Lie supine on incline board with torso elevated. Grasp feet hooks or sides of board for support.
Raise legs by flexing hips and knees until hips are completely flexed. Return until hips and knees are extended. Repeat.
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. It may be necessary to completely flex hips before waist flexion is possible, as in leg-hip raise. Also see Spot Reduction Myth and Lower Ab Myth.
Lower incline or lie on floor or bench to decrease intensity.
Elevate incline or straighten knees more to increase resistance. Once highest incline has been achieved, exercise can be performed vertically. Bent knees can also be raised to shoulders to dynamically involve Rectus Abdominis. Exercise can also be performed with added weight.