Too many days a week, inadequate rest periods.
https://www.soccerwire.com/soccer-blog/multiple-teams-in-a-season-a-total-disaster-for-youth-soccer-players/
Did you really think it was a good idea for a maturing kid with open growth plates, bone mineral density not ossified yet, adult stature not grown into either, to run them into the ground year-round with no off season and time for physical preparation? Did you really think it would end well to bolt from one team practice to the next, then to a Friday skills session, to Saturday game, to another Sunday skills session, and for them to come home to homework, too? Did you really think it would work in their favor for them to do this several days a week with no rest while the quadriceps muscle is pulling on the knee tendon during growth?
Remember, appropriate buckets in youth athletic development need to be filled equally: technical, tactical, and physical. Skills trainers aren’t totally necessary in-season if they take away from recovery days because they’re just adding more fuel to the overuse fire. They’re only keeping the muscles in a catabolic, not anabolic state. More breakdown, not enough build up.
Muscles require a new stimulus from resistance to grow, namely, in the gym. The same, repetitive movements in skills sessions, that have carryover to what’s already being done at practice, only provide more wear and tear on the muscles, bones and joints. There’s no progressive overload for the body to truly become strong. Pair with with the epidemic of malnourishment and lack of protein in kids, and it’s even worse.