Attach a resistance band to a door anchor. Place the door anchor at the top of a door. Connect the other end of the band to a D-ring on an ankle strap. Do the same for a pair of resistance bands. Lie supine on floor and palms facing downward flat at the sides.
Raise legs by flexing hips and knees until hips are fully flexed. Then raise hips from floor by flexing waist, hips and knees are extended and body is straight. Repeat.
The 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.