True or False: Adolescent Athletes at Risk of High Tendon Stress due to Non-Uniform Tendon/Muscle Adaptation

Not allowing young athletes to lift weights may in fact increase, not decrease, their injury risk and hamper their recovery.

I am not sure why, but people won't stop inventing new reasons why professional athleticism would be bad for adolescents. One of the more recently heard claims is that early resistance training will lead to a "non-uniform adaptation of muscle and tendon in young athletes" that may "result[] in increased tendon stress during mid-adolescence" (Mersmann. 2015).

In a recent longitudinal study Mersmann et al. investigated the development of the morphological and mechanical properties of muscle and tendon of volleyball athletes in a time period of 2 years from mid-adolescence to late adolescence and the results are quite unambiguous.
A total of eighteen elite volleyball athletes participated in magnetic resonance imaging and ultrasound-dynamometry sessions to determine quadriceps femoris muscle strength, vastus lateralis, medialis and intermedius morphology, and patellar tendon mechanical and morphological properties in mid-adolescence (16 ± 1 years) and late adolescence (18 ± 1 years).

Figure 1: Mean values ± standard deviation of the muscle volume of volleyball athletes in mid-adolescence and late adolescence; %-ages indicate relative mid-to-late differences (Mersmann. 2015).

As the data in Figures 1 and 2 indicates, the muscle strength, anatomical cross-sectional area (CSA), and volume showed significant (P < 0.05) but only moderate increases of 13%, 6%, and 6%, respectively. In contrast to the muscular development, the patellar tendon CSA (P < 0.05) which is under constant stress in (semi-)professional volleyball players showed a substantially higher degree of hypertrophy (27%) that wen in line with increased stiffness (P < 0.05; 25%) and reduced stress (P < 0.05; 9%). Accordingly, the scientists conclude that - in contrast to the commonly heard prejudice - exercise during early adolescence will lead to

"pronounced hypertrophy of the patellar tendon led to a mechanical strengthening of the tendon in relation to the functional and morphological development of the muscle - [...] adaptive processes [that] may compensate the unfavorable relation of muscle strength and tendon loading capacity in mid-adolescence and might have implications on athletic performance and tendon injury risk" (Mersmann. 2015).

You know what, I can read your minds: "What about resistance training, then?" That's the question that's preying on your mind, right now - right? Well, as one of the more recent reviews says, "there is evidence that resistance training may reduce injury in a young athlete’s chosen sport" (Myer. 2006). The authors of the review point out that ...

Heyna et al. have demonstrated as early as 1982 that young athletes who regularly perform resistance training exercises are not just less likely to be injured, they also recover faster (Hejna. 1982).

"[t]his evidence is based on the beneficial adaptations that occur in bones, ligaments, and tendons following training and is further supported by epidemiologic-based reports. Lehnhard and colleagues were able to significantly reduce injury rates with the addition of a strength training regimen to a male soccer team. [...] Hejna and coworkers reported that young athletes (13-19 years) who included resistance training as part of their exercise regimen demonstrated decreased injuries and recovered from injuries with less time spent in rehabilitation when compared with their teammates" (Myer. 2006).

Similar results have been found specifically for female athletes for whom strength training - especially when performed in theh preseason and as regular part of in-season conditioning - reduced injury risk factors and anterior cruciate ligament injuries significantly.
References:

• Lloyd, Rhodri S., et al. "Position statement on youth resistance training: the 2014 International Consensus." British journal of sports medicine (2013): bjsports-2013.
• Mersmann, F., et al. "Muscle and tendon adaptation in adolescent athletes: A longitudinal study." Scandinavian Journal of Medicine & Science in Sports (2015).
• Myer, Gregory D., and Eric J. Wall. "Resistance training in the young athlete." Operative techniques in sports Medicine 14.3 (2006): 218-230.