Hook feet under foot brace or secure low overhang. Lie supine on mat or bench with hips bent. Cross arms and place hands in front of shoulders.
Raise torso from mat or bench by bending waist and hips. Raise crossed arms over knees at top. Return until back of shoulders contact mat or bench. Repeat.
If upper back does not come completely down at end of movement, abdominal muscles may only be isometrically involved in exercise. Pectineus, Adductor Longus, and Brevis do not assist in hip flexion since hips are already initially bent. See Spot Reduction Myth. Also see Dangerous Exercise Essay.
Movement can be made easier extending arms down to sides of body. It can be made easier further by performing movement on decline board.
Exercise can be made more challenging by placing hands further up near head. See Arm Position During Waist Exercises. Exercise can also be performed on incline board or with additional weight.