The literature regarding concurrent training, which involves combining strength and endurance training into individual training sessions (2,8,20,25) has revealed conflicting results. It has been reported that training strength and endurance simultaneously can interfere with the optimal training adaptations of either parameter (strength or endurance) individually (2,25,29). The specific training adaptations for strength and endurance can induce antagonistic intracellular signaling mechanisms (2,25). Putnam et al. (26) found attenuated type I muscle hypertrophy with strength and endurance training vs. strength training alone in young adults. Short-term concurrent training (4 sessions) showed conflicting results between bench press and squats. Reed et al. (29) reported that the inclusion of cycle ergometry before either bench press or squats vs. the resistance exercise performed alone resulted in impaired back squat performance but no effect on bench press performance with young resistance-trained adult men. Longer-term training (8–16 weeks) failed to show interference effects at 8 weeks with untrained middle-aged men but did show lower maximal leg strength development with concurrent training after 16 weeks (19). A number of other researchers have also found mixed results. Schumann et al. (31) reported that endurance before strength training maintained strength but decreased testosterone sex hormone binding globulin with recreationally trained males.
In contrast, there was no evidence of muscle strength, hypertrophy, or neural activation training impairments with concurrently trained young adult men over 10 weeks (3 d·wk−1) (20). Concurrent training has also been shown to be effective for promoting strength and endurance improvements in male professional soccer players (15,16,32,38). Interestingly, the reported improvements were in line with those found for separate training interventions supporting the common practice of concurrent training in soccer (15,16,21,35,37,38). It must be noted that all the aforementioned studies used young adults, and the effect of concurrent training on youth is an important population to consider. This consideration is especially true, as youth tend to participate more in multiple sports with differing physiological and metabolic attributes and training practices. Youth also exhibit less fatigue, faster recovery periods, and less maximal anaerobic capacities than adults (24), which may affect their response to concurrent training.
The sequence of concurrent strength and endurance training may play a role with training facilitation vs. interference. The sequencing of strength and endurance training resulted in no group changes in strength, endurance, or hypertrophy but a decrement in neural adaptations with endurance before strength in adult males (11). Similar mixed results were reported by Häkkinen et al. (13) who showed similar improvements in isometric and dynamic strength and hypertrophy with strength and endurance vs. strength training alone but found concurrent training-related deficits in explosive strength. Chtara et al. (8) showed that training sequence may affect endurance performance when circuit strength training was performed after intermittent-endurance training in male sport students. However, training sequence had no effect on maximal and explosive strength variables, and the associated results were lower than in the strength-only condition (9). Conversely, McGawley and Andersson (21) studying male professional adult soccer players with a semiexperimental design (i.e., no control group), reported no effect of strength and endurance training intervention sequence on performance relevant variables. These contrasting results may suggest a population- or training-specific effect of concurrent training with a possible influence of players' fitness status and training on the outcome variables (8,9,21). Indeed, in soccer, it is common for concurrent training to be performed in parallel with technical-tactical training constituting an additional training stimulus (32).
Hence, based on the conflicts in the literature, the lack of concurrent training research on trained youth, the dearth of studies examining the sequencing of strength, and endurance activities within a training session, it is important to examine these variables. Population-specific differences would be of great interest as strength and endurance performance are considered relevant variables for talent development in soccer (33). Therefore, the aim of this study was to examine the effect of concurrent training and strength and endurance interventions sequencing in elite male young soccer players in conjunction with technical tactical soccer training. Training strength and endurance interventions were considered as intraday and interday sequences. It was hypothesized that there would be a superior effect of concurrent training with no sequence influence on physical performance variables (15,16,21).
Experimental Approach to the Problem
As youth athletes are often involved in multiple sports, school, homework, family responsibilities, social activities with friends, and other activities, there is limited time to engage in additional strength and endurance training programs outside the practice. Hence, the goal of this study was to examine whether concurrent training within single training sessions provided comparable training gains as alternate day training for strength and endurance. In this study, a short-term (12 weeks) randomized-parallel fully controlled with pre-to-post measurements design was employed. The intervention groups trained according to exercise intensities and strategies suggested by Wong et al. (38). Training sequence interventions involved (a) strength before endurance in a single training session (SE), (b) endurance before strength in a single training session (ES), and (c) strength and endurance training on alternate days (ASE). Training interventions were performed 2 and 4 times a week for intrasession (SE and ES) and ASE conditions, respectively. The ASE group performed endurance training the day before strength training. During the remaining weekly training sessions, players performed mainly technical-tactical drills for individual and team skill development. In this regard, the soccer technical-tactical skills development training was performed before the fitness interventions according to Wong et al. (38). In the ES and SE groups, this was performed on the 2 remaining days. Efforts were made to equate the total soccer training time across the experimental groups. All players were pre-to-post intervention tested for endurance, change of direction ability (CODA), lower and upper limbs' explosive-power, and maximal strength using soccer-relevant tests (1,7,27,32). The testing procedures were performed over 3 consecutive days at least 24 hours apart both before and after the training intervention. All the tests were performed at the same time of the day to avoid circadian effect on performance. The assessments took place during the hours usually considered for soccer training (i.e., 17–19 PM). Players' internal training-load was monitored using the Session-Rate of Perceived Exertion (RPE) method (17). In order to respect the training setup usually adopted in male youth soccer teams, no single endurance or strength training group was considered in this study (32).
Fifty-seven male elite-level field soccer players, members of a first division Tunisian soccer club (13.7 ± 0.5 years; 164 ± 8.3 cm; body 53.5 ± 8.6 kg; body fat; 15.6 ± 3.9%), volunteered for this study. Players were randomly allocated into a control group (n = 14, only soccer training, CG) and 3 experimental groups that differed for sequencing strength before (SE, n = 15) or after (ES, n = 14) endurance training within a training session (intrasession) and performing strength and endurance training on alternate days (ASE, n = 14). All players trained 4 times a week with a match played during the weekend over the entire intervention period (12 weeks). Study inclusion criteria for players were as per Castagna et al. (6). The procedures used in this study were familiar to all players as part of their usual seasonal fitness assessment. Written informed consent was obtained from each player and their guardian/parents before the commencement of this study. The Ethics Committee of the Tunisian National Centre of Medicine and Science in Sports approved the study.
Endurance performance was tested using the Yo-Yo intermittent recovery test level 1 (Yo-Yo IR1) and a progressive maximal field test (Vam-eval, VAM) (3,27,38). Maximal strength was assessed with squat and bench press player's heaviest weight lifted (1 repetition maximum [1RM]) (36). Lower-limb explosiveness was assessed using no-arm countermovement jump (CMJ) and squat jump (SJ) and the 5-jump test for distance (5JT) (1,7,36). The CMJ and SJ height was assessed using a force platform (Kistler 9281 C; Kistler, Winterthur, Switzerland). The medicine ball throw for distance (3 kg, MBT) was used for upper-limb explosiveness assessment (34).
Players' sprinting performance (time) was assessed using 10- and 30-m sprints (10,32). Change of direction abilities were assessed with the 15-m run with and without the ball (Agility-15 m and Ball-15 m, respectively) according to Mujika et al. (23). All the tests were timed with photocells gates (Brower Timing Systems, Salt Lake City, USA) placed 0.4 m above the ground and with players standing start 0.5 m behind the first timing gate. Players performed 2 trials of each test (2 minutes between trial passive recovery) with the best measure used for calculations. All the testing procedures were performed carefully controlling the food intake and hydration status of the players to warrant a proper carbohydrate and fluid assumption across the testing sessions.
The ES and SE groups performed both the endurance and strength training programs in a single session (Tuesday and Thursday). The only difference between the 2 training groups was the order in which they executed the training, either endurance training before strength or strength training before endurance. A 15-minute recovery period separated the training sessions. The ASE performed endurance only on Tuesday and Thursday and strength only on Wednesday and Friday.
For 12 weeks, the experimental groups followed an interval training program with 2 sessions per week of high-intensity intermittent-running exercises without interruption. The intensity of the proposed intermittent runs was individualized according to the peak speed attained at the end of the VAM test (MAS) for each player. They consisted of 2 series of 12–16 intermittent runs of 15 seconds at velocities ranging in intensity from 110 to 120% of peak speed attained during the VAM test alternated with 15 seconds of passive recovery (3,27,38). During the first 4 weeks, the number of repetitions per series was set at 10, and then the number of repetitions was increased to 16. The players were placed in different corridors according to their MAS. They had to cover the distance between the 2 extremities in 15 seconds with 15 seconds of passive recovery before turning back to perform the successive high-intensity bout. Players were allowed to stop running within the 3-m distance after the arrival line. Running paces were given by a manual timer producing a sound every 15 seconds from the start to the end of the exercise. The strength training used in the intervention groups was performed according to the procedures suggested by Wong et al. (37). Details of the used training protocols are reported in Table 1. The training study was performed during the first 12 weeks of competitive season.
Data are presented as mean ± SD with normality verified using the Shapiro-Wilk W test. A general multivariate linear-model analysis of variance (ANOVA) was used to examine intervention effects across time (pre-post) and groups (4 levels). Comparisons between groups session-RPE was performed with one-way ANOVA. Differences were reported as confidence intervals at 95%. Significance (p value) was set at p ≤ 0.05. Post hoc analyses were performed using Bonferroni test. Practical differences were expressed as effect size (partial η2). The intraclass correlation coefficient of the variables ranged from 0.89 to 0.94. Significance was set at p ≤ 0.05.
No baseline between-group differences were found (Table 2). A significant main effect for time (pre-to-post) was detected in all the experimental groups (Table 3). The CG showed significant (p ≤ 0.05) pre-to-post changes in the Yo-Yo IR1 (η2 = large), 30 m (η2 = medium), agility-15 m (η2 = medium), and squat (η2 = large). With the ES and SE groups, large pre-post changes were found in the endurance and strength variables (p < 0.02). In the sprint and jump tests, the SE and ES showed large to medium changes after training (p < 0.02). Agility-15m and Ball-15m showed medium to large changes in all the experimental groups. Small and medium changes were reported in the sprint, CODA variables in the CG after the training period. The MBT performance showed medium changes in all the groups but ASE (η2 = large). Large (p < 0.02) postintervention changes were found in ASE for all the variables but Ball-15m (η2 = small).
Posttraining changes between ES and SE were trivial (p > 0.05) for all variables. ES demonstrated large (10 m; p = 0.001) and medium improvements for 30 m, SJ, 5JT, bench press, and Ball-15m (p = 0.02) vs. ASE (Table 2). SE showed medium magnitude better results (p = 0.02) than ASE in 10 m, squat, and 5JT. The Yo-Yo IR1 postintervention performance difference was lower in CG compared with experimental groups with medium to large effects (p = 0.03). Lower posttraining changes were reported for VAM in the CG compared with experimental groups with medium effect (p ≤ 0.05). The posttraining 5JT performance in the CG was significantly lower (η2 = medium) than ES and SE.
Weekly session-RPE were 1473 ± 181, 1417 ± 159, 1570 ± 183, and 1296 ± 168 arbitrary units for the ES, SE, ASE, and CG, respectively. Weekly training load was significantly higher in the ASE compared with CG (p = 0.002, large). No significant and practical (trivial) intragroup and intergroup differences in body composition and mass were found pre-to-post intervention.
This is the first study that examined the effect of training sequence in male youth soccer players with a randomized fully controlled trial under ecological conditions over a medium-term training period (12 weeks). The results showed that concurrent training in the form used in this study was successful in promoting substantial improvements in fitness variables. Second, there was no effect of changing the sequence order of strength and endurance on training improvements. In addition, there was no advantage to training strength and endurance on separate days vs. within one session.
Altering the order of the intrasession strength and endurance exercises generally did not provide additional performance benefits. These findings are in line with those reported by McGawley and Andersson (21) that found no additional effect of strength and endurance intrasession sequence on fitness performance in adult professional-soccer players. However, other studies tend to show mixed results. Changing the sequence of strength and endurance exercises did not resulted in differing strength, endurance, or hypertrophy adaptations but did adversely affect neural responses with endurance before strength in adult males (11). Inconsistent results associated with strength and endurance sequencing were also reported by Häkkinen et al. (13) who showed similar improvements in isometric and dynamic strength as well as hypertrophy but reported concurrent training-related deficits in explosive strength. Training sequence affected endurance performance when circuit strength training was performed after intermittent-endurance training in male sport students (8). However, training sequence had no effect on maximal and explosive strength variables, and the associated results were lower than in the strength-only condition (9). In this study, there was some evidence of mixed results as ES but not SE had significantly greater training adaptations compared with control for 10-m sprint, and 1RM bench press. However, both sequences (ES and SE) shared similar results for 6 of the 8 measures.
The effectiveness of a training protocol should also satisfy the principle of dose response optimization. This study resulted in similar gains across the experimental groups responses (i.e., session-RPE) with a large difference only between ASE and CG. This suggests that independent of the strength and endurance sequence, the training loads were perceived as similar.
The lack of interference with concurrent training in this study contradicts classic literature that tend to report lower training effects with concurrent training compared with separate strength and endurance training (35). According to this literature, endurance and strength development may follow mutually competitive pathways (2,25). A rationale for the present results may relate to the age of the population in this study. The 13-year-old male soccer players trained and tested in this study were much younger than the typical young adult cohort used in other concurrent training studies (8,9,13,21). Children may have a more generalized rather than specific training response as compared with adults. For example, Murphy et al. (24) suggested that children had difficulty in perceiving the difference between maximal and submaximal intensity contractions. Children also present lower levels of fatigue, and thus concurrent training may be less likely to impose an overtraining burden (24). Youth recover quicker than adults, and thus the adult difficulties in handling concurrent training may not apply (12,28). The faster recovery of children can be attributed to a lower reliance on glycolysis, quicker phosphocreatine resynthesizes, faster acid base regulation, and increased fatigue resistance due to a lower power output (12,28,39). Hence, concurrent training may be more appropriate for a younger vs. more adult population. Indeed, concurrent training protocols are usually implemented with success in soccer at elite and subelite levels (15,16,32,38). However, soccer practice could supplement the endurance stimulus while impairing the strength stimulus thus resulting in a detrimental competitive effect (30). Further studies examining the dose response effect of concurrent training on the training responses of youth are warranted.
However, there was some evidence of a sequence effect on specific measures. Only the SE and ASE showed large changes in vertical jump performances with improvement of higher magnitude in the SJ than CMJ (10.5, 13.6% and 7.3, 8%, respectively). These findings are of great practical interest as they provide evidence of a selective sequence effect (strength before endurance) on lower-limb explosive strength development. It could be speculated that previous endurance training (ES) and fatigue could have produced a detrimental effect on neuromuscular activation leading to maladaptation in muscle firing rate (11,13,30). In this regard, further studies investigating the selective effect of endurance training on explosive strength are warranted. The ability to achieve high vertical jump heights is considered as valid and relevant procedure to test soccer players' lower-limb explosive strength (5,32). The CMJ and SJ are reported to have construct validity and to explain variance in squat 1RM in elite soccer (5,32,36). Furthermore, CMJ and SJ were reported as a variable of interest in soccer talent selection and development.
In this study, there was no advantage to training strength and endurance on separate days vs. within one session. For example, the experimental groups showed a large training effect on maximal strength variables with 1RM gains of +43% and +37% in squat and bench press, respectively. The reported improvements corresponded to the changes that are usually found when training strength and endurance separately (18,32). These 1RM squat findings in the experimental groups are in line with those previously reported with adult soccer players (14,15). In the CG, changes were large only in the squat (+21%) with medium but not significant changes (+16%) in 1RM bench press.
There was also evidence that the soccer training was as effective as the concurrent training in promoting training adaptations in some variables. Although there was a numerically greater improvement in Yo-Yo IR1 performance with the ES (+79%) vs. SE (+58.8%) and ASE (+54.52%), the large improvement with the CG (+42%) suggests an effect of weekly soccer technical-tactical practice on the ability to perform intermittent high-intensity activity. This information is of great practical interest as Yo-Yo IR1 was reported to possess good reliability and very large ecological validity in male youth soccer players (4). Furthermore, there were similar improvements in 30-m sprint performance for both the intervention groups (+2.5–6%) and the CG (+2.4%). Mujika et al. (22) reported that the combination of complex strength exercise with usual soccer training is effective for line sprint development in elite young soccer players. Endurance training studies have also showed that line-sprint abilities are conserved if high-intensity running or small-sided games are integrated in the usual soccer-training setup (14,18). These results suggest the complementary effect of strength training and soccer practice (22). Finally, the strength training circuit-involved exercises deemed to improve upper-limb strength and power that may lead to enhancements in throw-in ability in soccer players (32). In contrast to the squat 1RM, bench press and MTB exhibited medium to large improvements only in the experimental groups. The CG's 21% improvement in squat performance suggests the likelihood of a positive effect of soccer practice on lower-limb explosive and maximal strength.
This study showed that when training investments in strength and endurance are considered in youth soccer, they may be safely implemented with positive training consequences sequentially with intrasession or intersession. However, when the objective is to maximize the training effect, attention should be paid to strength and endurance sequence. Indeed, performing endurance before or after strength training proved to have a greater effect on intermittent high intensity mainly aerobic performance and maximal and explosive strength performance, respectively. In this regard, the crossover effect of instrumental changes in intrasession sequence warrant future studies for its potential practical effect.
A number of training studies that examined concurrent training with young adults have showed that endurance training inhibits or interferes with strength development (35). This study demonstrates that concurrent training produces similar or greater training advantages than alternate day training with young soccer players. As there was no training effect difference in the sequence of exercises for strength and endurance, coaches can add strength and endurance exercises to the tactical technical practice sessions in any sequence and expect training dividends. Training practices internationally may differ dramatically. Not all youth have the same access to training facilities outside practice time compared with other richer nations. Hence, concurrent training within practice time may be the only alternative in some parts of the world.
The results of the present study do not constitute endorsement of any products used in training or testing by the authors or the National Strength and Conditioning Association.
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