Seated Jackknife With Med Ball

Seated Jackknife With Med Ball challenges the abdominals and hip flexors while the trunk resists arching or rotating.

Primary muscle

Abs

Primary musclesRectus abdominis, Iliopsoas

Secondary musclesObliques, Rectus femoris, Tensor fasciae latae

Equipment

Medicine Ball

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How to perform Seated Jackknife With Med Ball

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

Sit near the front of a bench or mat, hold a medicine ball at the chest, lean back slightly, and lift the feet with the knees bent.
Brace the abdomen and extend the legs forward as the torso leans back without rounding or touching down.
Draw the knees and chest toward one another while keeping the medicine ball close to the chest or reaching it toward the shins.
Pause in the compact jackknife position without rocking on the pelvis.
Extend again under control and stop when the lower back or balance can no longer stay steady.

Common Seated Jackknife With Med Ball mistakes

Lower back arching

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

Swinging the legs

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

Shoulders losing support

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

Returning too quickly

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

When to modify or choose another exercise

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.

How to use Seated Jackknife With Med Ball in a training program

Where it fits in a session

Place Seated Jackknife With Med Ball after the main compound lifts, in a focused core block, or beside an exercise that does not require a fresh trunk.

How often to use it

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

Starting rep ranges by goal

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

Pairing and sequencing

Use Seated Jackknife With Med Ball 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