1. Lay on your back on a soft workout surface with your arms and legs outstretched away from the centreline of your body
2. Keeping your feet and hands close together, exhale and contract your abdominal muscles to bring your legs and arms up off the floor towards the centreline of your body, you should aim for legs to reach a 45-degree angle or more
3. At the highest point, reach up to touch your shins or toes before slowly relaxing your abdominal muscles to return to the starting position
The primary muscles used in Jackknife Sit Ups are the Abs.