Exercise guide
Jackknife Sit Up
- Intermediate
- Isolation
- Rep-based
- Waist
The jackknife sit-up is a high-intensity core exercise that simultaneously engages the upper and lower abdominals while utilizing the hip flexors and quadriceps for stability. It effectively builds explosive core strength and improves overall body coordination by requiring the upper and lower body to meet at a single point.
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 gap.
How to do it
- Exhale as you simultaneously lift your torso and your legs off the floor, hinging at the hips.
- Reach your hands toward your toes, aiming to form a 'V' shape while balancing on your glutes at the peak.
- 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 floor to maintain constant tension on the abdominals.
Form checklist
- Keep your legs as straight as possible throughout the entire range of motion.
- Avoid using momentum or swinging your arms to pull yourself up; initiate the move from the core.
- Ensure your lower back does not arch excessively as you lower your legs back down.
- Keep your neck neutral by looking toward your toes rather than tucking your chin to your chest.
Pro tips
- Focus on the 'crunch' at the top, pulling your ribcage toward your pelvis to maximize rectus abdominis engagement.
- To engage the obliques more effectively, imagine pulling your belly button toward your spine throughout the movement.
Make it harder
- Hold a light medicine ball or small dumbbell between your hands to increase the lever length and resistance.
- Slow down the eccentric phase to 4 seconds to significantly increase time under tension.
Frequently asked
- What muscles does the jackknife sit up work?
- The jackknife sit up primarily targets the abs and obliques, and also works the hip flexors as secondary muscles.
- What equipment do you need for the jackknife sit up?
- The jackknife sit up requires no equipment — just your body weight.
- Is the jackknife sit up good for beginners?
- The jackknife sit up is rated intermediate. Build a base with simpler variations first, then progress to it with light load and strict form.