Working in a PT gym full of kids using the batting cages or pitching, you sometimes still hear questions from parents about why is “P” on the gym side lifting, he's only 12 and you’ll get comments like, “don’t you know lifting stunts your growth, that's why all powerlifters are small.” What this fails to see is the natural selection of the sport, i.e. gymnastics doesn't make you short and similarly playing basketball doesn’t make you tall. These are just traits that lead to success in their respective sports and coaches tend to prioritize these things earlier in their careers as well as kids with these traits tend to have more success in their sport, leading them to gravitate their interest to those sports.
This theory has led to the belief that early resistance training in life leads to stunted growth secondary to compression of the growth plates but evidence has continued to disagree with the aforementioned idea. In 2012, Feigenbaum et al reported “resistance training can be a safe, effective and worthwhile activity for children and adolescents.” and in 2003, Falk and Eliakim reported “...contrary to the common misconception that resistance training may retard growth... Scientific evidence indicates that resistance training results in increased serum IGF-1 and that there is no detrimental effect on linear growth.”
To go along further, let's speculate on what would happen to the body due to the amount of force enacted on the body during sports. A study by McKay et al showed that adolescent boys and girls experienced a force of, on average, 5x their body weight upon landing from a jump. So even if you’re still skeptical, why would these compressive landing forces not make our basketball athletes shorter?
Children are clearly able to tolerate the high-velocity and explosive movements performed during their sport and these forces far exceed the forces applied to the lower extremities during regular gym and weight training activity. This is not to say there are no growth plate injuries, but most of these injuries occur during sport when the body is moving at rapid speeds like throwing, sprinting and jumping. If these athletes are able to tolerate such high forces of their sport, there's no reason they cannot be active participants in the weight room from an early age also.
It also might be important to realize that kids will not gain force output (read: strength) in the same manner as an adult will. While, in the adult population “strength” is gained through improved neural drive or the addition of muscle fibers or increased size of muscle fibers. In children, we see most of the increase in “strength” occurs through the first one listed, through neural adaptations and we keep strength in quotations as it’s more generally improved force output from more efficient or improved neural drive. While it is different, it is still an improved force output which is important for an athlete to jump higher, sprint faster, throw harder etc. Dahab and McCambridge showed that “children can improve strength by 30-50% in the first 8-12 weeks” of resistance training.
This leads to the question of what age should children begin to maximize the benefit of exercise, and what does that safe exercise look like?
The good news is the safe exercise looks almost like what anyone else would do in the gym, squats, push ups, deadlifts, box jumps, broad jumps, sprints etc. and these can start once the child is able to follow instructions. Training for kids (and probably everyone) should be fun and engaging, helping them to better learn how their body moves in space and helping them to produce more force. There’s no minimum requirement of when you can start to train and no known adverse effects to starting, get kids exposed to the gym early in life and find someone to challenge their athleticism and improve their performance while decreasing their potential injury risk.
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