Hang from a chin-up bar or sturdy rafter with knees bent and hands shoulder-width apart, palms facing forward. Raise the knees by flexing at the hips. At the same time, pull your knees to one side by laterally flexing the spine.
Raise the knees to slightly over 90 degrees relative to the ground. Lower the legs to starting position then alternate to the other side.
Primary: Internal oblique, external oblique, psoas, rectus femoris, lower rectus abdominis
Secondary: Upper rectus abdominis, anterior and posterior forearm muscles (such as the flexor carpi radialis and Palmaris longus), lower trapezius.