Lie supine on bench or mat. Place hands under lower buttock on each side to support pelvis. Raise bent leg up so lower leg is approximately horizonal above hip. If lying on floor, raise other leg slightly off of floor.
Simultaneously change positions of legs, lowering and straightening raised leg down just above floor while raising and bending lower leg to opposite position. Continue alternating leg positions.
Rectus Abdominis and Obliques only contract dynamically if actual waist flexion occurs. With no waist flexion, Rectus Abdominis and External Oblique will only act to stabilize pelvis and waist during hip flexion. Conversely, it may be necessary to completely flex hips before waist flexion is possible, which is not practical with alternating leg movements. Also see Spot Reduction Myth and Lower Ab Myth.
To decrease intensity, perform exercise on floor and allow alternating heels to make contact with floor each repetition.
Perform on incline board or perform with knees straight to increase resistance. Exercise can also be performed with added weight using ankle straps.