Jackknife Sit-Up

Jackknife Sit-Up is a beginner-level compound in the strength category that primarily works the abs. It is performed with bodyweight.

How to do it

1. 1Lie flat on the floor (or exercise mat) on your back with your arms extended straight back behind your head and your legs extended also. This will be your starting position.
2. 2As you exhale, bend at the waist while simultaneously raising your legs and arms to meet in a jackknife position. Tip: The legs should be extended and lifted at approximately a 35-45 degree angle from the floor and the arms should be extended and parallel to your legs. The upper torso should be off the floor.
3. 3While inhaling, lower your arms and legs back to the starting position.
4. 4Repeat for the recommended amount of repetitions.

Muscles worked

Primary muscles in filled badges, secondary in grey. Tap for more exercises per muscle.

Tips and common mistakes

• •Keep your core braced and your spine neutral throughout the movement.
• •Add load only once you complete all reps with good form — progressive overload.
• •Initiate the pull from the target muscle rather than jerking with body momentum.
• •Prioritize full range of motion over heavy weight with partial reps.

Sets and reps

For hypertrophy: 3–4 sets × 8–12 reps with 1–2 reps in reserve (RPE 7–9) and 1–2 minutes rest. For strength: 3–5 × 4–6 with longer rest.