How to Do Jackknife Sit-Up
Start and end of one clean rep of Jackknife Sit-Up.
Quick facts
| Primary muscle | Core |
|---|---|
| Secondary muscles | — |
| Equipment | Bodyweight |
| Category | Strength |
How to perform Jackknife Sit-Up
- Lie 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.
- As 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.
- While inhaling, lower your arms and legs back to the starting position.
- Repeat for the recommended amount of repetitions.
Muscles worked
- Core (primary): braces the trunk and resists twisting to protect the spine.
Sets & reps by goal
| Goal | Sets × reps | Load | Rest |
|---|---|---|---|
| Strength | 3–5 × 3–5 | Heavy (~85%+) | 2–3 min |
| Hypertrophy | 3–4 × 8–12 | Moderate (~65–75%) | 60–90 s |
| Endurance | 2–3 × 15–20 | Lighter (~50–60%) | 30–45 s |
Bodyweight movement: once you own the top of a rep range, progress by slowing the tempo, pausing at the hardest point, or moving to a harder variation instead of adding load.
Form tips
- Brace as if about to take a light punch and breathe shallowly — don't hold your breath for the whole set.
- Own the largest range you can control with clean form before you chase extra reps.
- Treat every rep the same — a repeatable setup and bar path is what lets you add weight safely over time.
- Control the lowering phase — a 2–3 second descent builds more than dropping the weight.
Common mistakes
- Cranking with the hip flexors or neck instead of moving from the trunk.
- Cutting the range short to squeeze out extra reps instead of owning each full one.
- Chasing weight jumps before the current load feels solid on every rep.
Frequently asked questions
What muscles does Jackknife Sit-Up work?
Jackknife Sit-Up is a focused movement that primarily works the core.
What equipment do I need for Jackknife Sit-Up?
None — Jackknife Sit-Up is a bodyweight movement, so you only need your bodyweight and a little space.
How many sets and reps should I do for Jackknife Sit-Up?
It depends on your goal: about 3–5 sets of 3–5 heavy reps for strength, 3–4 sets of 8–12 for muscle growth, or 2–3 sets of 15–20 for endurance. Bodyweight movement: once you own the top of a rep range, progress by slowing the tempo, pausing at the hardest point, or moving to a harder variation instead of adding load.
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