To develop a basic understanding of sports training, you have to start with injuries. Because injuries stop play they eliminate any gains in performance. Particular to youth sports, the three most important concepts to understand in injury prevention are age, degree of sport specialization, and training intensity.
Growing athletes are more vulnerable to damaging sensitive body part. Growth plates, cartilage, bones, and ligaments are just a few examples of developing structures that should not be overused.
Doing the same thing over and over definitely improves skill but dramatically increases sports-specific injuries. Many athletes are focusing their attention on one sport, often times playing year round. This is okay if you are a collegiate athlete, but not for a developing adolescent. The key to being a great athlete - regardless of sport - is athleticism. If an athlete never takes time off from a sport, his body doesn't have a chance to heal. The result: gains in skill are offset by increased risk of injury.
More is NOT better
The belief that "if more is better, than even more than that will get even better results" plagues today's youth sports. It's as if a cook decided, even though the directions said to cook the turkey for 4 hours at 400 degrees, to cook it instead for 2 hours at 800 degrees. Chances are he won't be eating turkey tonight.
Same thing holds true in sports. Having practices, games, training sessions, and conditioning drills everyday with no chance for any rest is counterproductive. Again, for an adult athlete who has finished growing, high training intensity may be okay. Younger athletes pushing through injuries in the belief that "it is because I am weak, I need to train more" are simply jeopardizing their futures.
With so much to learn about sports, training, and performance, it is important to have a sound foundation. Understanding and applying the concepts of age, specialization, and intensity to reduce risk of sports injuries is a great place to start.