The supine knee-in is an effective abdominal exercise when performed
correctly. In most gyms you’ll see a medicine ball or dumbbell held
between the feet for resistance during knee-ins, but since the weight is
acting downward, this tends to over-activate the hip flexor muscles to
keep the weight up off the ground. A much better option is to use
horizontal loading in the form of a cable, which provides resistance in
the direction of the movement.
1. Hook your feet into two handles (as shown in the photo above) or secure both feet onto a weight belt attached to a low pulley.
2. Place your arms at your sides with palms down. Keep your chin on your chest. This will help flatten your lower back against the ground to encourage greater lower abdominal activity over the strong pull of the iliopsoas muscle.
3. Pull the knees toward the chest, then extend your legs back to the original position.