Perform this routine as a circuit, says its creator, Martin Rooney, P.T., C.S.C.S., author of Ultimate Warrior Workouts.
Do 10 reps of each exercise, and complete as many circuits as you can in 15 minutes. Rest briefly when you need to, and resume working until the time is up. As your conditioning improves, increase reps or decrease the amount of rest.
1. Body-weight squat
Stand with your feet shoulder-width apart. Lower your body as far as you can by pushing your hips back and bending your knees until your thighs are parallel to the floor. Pause, and slowly stand back up. For the safest, fastest way to master this movement, learn the secret to the perfect squat.
2. Judo pushup
Begin in a pushup position but move your feet hip-width apart and forward, and raise your hips so your body almost forms an upside-down V. Lower the front of your body until your chin nears the floor. Then lower your hips as you raise your head and shoulders toward the ceiling. Now reverse the movement and return to the starting position.
3. Sprinter sit-up
Lie on your back with your legs straight and arms at your sides, keeping your elbows bent at 90 degrees. As you sit up, twist your upper body to the left and bring your left knee toward your right elbow while you swing your left arm back. Lower your body to the starting position, and repeat to your right. That's 1 rep. For a 15-minute workout that will target your abs, check out Carve Your Core.