Reverse situps -- or reverse crunches -- are the opposite of traditional situps. Instead of raising and lowering your upper body, only your lower body moves. The exercise targets your transversus abdominis, the lower portion of your rectus abdominis and are driven by the psoas muscles, which are hip flexors that lie deep below your abdominal muscles.
Lie on the floor, on your back. Spread your arms out to the sides of your body and straighten your legs out. Your body should resemble a "T" shape.
Raise your legs off the floor. Bend your knees at a 90-degree angle, lift your hips off the floor and pull your knees into your chest while holding your upper body still.
Lower your hips to the floor. Straighten your legs and lower them toward the floor, but not quite touching the floor. Repeat the whole maneuver and complete as many reps and sets as your fitness level allows.