Date Published: October 10, 2018
Publisher: Public Library of Science
Author(s): Matti Peitz, Michael Behringer, Urs Granacher, Johnny Padulo.
To date, several meta-analyses clearly demonstrated that resistance and plyometric training are effective to improve physical fitness in children and adolescents. However, a methodological limitation of meta-analyses is that they synthesize results from different studies and hence ignore important differences across studies (i.e., mixing apples and oranges). Therefore, we aimed at examining comparative intervention studies that assessed the effects of age, sex, maturation, and resistance or plyometric training descriptors (e.g., training intensity, volume etc.) on measures of physical fitness while holding other variables constant.
To identify relevant studies, we systematically searched multiple electronic databases (e.g., PubMed) from inception to March 2018. We included resistance and plyometric training studies in healthy young athletes and non-athletes aged 6 to 18 years that investigated the effects of moderator variables (e.g., age, maturity, sex, etc.) on components of physical fitness (i.e., muscle strength and power).
Our systematic literature search revealed a total of 75 eligible resistance and plyometric training studies, including 5,138 participants. Mean duration of resistance and plyometric training programs amounted to 8.9 ± 3.6 weeks and 7.1±1.4 weeks, respectively. Our findings showed that maturation affects plyometric and resistance training outcomes differently, with the former eliciting greater adaptations pre-peak height velocity (PHV) and the latter around- and post-PHV. Sex has no major impact on resistance training related outcomes (e.g., maximal strength, 10 repetition maximum). In terms of plyometric training, around-PHV boys appear to respond with larger performance improvements (e.g., jump height, jump distance) compared with girls. Different types of resistance training (e.g., body weight, free weights) are effective in improving measures of muscle strength (e.g., maximum voluntary contraction) in untrained children and adolescents. Effects of plyometric training in untrained youth primarily follow the principle of training specificity. Despite the fact that only 6 out of 75 comparative studies investigated resistance or plyometric training in trained individuals, positive effects were reported in all 6 studies (e.g., maximum strength and vertical jump height, respectively).
The present review article identified research gaps (e.g., training descriptors, modern alternative training modalities) that should be addressed in future comparative studies.
For many years, the effects of resistance and plyometric training in youth and its potential benefits and harms were among the most debated research topics in exercise science and physiology. During the 1970s and 80s, researchers and scientific societies postulated an increased risk of sustaining injuries when conducting resistance training because of the immaturity of the skeletal system [1–3]. In addition, it was argued that resistance training in youth is ineffective due to a lack of circulating anabolic hormones . By contrast, nowadays resistance and plyometric training is deemed to be a crucial component of a health promoting lifestyle in young individuals . There is compelling evidence that resistance and/ or plyometric training improves muscular fitness (i.e., muscular strength, muscular power, local muscular endurance) [6–10], bone mineral accrual [11–13], body composition [12,14], motor performance skills [15–17], and lipid profiles . Furthermore, muscular fitness is positively associated with the subjective evaluation of the own worth regarding self-esteem . The current position statement on youth resistance training of the UK Strength and Conditioning Association even concludes that children and adolescents may increase their risk for negative health outcomes during adulthood if they do not participate in physical activities that build up strength and improve motor performance skills .
Initially, our search syntax identified 2,459 potentially relevant studies (see Fig 1). Five hundred and eighty studies remained after the screening of titles. Abstract screening reduced the number to 146 potentially relevant studies. This number was brought down to 86 comparative studies after perusal of the full texts. Another 11 comparative studies were excluded because they did not apply either single-mode resistance or plyometric training (Fig 1). Finally, 75 comparative studies were eligible for inclusion in this literature review. Overall, 5,138 participants (24% female, 76% male) were enrolled in the identified studies. On average, duration of resistance and plyometric training lasted 8.9 ± 3.6 weeks and 7.1±1.4 weeks, respectively. Both, the influence of maturation (resistance training: 9 studies, plyometric training: 5 studies) and the influence of sex (resistance training: 8 studies, plyometric training: 4 studies) on resistance and plyometric training-related effects on components of physical fitness in youth have been examined (resistance training: Tables 1 and 2, plyometric training: Tables 8 and 9). Forty studies investigated the effects of training descriptors (e.g., intensity and volume, periodization) on physical fitness outcomes (e.g., muscle strength [1RM] and power [jump height]). Tables 4–6 illustrate training descriptors for resistance training, Tables 10 and 11 for plyometric. Another 13 studies examined how training type influenced physical fitness outcomes (resistance training, plyometric training, complex training, see Table 12). Additionally, three studies each were identified that investigated the influence of alternative resistance training methods (Table 13) or supervision (Table 7) on outcomes of physical fitness in youth. The explanation for these numbers exceeding the total of 75 studies is due to inclusion of some studies (e.g.,) to perform multiple sub-analyses (e.g., maturity and sex).
In line with previous reviews [1,12] and meta-analyses [6,9,23], the present systematic literature review revealed that resistance and plyometric training are effective in increasing a wide range of physical fitness outcomes in youth.