Jackknife Sit-Up

Jackknife Sit-Up trains the abdominal muscles by bringing the rib cage and pelvis closer together through a controlled range.

Primary muscle

Abs

Primary musclesRectus abdominis, Iliopsoas

Secondary musclesObliques, Rectus femoris, Tensor fasciae latae

Equipment

Bodyweight

Set up for Jackknife Sit-Up

Use a level, non-slip surface and clear enough room for the complete movement.
Place any required mat, bench, or stable hand support before taking the starting position.

Establish the named start position for Jackknife Sit-Up, secure the training area, and rehearse the complete pain-free range before adding effort.
Set the pelvis and lower ribs before moving the legs, and shorten the lever if the lower back cannot remain controlled.

Choose the easiest Jackknife Sit-Up variation that preserves the required body line and full controlled range; use elevation, support, or assistance as needed.
Perform one slow rehearsal repetition and change the setup if the equipment shifts, the range causes pain, or the target position cannot be maintained.

Follow the sequence in order, paying attention to the starting position, movement path, and finish described in each step.

For Jackknife Sit-Up, lie with the head and trunk supported, arrange the legs for the named variation, and press the lower ribs gently toward the floor.
Brace before moving and keep the ribs, shoulders, and support points quiet.
Draw the knee or legs toward the torso without swinging or allowing the lower back to arch away from support.
Pause when the pelvis begins to curl or the legs reach the strongest controlled position; keep the shoulders quiet.
Return the legs slowly without losing trunk position. Repeat without changing the established position.

These are the form changes most likely to move work away from the target muscles or make the repetition harder to control.

Shorten the leg lever or range as soon as the pelvis and ribs can no longer remain controlled.

Pause between repetitions and initiate with the abdominals instead of momentum.

Keep the upper body active against the floor, bar, bench, or straps.

Lower the legs slowly; the eccentric phase is part of the exercise.

Reduce the resistance, range, lever length, or balance demand if Jackknife Sit-Up cannot be completed without compensation.
Stop when the exercise produces sharp, catching, radiating, or increasing joint pain rather than the expected muscular effort.
Shorten the hold, range, or lever if the lower back or neck takes over; choose a different core pattern if symptoms continue.

Programming guide

Use Jackknife Sit-Up in a focused core block after the main lifts, or pair it with an exercise that does not depend heavily on trunk freshness.

Place Jackknife Sit-Up after the main compound lifts, in a focused core block, or beside an exercise that does not require a fresh trunk.

Use two to four times per week while accounting for trunk work already supplied by squats, hinges, carries, and overhead lifting.

Goal	Sets	Reps or time	Rest	Effort and execution
Control / technique	2–3	5–8 reps	45–75 sec	Stop well before position changes
Strength / endurance	2–4	8–15 reps	60–90 sec	Finish with 1–3 technically clean reps in reserve

Increase the controlled range, lever length, or hold duration before adding load. End every set when rib, pelvis, or lower-back position changes.

Pair with upper-body accessory work when neither movement depends on an exhausted trunk.
Avoid fatiguing the core immediately before heavy squats, hinges, carries, or overhead work.

Start with 2–4 sets of 6–15 controlled repetitions; use shorter sets for rollouts or long-lever variations.
Stop the set when the pelvis, ribs, or lower back can no longer stay in the intended position.
Progress range or leverage before adding resistance and avoid turning a controlled core drill into a speed test.

Scale the movement