As the name implies, reverse crunches are the same as regular ones, except that they require you to raise your legs rather than your torso. Lie on your back with your arms at your sides, and lift your legs off of the ground. Keep your knees bent at a 90-degree angle so your thighs point up and your lower legs point straight forward. Next, roll your pelvis backward, and momentarily hold your hips a few inches off the floor to improve thrusting power and endurance, says Hays. Do three sets of 15 to 20 crunches, four times per week.