1. Sit on the edge of a flat bench, legs hanging down with knees slightly bent, and grip the bench behind you.
2. Raise your knees up toward your chest, keeping your legs together.
3. Lower your legs back down until your heels almost touch the floor.
Primary: Rectus abdominis.
Secondary: Obliques, hip flexors (iliosoas, rectus femoris).
Range of motion:
Raise your knees up until your thighs almost touch your abdomen. As you lower your legs down, stop before your heels make contact with the floor to keep tension on the muscles.