How to do the Jackknife Sit Up
CoreAbsBodyweightIntermediate
The jackknife sit-up demands full-body coordination, forcing your abs to work as a true hinge point rather than just a flexor. Master this movement and you build the kind of deep core strength that transfers to every athletic demand you face.
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Primary
AbdominalsObliques
Secondary
Hip flexorsLower back
Step-by-step
- Lie flat on your back with arms extended overhead and legs straight, pressing your lower back gently into the floor.
- Simultaneously lift your straight legs and reach your arms forward, folding your body at the hips to meet hands to feet at the top.
- Hold the peak contraction for a brief moment, squeezing your abs hard before controlling the descent.
- Lower both arms and legs slowly back to the start without letting them crash to the floor, maintaining tension throughout.
Common mistakes
- Using momentum to swing the arms and legs up instead of initiating from the abs — slow the movement down and focus on the contraction driving the fold, not the swing.
- Bending the knees to make the lift easier, which removes the lever challenge the straight leg creates — keep legs locked out to maximize the demand on your lower abs.
- Letting the lower back arch aggressively at the top or bottom, which shifts stress off the abs — consciously posteriorly tilt your pelvis to keep the spine neutral throughout the rep.
Pro tip — Exhale forcefully as you fold upward, drawing your navel toward your spine at peak contraction — this activates the transverse abdominis and turns a surface-level crunch into a full deep-core movement.
Sets & reps by goal
Build muscle3–4 sets × 10–15 reps
Get stronger3–4 sets × 6–8 reps
Lose fat / tone3 sets × 12–20 reps
Rest: 60–90 sec between sets.
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