Attach cable ankle straps to both ankles, then attach ankle straps to cable. Lie supine on floor or mat, back far enough so cable is taut. Grasp support behind head.
Raise legs by flexing hips until fully flexed. Raise hips from floor by flexing waist. Return until waist and hips are extended downward. 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. Waist flexion will occur earlier in lift if legs are straighter or Hamstrings are not that flexible. See Spot Reduction Myth and Lower Ab Myth. Also see Cable Lying Straight Leg Raise.