Distance running can decrease muscle mass and strength training can help to balance that. Distance runners should focus on developing strength-endurance, the capacity to produce moderate-intensity muscle contractions over long intervals without tiring, rather than bulking up. Sprinters will want to develop power and speed; while bigger muscles help, runners should not focus on bulking up at this age.
Strength training is important for increasing stability and power and balancing muscle strength throughout the body—all of which are critical for efficient and injury-free running. Begin including strength training for middle school athletes, particularly toward the end of their growth spurt. At this age, with developing bodies, students should only use their body weight as resistance.