What is Jack Knife?

The Jack Knife is an easy bodyweight core exercise that raises both straight arms and legs from a lying position to engage the rectus abdominis and obliques. It builds core strength and endurance with no equipment; focus on controlled movement and a neutral spine to protect the lower back.

How to Do Jack Knife

  1. Start lying flat: Lie on your back with arms and legs fully extended, shoulders relaxed, and a neutral spine. Breathe normally and prepare to engage your core.
  2. Brace your core: Exhale and draw your belly button toward your spine, tightening the abs to protect the lower back before initiating movement.
  3. Lift arms and legs: Simultaneously raise straight arms overhead and legs up to about 35–45 degrees, keeping knees slightly soft and movement controlled.
  4. Reach and hold: At the top, aim to bring hands toward feet, hold the contraction 1–2 seconds while breathing steadily and avoiding neck strain.
  5. Lower with control: Inhale and slowly lower arms, legs, and torso back to the mat, maintaining tension in the core and a neutral lower back.

Muscle Groups

Core

Description

Lie flat on the floor with your arms and legs extended. Your arms should be lifted so that they’re above your head. Make sure your spine is in a neutral position so your lower back isn't arched or rounded.

Exhale and contract your abs, raising both arms and legs. Try to keep your legs straight and bring them up so they’re at about a 35-45 degree angle to the floor. At the same time, bring your arms up and over your head so that they’re parallel to your legs. Your upper body should be raised slightly off the floor.

Inhale as you lower your arms and legs slowly to the starting position. Take a deep breath, then slowly return your arms, legs, and torso to the mat. Try to keep your arms and legs as straight as you can as you lower them.

Repeat for the required amount of repetitions.

Movement Group

Core

Required Equipment

None (bodyweight only)

Progressions and Regressions

None

Frequently Asked Questions

What are the benefits of the Jack Knife?

The Jack Knife strengthens the rectus abdominis and obliques, improves core control, and enhances hip flexor coordination without equipment. It supports better posture, spinal stability, and transfers to sports and compound movements when performed with proper form.

What are common mistakes when doing the Jack Knife?

Common mistakes include arching the lower back, using momentum, bending the knees, and tensing the neck. To fix them, slow the descent, keep a neutral spine, maintain core tension, and focus on controlled breathing and full-range movement.

How can I progress or modify the Jack Knife?

Modify by bending knees or performing single-leg jack knives to reduce load. Progress by increasing reps, adding a pause at the top, trying V-sits or weighted jack knives, or working hollow holds and leg raises to build strength.