Exercise guide
Jack Knife Sit Up
- Intermediate
- Compound
- Rep-based
- Waist
The Jack Knife Sit Up is a dynamic core exercise that simultaneously targets the upper and lower rectus abdominis and obliques by folding the body into a 'V' shape. It builds explosive core strength and improves coordination between the hip flexors and abdominal muscles.
Reviewed by the Crucible Fit 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 ground, eliminating any arch.
How to do it
- Exhale sharply as you simultaneously lift your legs and your upper body off the floor, hinging at the hips.
- Reach your hands toward your toes, aiming to form a 'V' shape with your body at the peak of the movement.
- Inhale as you slowly lower your arms and legs back toward the floor with a controlled 2-second tempo.
- Stop just before your heels and shoulder blades touch the ground to maintain constant tension on the abdominals.
Form checklist
- Keep your arms and legs as straight as possible throughout the entire range of motion.
- Avoid using momentum or swinging your arms aggressively to pull yourself up.
- Ensure your movement is symmetrical, with the upper and lower body meeting at the same time.
- Keep your neck neutral by looking toward your toes rather than tucking your chin to your chest.
Pro tips
- Focus on the 'hollow body' position at the bottom of the rep to keep the core engaged and protect the spine.
- Pause for a split second at the top of the movement to emphasize the peak contraction of the rectus abdominis.
Make it harder
- Hold a light medicine ball or dumbbell between your hands to increase the resistance on the upper core.
- Slow down the eccentric phase to a 4-second count to significantly increase time under tension.
Frequently asked
- What muscles does the jack knife sit up work?
- The jack knife sit up primarily targets the abs and obliques, and also works the erector spinae as secondary muscles.
- What equipment do you need for the jack knife sit up?
- The jack knife sit up requires no equipment — just your body weight.
- Is the jack knife sit up good for beginners?
- The jack knife sit up is rated intermediate. Build a base with simpler variations first, then progress to it with light load and strict form.