledge, no study has investigated the effect of different volumes of plyometric training on the testosterone and COR responses. Surprisingly, in our study, the 3 jump volumes resulted in similar hormonal responses. Regarding testosterone, the similar responses between the different exercise volumes can be associated with the similar lactate responses to these protocols. It has been shown that the testosterone response to resistance exercise is influenced by a direct stimulation of the testes by lactate, as demonstrated in a study by Lu et al. (22), who observed a correlation between the increase in testosterone and the increase in lactate during an incremental protocol in rats. Furthermore, these authors demonstrated in vitro that a direct infusion of lactate into the testes results in a dose-dependent increase in testosterone. Thus, it is possible that the absence of an additional increase in lactate when the exercise volume was increased might explain the absence of an additional increase in testosterone. Both responses might be related to the long rest time used when 200 and 300 jumps were performed (i.e., 5 minutes). In fact, it has been shown that lactate and testosterone responses are lower with long rest intervals between sets when compared with short rest intervals (18,19,29). In addition, explosive strength training has been shown to result in low increases (i.e., ∼10%) in the testosterone (2,24) and lactate responses after exercise. Although testosterone responses to resistance exercise have been postulated as an important factor to optimize strength and hypertrophy (4,12,19,30), the role of this hormone in power development during plyometric training needs to be investigated further.
With respect to basal circulating COR levels, this catabolic hormone is responsible for lipid and protein degradation and the subsequent mobilization of energy substrates during exercise (19). In addition to the metabolic impact related to intensity, volume, and rest interval (18,29,34), it has been demonstrated that training status directly influences the magnitude of the adrenal cortical response, with trained individuals having a significantly lower acute COR response compared with untrained individuals (4,17). Thus, the fact that the subjects in this study were strength trained may be another factor that influenced the weak COR response to the exercise protocols performed in this study.
Our study has some limitations. First, our results showed no differences between acute neuromuscular, lactate, and hormonal responses after 100, 200, or 300 hurdle jumps. However, no training adaptations were investigated in this study, and the potential chronic neuromuscular adaptations induced by these 3 different plyometric exercise volumes must be investigated in a long-term study. Second, this study examined only 1 type of plyometric exercise (i.e., hurdle jumps), which involves minimal contact time and fast SSC, and it is not appropriate to extrapolate these results to other types of plyometric exercises (i.e., countermovement jump, SJ). Another possible limitation of this study was the absence of strength and power tests after the first 24 hours (i.e., 48 and 72 hours postexercise) because possible differences between the plyometric exercise volumes performed could appear if a longer recovery period was monitored. However, this hypothesis remains speculative and must be investigated.
In summary, the present results expand the data regarding the acute neuromuscular effect of SSC exercise because it shows that different plyometric exercise amounts ranging from 100 to 300 ground impacts impaired the neuromuscular performance in power-trained rugby players. In addition, these impairments in neuromuscular performance remained until 24 hours after plyometric exercise. Moreover, similar neuromuscular, metabolic, and hormonal effects were observed after all training volumes tested. Thus, a low volume (100 jumps) of plyometric training seems to be a sufficient stimulus to result in marked neuromuscular, metabolic, and hormonal acute responses, and these responses are not enhanced for the training sessions of up to 300 jumps. The rest interval between sets of 100 jumps used in this study (i.e., 5 minutes) might help to explain the lack of differences between the results of the different exercise volumes.
The results of this study suggest that coaches should carefully monitor the volume of plyometric training sessions during microcycles because athletes might not be able to perform strength or power activities at their best until 24 hours after plyometric exercise sessions. Our results showed that the jump height, PT, and RFD were not recovered 24 hours after completing 100, 200, or 300 hurdle jumps. Thus, even when only 100 ground impacts are performed, plyometric exercise sessions must be followed by more than 24 hours of recovery to allow rugby athletes to recover their strength and power. In addition, if greater exercise volumes are necessary to improve power performance in highly trained rugby athletes, 200 and 300 ground impacts can be performed with the same level of fatigue and neuromuscular impairments as 100 jumps, at least in the first 24 hours with long rest intervals between sets, such as the intervals used in this study (i.e., 5 minutes). However, caution should be exercised when applying the present results because the time of recovery was monitored only in the first 24 hours and differences between the plyometric exercise volumes investigated might appear after this period. Furthermore, the subjects of this study were elite rugby athletes who often perform high volumes of plyometric training, and it is possible that the same pattern of neuromuscular responses would not be observed in untrained or less trained subjects.
This study was partially supported by the National Council of Technological and Scientific Development (CNPQ), Coordination of Improvement of Higher Education Personnel (CAPES) and Brazilian Sports Ministry.
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