Lie on your back with your arms extended on the floor above your head and your knees bent at a 90Â° angle with your feet on the ground.
Contract your core muscles and push your lower back against the mat. Raise your knees up above your hip joint, then extend your legs straight. Simultaneously crunch upwards, reaching towards your toes.
Slowly lower back down; keep your arms straight and bend your knees back to a 90Â° angle, hovering them just off the ground before repeating the motion. Continue the motion for a set number of repetitions or a set time period.
The Jack Knife Crunch Level 1 is a very challenging move that strengthens and tones your abdominal muscles by engaging the range of motion through both your upper and lower abs. This intermediate core exercise should not be done if proper form cannot be kept throughout the motion. The Jack Knife Crunch is a great core exercise for a regular abs routine and makes a perfect core component in a boot camp workout.