In a recently published narrative review, Faigenbaum et al. (2009) emphasized the benefits of resistance training for young athletes to stimulate their athletic development, to tolerate the demands of long-term training and competition, and to induce long-term health promoting effects that are robust over time and track into adulthood. In a recent scoping review, Granacher et al. (2016) introduced a conceptual model for the implementation of resistance training during the stages of long-term athlete development. The authors recommended applying balance and plyometric training during the stages 'learning to train', 'training to train', and 'training to compete'.
It has previously been purported that balance is a fundamental motor skill for the development of athletic performance (Behm and Colado Sanchez, 2013). Taube et al. (2007) observed significant balance improvements following six weeks of balance training with three training sessions per week in male elite athletes (i.e., ski jumpers and Nordic combined skiers with a mean age of 14 years). Despite the fact that the balance training protocol did not contain any strength-building or plyometric exercises, there was still an increase in squat jump and countermovement jump (CMJ) performance after training. Other studies using adult participants have also reported improved jump height performance following balance training with no additional resistance training (Bruhn, 2001; Kean et al., 2006). The positive training effects of balance on jump height are presumably related to the strong correlations between balance measures and power (r = 0.511-0.827) in youth (Hammami et al., 2016a).
There is evidence in the literature that plyometric training is well-suited to enhance muscular power (de Villarreal et al., 2009) and/or proxies of muscular power because plyometrics (e.g., hops, jumps) refer to exercises that link strength with speed of movement (Faigenbaum and Chu, 2001). A position stand / review by the Canadian Society for Exercise Physiology (CSEP) recommended that resistance training exercises for youth can range in complexity from simple body mass, dumbbell, or machine-type resistance exercises to more advanced techniques such as plyometrics and Olympic-style lifting (Behm et al., 2008). Consequently, a large number of studies have examined the effectiveness of plyometric training on muscle power and athletic performance particularly with youth soccer athletes (Chaouachi et al., 2014a; 2014b; Granacher et al. 2015; Meylan and Malatesta, 2009; Ramirez-Campillo et al., 2014). Chaouachi et al. (2014a) trained younger children (10-12 years) with plyometrics and traditional resistance training for 12 weeks. Using precision estimates for inferences, they found that plyometric training was 78% likely to elicit substantially better training adaptations than traditional resistance training for balance, isokinetic force, and power, as well as 5- and 20-m sprints. In a recently published systematic review and meta-analysis, Lesinski et al. (2016) demonstrated that plyometric training is effective in improving muscular strength, vertical jump height, linear sprint performance, agility, and sport-specific performance in the range of small-to-medium effect sizes in youth athletes. As both balance and plyometric training are effective training modalities for youth, it is possible that a program integrating both these physical training components may lead to an enhanced performance adaptation.
It has previously been reported that balance and plyometric training can be implemented in the training routine of youth athletes as separate training regimens or in different combinations. Chaouachi et al. (2014b) demonstrated that an eight week training program with 12-15 year old boys that combined plyometric and balance exercises was as or more effective (sprint, shuttle run, leg stiffness) than a plyometric only training program, even though the combined program had less than half of the volume of plyometric exercises compared to the plyometric only program. Hammami et al. (2016b) examined the sequencing effects of balance and plyometric training on measures of physical fitness in youth soccer athletes aged 12-13 years. The authors were able to demonstrate that the blocked combination of four weeks of balance training with subsequent four weeks of plyometric training produced similar (medicine ball throw, horizontal and vertical jumps, agility and sprints) or superior (reactive strength, leg stiffness, triple hop, balance) performance enhancements as compared to the blocked combination of four weeks of plyometric training prior to four weeks of balance training. Besides the blocked / sequenced combination of balance and plyometric training, these two training regimens can also be integrated as same-session combined training. This training format using balance and plyometric exercises has not been previously investigated.
In other words, balance training could be conducted during the first part of the training session and plyometric training during the second or balance and plyometric exercises could be implemented in pairs (i.e., balance exercise always precedes a plyometric exercise). However, as plyometric training can be an intense, fatiguing activity (Wadden et al., 2012); pairing balance and plyometric exercises could have fatigue-related deleterious effects upon the quality of the later training exercises (Romero-Franco and Jimenez-Reyes, 2015). Conversely, prior non-fatiguing, high intensity exercises such as plyometrics could also have potentiating effects on the subsequent training activity (Sale, 2002).
Therefore, the purpose of this study was to examine the effects of eight weeks of balance and plyometric training when either conducted in the form of blocked training sessions or when applied as alternating exercise pairs on measures of physical fitness in youth soccer athletes. It was hypothesized that the blocked form (BBPT: block of all balance exercises before plyometric training exercises) would result in larger performance enhancements because blocked balance exercises are conducted in a rested state while the alternated form (ABPT: a series of two paired exercises, which alternate balance exercises before plyometric training exercises) may induce fatigue due to the combination of a balance with a plyometric exercise.
Young male elite-level soccer players, between 13-14 years of age, and members of a first division Tunisian soccer club (Esperance Club Tunis, Tunisia), volunteered to participate in this study and were either enrolled in ABPT (n=13) or BBPT (n = 13) training program. With reference to the study of Hamammi et al. (2016b) an a priori power analysis (Faul et al., 2007) with an assumed Type I error of 0.01 and a type II error rate of 0.10 (90% statistical power) was conducted for results in the Y balance test as a proxy of dynamic balance and revealed that 26 persons would be sufficient to observe a medium group x test interaction effect. Participant characteristics are listed in Table 1. All participants were from similar socio-economic status and had the same daily school and soccer team-training schedules. All participants trained four times a week with a match played during the weekend over the entire intervention period (8 weeks). Since they lived in the same city, environmental conditions for testing and training were similar for all individuals. Participants were not involved in any after-school activities or any formalized strength and conditioning training programs other than their formal soccer training. To estimate the maturity status of participants, a maturity index (i.e., timing of maturation) was calculated (Table 1) (Mirwald et al., 2002). This assessment is a non-invasive and practical method of predicting years from peak height velocity (PHV) as a measure of maturity offset using height and age as variables (PHV = - 7.999994 + [0.0036124 x age x height]). The equation has previously been validated with standard error of estimates reported as 0.57 and 0.59 years, respectively (Mirwald et al., 2002).
Before participation in this study, the participants were given a letter that included written information about the study and a request for consent from the parents or their legal representatives to allow their children to participate in the study. Parents or legal representatives and participants provided informed consent after thorough explanation of the objectives and scope of this project, the procedures, risks, and benefits of the study. The study was conducted according to the Declaration of Helsinki and the protocol was fully approved by the Ethics Committee of the National Centre of Medicine and Science of Sports of Tunis (CNMSS) before the commencement of the assessments. No player had any history of musculoskeletal, neurological, or orthopedic disorder that might impair their ability to execute plyometric or balance training or to perform power or balance tests.
Young male adolescents participated in an 8-week training program with two training sessions per week. Participants were randomly allocated to a program that either alternated a series of paired individual balance and plyometric exercises (n = 13, ABPT: Table 2a) or they performed a block of balance exercises prior to a block of plyometric exercises (n = 13, BBPT: Table 2b) within each training session. Similar to other previously published training studies (Granacher et al. 2015, Hammami et al. 2016a; 2016b), a true control group could not be incorporated as the two experimental groups were national level elite athletes and there were no comparable athletes available that would provide similar baseline values. Pre- mid- and post-training measures included proxies of strength, power, agility, sprint, and balance such as CMJ, reactive strength index (RSI), maximum voluntary isometric contraction (MVIC)...