Exercise guide
JackKnife
- Intermediate
- Compound
- Timed hold
- Upper legs
- Waist
The JackKnife is a dynamic core exercise that targets the entire abdominal wall and hip flexors by simultaneously lifting the upper and lower body. It builds explosive core strength and improves coordination through a synchronized folding movement.
Reviewed by the Crucible team · Updated June 2026
Muscles worked
Setup
- Lie flat on your back on a mat with your legs fully extended and feet together.
- Extend your arms straight back behind your head, resting them on the floor.
- Engage your core to press your lower back firmly into the mat, eliminating any arch.
How to do it
- In one fluid motion, exhale and lift your torso and legs simultaneously to form a 'V' shape.
- Reach your hands toward your toes or shins, keeping your arms and legs as straight as possible.
- Inhale and slowly lower your limbs back to the starting position with control, stopping just before they touch the floor to maintain tension.
- Maintain a controlled 2-0-2 tempo, focusing on the contraction at the top.
Form checklist
- Keep your legs and arms straight throughout the entire movement.
- Avoid using momentum or swinging your arms to pull yourself up.
- Ensure your lower back stays flat against the floor during the descent.
- Focus on 'folding' at the hips rather than just lifting the shoulders.
Pro tips
- Imagine trying to touch your belly button to your spine to maximize deep core engagement.
- Pause for a split second at the peak of the contraction to emphasize the 'V' position and peak tension.
Make it harder
- Hold a light medicine ball or dumbbell between your hands to increase resistance.
- Slow down the eccentric lowering phase to 4 seconds to increase time under tension.
Frequently asked
- What muscles does the jackknife work?
- The jackknife primarily targets the abs and obliques, and also works the erector spinae, glutes, and hip flexors as secondary muscles.
- What equipment do you need for the jackknife?
- The jackknife requires no equipment — just your body weight.
- Is the jackknife good for beginners?
- The jackknife is rated intermediate. Build a base with simpler variations first, then progress to it with light load and strict form.