Jackknife Sit Up: How To, Form & Muscles Worked

Name: Jackknife Sit Up — exercise demonstration
Uploaded: 2026-09-06T00:00:00+00:00
Description: Fold at the hips by lifting your torso and drawing your knees toward your chest in a coordinated movement that primarily works the abdominals.

Fold at the hips by lifting your torso and drawing your knees toward your chest in a coordinated movement that primarily works the abdominals.

How to do the Jackknife Sit Up#

Lie on your back with your arms at your sides and your legs extended, feet resting on the floor.
If using resistance, hold a dumbbell or medicine ball overhead with your arms extended.
Brace your core and simultaneously curl your upper body up while bringing your knees toward your chest; allow a small bend in the knees if needed.
Lower both torso and legs with control until you return to the starting position and repeat for the desired reps.

Muscles worked#

Primary: Abs (Rectus Abdominis), Hip flexors (Iliopsoas (Hip Flexors))
Secondary: Obliques (External & Internal Obliques)

Common mistakes#

Swinging the torso and legs instead of moving deliberately.
Pulling on the neck or tensing the head with the hands.
Dropping quickly on the descent instead of controlling the return.

Frequently asked questions#

What muscles does the Jackknife Sit Up work?

The Jackknife Sit Up primarily works the abs, hip flexors. The obliques assist as secondary muscles — they stabilise the movement and take over when the main target fatigues. If you feel it mostly in the obliques, lighten the load and slow the lowering phase.

What equipment do I need for the Jackknife Sit Up?

None. The Jackknife Sit Up is a bodyweight exercise, so all you need is enough space to move through the full range of motion.

Is the Jackknife Sit Up suitable for beginners?

Not as one of your first exercises. The Jackknife Sit Up is rated intermediate, so it suits lifters who already have a few months of consistent training behind them. Build the pattern on an easier variation first, then come back to it.

What are good alternatives to the Jackknife Sit Up?

Ab Coaster, Ab Wheel, Air Bike all train the same primary muscles as the Jackknife Sit Up. Pick whichever one matches the equipment in front of you.

How to do the Jackknife Sit Up#

Muscles worked#

Common mistakes#

Plan your sets

Frequently asked questions#

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