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

How to perform Jackknife Sit-Up

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

Lie long with the legs straight and arms reaching overhead while the lower back stays controlled.
Brace the abdomen and lift the torso as both legs rise toward the ceiling.
Reach the hands toward the shins or feet so the body closes into a V without jerking from the neck.
Pause at the balanced top position with the pelvis steady.
Lower the torso, arms, and legs together under control, stopping before the lower back arches.

Bend the knees or use a smaller range as soon as the lower back starts to lift or arch.

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

Keep the head, shoulders, and upper back supported while the legs move.

Lower the legs slowly. The lowering part of the repetition still trains the muscles.

Lower back arching: Bend the knees or use a smaller range as soon as the lower back starts to lift or arch.
Shoulders losing support: Keep the head, shoulders, and upper back supported while the legs move.
Stop when the exercise produces sharp, catching, radiating, or increasing joint pain rather than the expected muscular effort.
Alternative Bent-Knee Raise: Bend the knees to make the raise easier while keeping the hips and ribs still.

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	Stop while 1–3 good-form reps are still possible

Use Jackknife Sit-Up after Deadlifts or Barbell Back Squats so abdominal fatigue does not affect the main lift.
Pair it with Bird Dog or Cable Pallof Press for a focused core block.

Scale the movement