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."
Lie 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.
Get in plank position on your elbows. Your body should form a straight line from your head to your ankles. Once in position, time how long you can maintain it with perfect form (don't let your hips hike up).
You can hold the position for two minutes without breaking perfect form.
You can hold the position for 90 seconds without breaking perfect form.
You can hold this position for less than 90 seconds.
A 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.
Complete as many standard push-ups as possible, maintaining good form (don't let your back sag).
Push-Up and Ball-Crunch Combo
Get in push-up position with your shins on a stability ball. Complete a push-up, then pull the ball toward your chest. Return to start. Do two or three sets of 10 to 25 reps, resting 30 seconds between sets.
Age 20-29 30-39 40-49 50+
Great >23 >22 >18 >15
>30 >25 >21 >18
Good 12-22 10-21 8-17 7-14
17-29 13-24 11-20 9-17
Fair 0-11 0-9 0-7 0-6
0-16 0-12 0-10 0-8