If you want to improve your performance at sports, you need to strength train, work on your speed, balance/coordination and flexibility.
The first is strength. Muscle strength is the foundation upon which you build. The most important. Being quick and developing your speed is also important. As is flexibility training.
Our sessions consist of speed training outdoors (40 yard sprints) timed, body weight exercises (push-ups, pull-ups, squats) etc. Training with pre-teens is a blast! They have so much energy and love trying new exercises and competing with each other. Brett & Michael are two of my many younger students. They are both trying out for the football team this year. I am teaching them basic weight training exercises, along with speed work and some boxing (they beg for it) at the end of our sessions. They love it!