In addition to sport-specific technical and tactical skills, strength, explosive power, speed, and endurance, repeated sprint ability (RSA) has been shown to be an important factor for determining success in football (20,25). In top-level professional soccer players, significant correlations have been reported between running distances covered during a match and mean sprint times on an RSA test (25). Results on an RSA test have also been shown to discriminate professional from amateur players (1,20). Consequently, the optimal design and implementation of training strategies that enhance RSA in talented young players is of significant interest to football coaches and players.
From a physiological viewpoint, RSA is a complex quality believed to be related to both neuromuscular related (determining maximal sprint speed, e.g., neural drive or motor unit activation) and metabolic related (involved in the ability to repeat sprints, e.g., oxidative capacity for PCr recovery, H+ buffering) factors (13,28). It is intuitive, thus, that training strategies targeting the development of maximal sprint velocity, metabolic function, or both simultaneously, may account for an improvement in RSA (13,28). To date, most studies in young team sport players have investigated the effect of either metabolic oriented (i.e., high-intensity aerobic training) (3,7,9) or “mixed” (i.e., repeated shuttle sprints, RS) (3,9) programs. Results have been encouraging because all methods have been shown to improve RSA significantly from 1.5 to 3%.
Explosive-type strength training (ExpS) (15) is commonly used to improve neuromuscular qualities (e.g., maximal sprinting speed, muscle explosive power) for athletic performance in both adults (17) and young athletes (14,22). For example, after 8 weeks of explosive strenght training, Mikkola et al. (22) have reported significant improvement in 30 m sprint times in young long-distance runners. Great improvements in jumping ability have also been observed in young soccer players when adding explosive strength exercise to a normal soccer training session (14). However, to our knowledge, no study has been directed toward determining the effects of periodized explosive-type training on RSA.
The aims of the present study were to examine the effects of adding explosive strength training to normal training sessions on RSA in young elite soccer players and to compare these results with those observed after an RS program. In-season RS training has proven to be effective at improving RSA and thus served as a reference (control) intervention. We hypothesized that explosive-type strength training combined with soccer training would result in substantial improvements in neuromuscular performance (as inferred by changes in jumping ability and peak sprinting speed) and consequently RSA.
Experimental Approach to the Problem
Using a controlled study design, participants were divided into 2 training groups who performed either exclusive ExpS (n = 10) or RS (n = 10) in addition to their normal training sessions. Players within each group were matched according to their initial athletic performance and years of practice, assuring that both groups displayed equivalent pretraining mean values for each of the performance parameters. Tests were performed on an outdoor synthetic soccer field 1 week prior to the commencement of training and 1 week following the training period. Tests included 10- and 30-m sprints, an RSA test, a countermovement jump (CMJ), and a hopping test. Players were familiarized with the exercise procedures prior to commencement of each test. They were told not to perform intense exercise on the day prior to a test and to consume their last meal at least 3 hours before the scheduled test time.
The sample size was estimated using acceptable precision or confidence intervals (CI) a priori using the approach developed for magnitude-based inferences (18). Based on the assumption that a between-group difference in mean RSA time of 1.2 ± 1.1% is meaningful (7,9) and considering a within-subject standard deviation (typical error) of 0.8% (20), a sample size of >7 participants per group would provide maximal chances of 0.5 and 25% of type I and type II errors, respectively. Twenty elite, male, adolescent players were recruited (age: 14.5 ± 0.5 years, body mass: 64 ± 8 kg, height: 1.74 ± 0.10 m) and estimated (via the speed reached at the end of the 30- to 15-intermittent fitness test (5)) (maximal oxygen uptake: 51.2 ± 1.5 ml·kg− 1·min− 1). All the players were training in a soccer club for at least 6 years and participated in ∼9 hours of soccer training plus 1 competitive game per week. None of them had already participated in either a periodized ExpS or an RS training program. Their maturational status was estimated at pubertal stage III (n = 12) or stage IV (n = 8) according to Tanner classification (29) by an experienced investigator via direct visual observation of primary and secondary sexual characteristics (i.e., abdominal, shoulder, chest and facial hair, Adam's apple, and voice, adapted from previous data in girls (16)). They were all free of cardiovascular and pulmonary disease and were not taking any medications. The present study, which was approved by the institutional research ethics committee, conformed to the recommendations of the Declaration of Helsinki. Participants and their parents gave voluntary written informed consent to participate in the experiment.
Subjects performed one specific training session per week (every Thursday afternoon (4 pm), at the start of the session, after a standardized warm-up for both groups), in addition to their normal training requirements for 10 consecutive weeks. Eighty-minute football games (6) were played once per week during the experimentation period (every Sunday). Because football coaches in charge of the boys wanted to place their training priorities on technical/tactical contents, only 1 specific session per week could be programmed. The remaining training time was thus exclusively devoted to football training (i.e., technical/tactical). Pilot studies conducted before the present study showed that technical and tactical training sessions can be qualified as intermittent moderate-intensity aerobic exercise (45-75% maximal heart rate for 1-1.5 hours), with regular occurrence of short sprints, as in real games at this age (6). Although football training contents were similar each week for both groups, the experimental training programs were matched by total duration and followed a typical periodized plan (9) that incorporated progression and used a 7-day tapering period (i.e., total training volume was reduced by 35%, and sessions included low-intensity football training contents only). RS training consisted of 2-3 sets of 5-6 × 15- to 20-m shuttle sprints (interspersed with 14 seconds of passive recovery or 23 seconds of active recovery [∼2 m·s− 1 (9)]). ExpS was adapted from previously published programs, with exclusively body weight exercises (14,22). While training across the entire force velocity curve is common in adults (2) who do strength training using gym equipment (e.g., free weights, specific machine for loaded jumps), we choose to restrict our training to the right part of the spectrum (i.e., high velocities and low loads only) in our players with no previous experience in strength training. Using body weight only is also easier to implement ExpS exercises directly on the football pitch. ExpS training consisted of 4-6 series of 4-6 exercises, that is, maximal unilateral CMJs to box, horizontal (hop) and depth (hurdles) plyometric jumps, calf jumps, agility drills (ladders), and repeated standing start sprints. Each repetition and series were interspersed with at least 45 seconds and 3 minutes of passive recovery, respectively.
Running speed was evaluated by 30 m sprint times (standing start) with a 10 m split time. The front foot was placed 5 cm before the first timing gate. Time was recorded with photoelectric cells placed 10 m apart (Brower Timing System, Draper, UT, USA). The 30-m sprint was performed 3 times, separated by at least 2 minutes of passive recovery. The best performance was recorded.
Repeated Sprint Ability
The RSA test involved 6 repetitions of maximal 2 × 15-m shuttle sprints (∼6 seconds) departing every 20 seconds (9) (adapted from a previous running test that has been shown to be reliable and valid in estimating RSA (20)). During the ∼14-second recovery between sprints, subjects were required to stand passively. Two seconds before starting each sprint, the subjects were asked to assume the start position as detailed for the 10-m sprints and await the start signal from a compact disc. Strong verbal encouragement was provided to each subject during all sprints. Two scores were calculated for the RSA test: the best sprint time (RSAb, seconds), usually the first sprint, and the mean sprint time (RSAm, seconds).
Lower Limb Explosive Power Test
Lower limb explosive power was assessed using a vertical CMJ (centimeters) and a hopping test (Hop, centimeters) with flight time measured by an Optojump (Optojump, Microgate, Bolzano, Italy) to calculate jump height. Each trial was validated by visual inspection to ensure that each landing was without any leg flexion, and participants were instructed to keep their hands on their hips during both CMJ and Hop jumps. The depth of the countermovement was self-selected. For the Hop test, players were asked to perform 7 plyometric calf jumps in a raw (i.e., bouncing 7 times) at self-selected frequency between 1.5 and 2 Hz (12). Only the last 6 jumps were retained, and jumping height was averaged. Hopping frequency was recorded during pre- and posttests. During posttest, if hopping frequency was different from that of pretest of more than 0.05 Hz, players were asked to perform the test again after 45 seconds. All athletes were verbally encouraged throughout the test and asked to jump as high as possible during both jumping tests. Each test was performed 3 times, separated by 45 seconds of passive recovery, and the best performance was recorded.
Data in text and figures are presented as mean ± SD. Relative changes (%) in performance are expressed with 90% CI (90% confidence interval). The distribution of each variable was examined with the Kolmogorov-Smirnov normality test. Student's unpaired t-tests were used to examine differences between groups for baseline and final measurements. Data were first analyzed using a 2-factor repeated measures analysis of variance with 1 between factor (training type; RS vs. ExpS) and 1 within factor (period; pretraining vs. posttraining). Each of these analyses was carried out with Minitab 14.1 Software (Minitab, Inc., Paris, France), and the level of significance was set at p ≤ 0.05. In addition to this null hypothesis testing, these data were also assessed for clinical significance using an approach based on the magnitudes of change (19). The standardized difference or effect size (ES) of changes in each fitness parameter between the ExpS and RS groups was calculated using the pooled pretraining standard deviation (11). Threshold values for Cohen ES statistics were >0.2 (small), 0.5 (moderate), and >0.8 (large). For within/between-group comparisons, the chance that the true (unknown) values for each training program were beneficial/better (i.e., greater than the smallest practically important effect or the smallest worthwhile change [0.2 multiplied by the between-subject standard deviation, based on Cohen ES principle (11)]), unclear or detrimental/worse for performance was calculated. Quantitative chances of beneficial/better or detrimental/poorer effect were assessed qualitatively as follows: <1%, almost certainly not; 1-5%, very unlikely; 5-25%, unlikely; 25-75%, possible; 75-95%, likely; 95-99%, very likely; and >99%, almost certain. If the chance of having beneficial/better or detrimental/poorer performances was both >10%, the true difference was assessed as unclear (19).
Only players who participated in >85% of all training sessions were included in the final analysis. As a result, 5 of the 20 participants (25%) were excluded from analysis. One player was injured during a game (week 4), whereas 4 others could not attend to several sessions because of school commitments. None of the player got injured during the specific fitness sessions. Accordingly, 15 players (14.5 ± 0.1 years, 64 ± 8 kg, and 1.74 ± 0.09 m, Tanner stage III = 8, IV = 7) were included in the final analysis. The final sample size for each training group was n = 7 (14.5 ± 0.5 years, 64.7 ± 10 kg, and 1.74 ± 0.12 m, Tanner stage III = 4, IV = 3) for RS and n = 8 (14.5 ± 0.5 years, 64.2 ± 6 kg, and 1.73 ± 0.07 m, Tanner stage III = 4, IV = 4) for ExpS. As a result of the participants' exclusions, some players were no longer matched within both training groups; however, there were no significant differences between mean initial athletic performance and years of practice between final groups before and after training. The baseline anthropometric and physical performance measures of the study dropouts were not significantly different from those who completed the study.
Changes in Physical Performance Parameters After Training
Raw values for all performance parameters are presented in Table 1. After training, except for 10 m (p = 0.22), all performances were significantly improved in both groups (all p < 0.05 for main “period” effect). There was no “training type × period” interaction for 30 m (p = 0.90), RSAb (p = 0.21), and Hop (p = 0.13). A significant interaction was however observed for CMJ (p = 0.05), and a trend toward a significant interaction was noted for RSAm (p = 0.09). Practically worthwhile differences between the training groups were also evident, as supported by large ESs and qualitative outcomes, suggesting probably to almost certainly true changes.
Relative changes and qualitative outcomes resulting from the within-group analysis are presented in Figure 1. For 10 m sprint time, chances that the true changes were beneficial/unclear/detrimental were 65/21/14 and 40/32/29% for RS and ExpS training, respectively. For 30 m sprint time, chances were 75/16/9 and 64/24/12% for RS and C training, respectively. For RSAb, chances were 90/3/7 and 36/32/31%. Chances were 88/8/4 and 52/29/19% for RSAm. For CMJ, chances were 62/22/15 and 81/14/5% for RS and ExpS training, respectively. At last, chances that the true changes were beneficial/unclear/detrimental for Hop were 85/10/5 and 93/6/2% for RS and ExpS, respectively.
Results from between-group analysis are presented in the Table 1 and illustrated in Figure 2.
Relationships Between Physical Performance Indices
When data from both groups were pooled, the relative decrement of 30 m sprint time was significantly correlated to the relative improvement jumping height during CMJ (r = −0.63, p = 0.01; Figure 3), but there was no association with changes in Hop (r = −0.07, p = 0.81). There was no relationship between changes in all athletic performances and RSA variables.
The present study is the first to use specific field tests and a controlled study design to compare the effectiveness of the addition of 2 distinct training approaches (i.e., repeated sprint and explosive strength training) to normal soccer training sessions on RSA in elite adolescent soccer players. Our results show different and specific adaptations to both training regimens; improvements in the RSA test were only observed after RS, whereas jumping height was only increased after ExpS. Because RS and ExpS were equally efficient at enhancing maximal sprinting speed, RS training-induced improvements in the RSA test used in the present study were likely related to improvements in the ability to change direction.
As expected, ExpS training significantly improved jumping height during the CMJ and Hop tests (Figure 1), and these improvements were probably greater than those for the RS program (Figure 2). While this study is the first to report the effect of explosive strength training on RSA in elite young soccer players, the present results are in agreement with previous studies investigating the effect of similar type of training in young runners (22) and soccer players (14), which had reported beneficial effects on maximal sprinting and jumping performance, respectively. This was likely a result of increases in leg muscles (both quadriceps and calf, inferred from CMJ and Hop tests, respectively), explosive power through improvements in motor unit synchronization, stretch-shortening cycle efficiency, or musculotendinous stiffness (15). It is also possible that a better synchronization of body segments or an increase in specific jumping coordination level may have resulted from the ExpS training compared with the RS training (30). These gains in explosive power could also explain the parallel increase in maximal sprinting speed in the present study (i.e., 30-m sprint, Figure 1), which is in line with previous studies that have reported strong correlations between these attributes in elite adults (10) and young soccer players (14). It was, however, surprising to observe no change in RSA, despite an increase in maximal sprinting speed because strong correlations between RSA and sprint qualities have been previously reported (24). Because the RSA test used here involved shuttle runs and because there is apparently no relationship between straight-line and shuttle sprint performance (4,10), it is possible that players did not benefit from the favorable effect of explosive strength training on leg power and maximal sprinting speed when performing the RSA test. Biarticulate muscle (e.g., biceps femoris, rectus femoris, hip adductors, illiosoas, and gastrocnemius), known to be determinant for multi-joint movements that involve deceleration and acceleration as in the shuttle protocol (21), might have not been stimulated enough with the current explosive strength training. In addition, particular coordination and agility (31) might have not been targeted too, explaining the lack of improvement in shuttle (repeated) sprint performance. This illustrates the idea that physiological adaptations and associated changes in performance after a training intervention are to a certain extent training specific, with the energy system, the muscle group, the contraction force, or the movement patterns engaged each playing a role in determining the final adaptations (15,26). Nevertheless, because of the improved 30-m sprinting speed, we can speculate that performance on a repeated straight-line sprint ability test would have been improved after explosive strength training (22,24). In a development perspective, explosive strength training should thus be associated with runs involving changes of direction. This allows for a greater cover and potentially development of the athletic needs for elite youth soccer players (23). It is also worth noting that, while a training frequency of 2 (14) or 3 times (22) per week is typically recommended for young athletes engaged in power/sprint training, significant improvements in performance were observed in the present study using only 1 specific training session per week. This could be related to the fact that our young players, even already well trained, had never participated in such periodized training programs before. This is of interest for coaches dealing with young players, who could thus have more training time to develop technical/tactical skills.
The present results indicate that the RS training program had a likely beneficial impact on maximal sprinting speed, single and repeated shuttle sprint performance, and hopping (jump) height (Figure 1). Moreover, the observed effects on RSA were almost certainly and probably higher than those of explosive strength program (Table 1 and Figure 2). The improvement in RSA (2.5% for mean sprint time) observed in the present study was slightly higher and comparable to what have been previously reported in young handball (9) and soccer (3) players after similar training regimens. The lack of significant improvements in jumping height in the RS group (Figure 1) and the similar improvements in maximal sprinting speed (i.e., 30 m) in both training groups suggest that the observed improvements in RSA in the present study were likely related to changes in specific coordination and agility (31) rather than due to enhancements in explosive force or sprinting mechanisms (27). Given the great importance of changing direction while sprinting at near-maximal speeds in soccer (23), the present data suggest that these training drills should be part of the training program in soccer. Despite the proven efficiency of the repeated sprint training program in improving RSA (present and previously reported results, e.g., (3,9)), the complementary use of high-intensity aerobic exercises, which have also been shown to be greatly effective at improving RSA in young elite team sports players, should be considered as training alternatives (7-9).
The addition, once per week, of either explosive strength or repeated sprint training regimens to normal soccer training sessions represents effective means to increase performance-related physical fitness traits in young elite soccer players. Explosive strength training is likely to improve lower limb explosive power and straight-line maximal sprinting speed, with no impact on repeated shuttle sprint ability. On the other hand, repeated shuttle sprint training is likely effective at improving maximal sprinting speed and repeated shuttle sprint ability. The improvements in different athletic qualities (e.g., shuttle sprinting speed or jumping abilities) in response to the 2 exercise regimens tested in the present study illustrate the concept of training specificity (26) and suggest that both training contents could be part of the training program in young soccer players. These training-specific adaptations offer coaches and practitioners the possibility to individualize training content specific to the athletic qualities in youth soccer. However, whether cumulated improvements are likely to be obtained while combining both training types is still to be investigated.
The authors would thank the players for their enthusiastic participation. The results of the present study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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