Jackknife Sit-Up

How to do it

1. Lie flat on your back with your arms extended overhead and your legs straight.
2. Brace your stomach and press your lower back toward the floor.
3. Lift your arms and legs at the same time, reaching your hands toward your feet.
4. Meet above your hips in a V, balancing briefly on your seat.
5. Lower both under control back toward the floor.
6. Stop just before the heels and hands touch down, then go again.

Common mistakes

• Letting the lower back arch as the arms and legs lower. That is the point where the abdominal wall stopped holding, and it is the range to stop at rather than push through.
• Bending the knees to make the legs lighter. Bending them is a fine way to shorten the lever deliberately; doing it halfway through a set is the exercise changing while nobody decided to.
• Throwing the arms to generate the lift. A jackknife closes because the trunk flexes; an arm swing is momentum standing in for that.
• Treating it as an abdominal exercise only. Half of what raises the legs is hip flexion, and this movement asks the trunk and the hip flexors to work at the same time — which is what makes it harder than a crunch and less isolated.

Muscles worked

Primary

• Abs

Secondary

• Hip Flexors