CORE STRENGTH
A strong core—the muscles in your abdominals, back, and glutes—gives you stability, power, and endurance. "If your core muscles can't support your pelvis, it will drop, which causes your hips, knees, and ankles to lose proper alignment," says Michael Fredericson, M.D., a professor of sports medicine at Stanford University. "When this happens, you can't absorb forces appropriately, and your muscles fatigue quickly." 

IMPROVE IT The SprinterLie on your back with your hands at your sides, legs straight, and heels hovering six inches off the floor. Start sitting up while elevating your left arm with the elbow bent so it resembles a sprinter's pumping motion. At the peak of the sit-up, bring your right knee toward your chest. Return to the starting position, keeping your legs raised, and repeat with the opposite arm and leg. That's one rep. Do up to 20.

UPPER-BODY STRENGTH
strong upper body makes it easier for a runner to hold good form, which can improve running economy—how efficiently you use oxygen while running. "The more economical you are, the less oxygen you will use, and the longer you can sustain a given pace," says Tom Holland, an exercise physiologist in New Canaan, Connecticut.