Any one sport focused athlete is not just training in their sport- their workouts include cross training- so while they may not be actually on another field or rink playing a game- they are in fact cross training and using muscles in different ways. My sons hockey team does three on land practices a week. Sometimes it includes a a ball, and sometimes track and field activities. Ballet, yoga and pilates is also beneficial to the athlete and strengthens, lengthens and stretches the muscles in another way. While it is fun for a kid to play with friends in multiple sports, not everyone needs to play 3-4 sports. Some kids enjoy just two and some just one. As long as they are getting trained in a variety of ways, the body will respond and so will the brain.