Waist Exercise · Body Weight

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Jackknife Sit-Up

The jackknife brings the torso and legs up to meet in the middle simultaneously, which demands more from the abs than a standard sit-up where only the torso moves. It's a harder variation precisely because both ends of the body are working against gravity at once instead of one end staying anchored.

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Category

Waist

Equipment

Body Weight

Target

Abs

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Exercise reference published by PlanWorkouts; see our editorial standards.

How to Do the Jackknife Sit-Up

1. Lie on your back with your legs straight and your arms extended overhead.
2. Brace your core and lift your legs and upper body together, reaching your hands toward your toes.
3. Pause momentarily at the top, then lower your legs and torso back down together.
4. Continue for the prescribed repetitions while maintaining full control through both the lift and lower phases.

Technique & Form Tips

Focus on trunk flexion, rotation, or anti-movement stability as required by your specific variation. Maintain the suggested pelvic and spinal orientation to keep tension focused on the abdominal area throughout each repetition or hold.

Common Mistakes to Avoid

• Copying the wrong trunk action: Follow the demonstrated movement. A held plank, rotation and crunch require different actions rather than the same repeated trunk curl.
• Lower back arching: Keep the spine controlled to prevent the lower back from hyperextending under the load.
• Excessive speed: Perform movements with deliberate control rather than relying on rapid, bouncing motions.

How to Breathe During This Exercise

Maintain a steady breathing rhythm during the effort.

Comfort and Control

Choose a load and range you can control. Stop if the movement causes pain. If discomfort persists, stop and ask a qualified professional about an appropriate exercise and range.

General guidance: Mayo Clinic on weight-training technique. Follow the specific movement steps above for this variation.

Muscles Worked

The Jackknife Sit-Up mainly targets your Abs, with your Hip Flexors helping support the movement.

More Waist Exercises

3/4 Sit-Up Body Weight · Abs 45° Side Bend Body Weight · Abs Air Bike Body Weight · Abs Alternate Heel Touchers Body Weight · Abs Arm Slingers Hanging Bent Knee Legs Body Weight · Abs Arm Slingers Hanging Straight Legs Body Weight · Abs

More Abs Exercises

Arms Overhead Full Sit-Up (male) Body Weight · Abs Assisted Hanging Knee Raise Assisted · Abs Assisted Hanging Knee Raise With Throw Down Assisted · Abs Assisted Lying Leg Raise With Lateral Throw Down Assisted · Abs Assisted Lying Leg Raise With Throw Down Assisted · Abs Assisted Motion Russian Twist Medicine Ball · Abs

Equipment Alternatives

Don't have body weight? These target the same abs with different equipment.

Assisted Hanging Knee Raise Assisted · Abs Assisted Motion Russian Twist Medicine Ball · Abs Band Alternating V-Up Band · Abs Barbell Press Sit-Up Barbell · Abs

FAQ

What muscles does the Jackknife Sit-Up work?

The Jackknife Sit-Up mainly works your abs. Your hip flexors also works to help support and steady your body. Using good, smooth form makes sure you work the right muscles and avoid unnecessary strain on your joints.

What equipment do I need for the Jackknife Sit-Up?

You do the Jackknife Sit-Up with no equipment — just body weight. Make sure your equipment is set up safely before starting. If you work out at home or have limited gear, check out our alternative exercises below.

What form mistake should I watch for on the Jackknife Sit-Up?

One mistake to watch for is copying the wrong trunk action: Follow the demonstrated movement. A held plank, rotation and crunch require different actions rather than the same repeated trunk curl.. Use a comfortable range and a controlled return; reduce the load if you cannot maintain the demonstrated setup.

How many sets and reps should I perform on the Jackknife Sit-Up?

Use a short set of controlled repetitions, or a manageable hold when the movement is static. Stop when you cannot maintain the demonstrated position. Build control before adding duration, repetitions or resistance; the appropriate amount depends on the exercise and your experience.

What's a good alternative to the Jackknife Sit-Up?

The Assisted Hanging Knee Raise trains your abs with assisted and a similar movement pattern. Review its form guide before substituting it, since setup and range of motion can differ.