15 Minute Workout
5 of 5
An alternative to sit-ups, this exercise strengthens the muscles that support your spine while giving you rock-hard abs. Lie on your stomach with your elbows on the floor in front of you, then slowly rise up onto your toes and straighten your body like a plank. Hold for ten to 20 counts, then lower yourself back down. Daily dose: One or two sets of ten.