Jackknife Sit-up
Jackknife Sit-up turns pure bodyweight leverage into bracing strength that carries over — simple enough to learn in one session.
How to do Jackknife Sit-up
- Lie flat on your back with your legs extended and your arms overhead.
- Engage your core and lift your legs and upper body simultaneously, reaching your hands towards your toes.
- Pause for a moment at the top, then slowly lower your legs and upper body back down to the starting position.
- Repeat for the desired number of repetitions.
Jackknife Sit-up Sets and Reps by Goal
Strength
3 sets of 6-8 hard reps with 2 minutes of rest. Change leverage or elevation before you add load.
Hypertrophy
3 sets of 10-15 reps with 60-90 seconds of rest. Hold the end position for a beat once reps come easily.
Endurance
2-3 sets of 20-25 reps with 45 seconds of rest. Add reps first, then slow the lowering phase.
Common Jackknife Sit-up Mistakes
Shortening the range as soon as the set gets hard
End the set on the last full-range rep instead of grinding out halves.
Pulling with the hip flexors while the abs stay slack
Start every rep by flattening the lower back down, then move.
Easier and Harder Than Jackknife Sit-up
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Jackknife Sit-up — Frequently Asked Questions
How do you do the Jackknife Sit-up?
Lie flat on your back with your legs extended and your arms overhead. Engage your core and lift your legs and upper body simultaneously, reaching your hands towards your toes. Pause for a moment at the top, then slowly lower your legs and upper body back down to the starting position. Use your bodyweight only and keep the movement controlled through the full range. The animated GIF above shows the complete rep at normal speed.
What muscles does the Jackknife Sit-up work?
The Jackknife Sit-up primarily targets the abs. It is classified as a core exercise and is performed with your bodyweight only.
How many sets and reps of the Jackknife Sit-up should I do?
For muscle growth: 3 sets of 10-15 reps with 60-90 seconds of rest. Hold the end position for a beat once reps come easily. Adjust the load rather than the rep target if form breaks down before the final rep.
Is the Jackknife Sit-up good for beginners?
Yes. The Jackknife Sit-up is a beginner-level movement, so it is a reasonable place to start. Keep the load light enough that you can control every rep and stop 2-3 reps short of failure while you learn the pattern.
What can I do instead of the Jackknife Sit-up?
Try the Cable Standing Lift, Band Bicycle Crunch or the Lever Seated Leg Raise Crunch. Each trains the abs with a similar movement pattern, so you can swap based on the equipment you have available.
