Lie supine on bench or floor. Grasp sides of bench for support.
Raise legs by flexing hips while flexing knees until hips are fully flexed. Continue to raise knees toward shoulders by flexing waist, raising hips from board. Return until waist, hips and knees are extended. Repeat.
When raising hips, keep knees fully flexed, so as not to throw weight of lower legs over head. Rectus Abdominis and Obliques dynamically contract only if actual waist flexion occurs. With no waist flexion, Rectus Abdominis and External Oblique will only isometrically contract to stabilize pelvis and waist during hip flexion. It may be necessary to completely flex hips before waist flexion is possible. See Spot Reduction Myth and Lower Ab Myth.
To decrease intensity, perform exercise on floor and allow heels to make contact with floor each repetition or perform on slightly declined surface.
Perform on incline board or exercise ball to increase resistance. Exercise can also be performed with added weight.