Also known as: v-up, v sit-up, jackknife situp, jackknife crunch

What is Jack Knife?

The Jack Knife is an easy bodyweight core exercise that raises straight arms and legs together to target the rectus abdominis and obliques. It builds core stability, hip flexor control, and abdominal endurance with no equipment—ideal for beginners refining technique.

How to Do Jack Knife

  1. Starting position: Lie flat with arms extended overhead and legs straight, keeping a neutral spine and lower back lightly touching the floor. Breathe in and prepare.
  2. Brace core: Exhale and draw your navel to your spine, tightening the abs and obliques. Keep ribs down, engage pelvic floor, and avoid holding your breath.
  3. Simultaneous lift: Lift both legs and upper torso together until legs reach about 35–45 degrees and arms align with legs; try to keep knees as straight as possible.
  4. Hold briefly: Pause 1–2 seconds at the top while maintaining core tension and neutral neck alignment—avoid neck strain by looking slightly forward, not tucking the chin.
  5. Controlled descent: Slowly lower arms, legs, and torso for 3–4 seconds while inhaling. Keep abdominal tension and prevent the lower back from arching or collapsing.
  6. Repeat safely: Rest briefly between reps and aim for controlled sets of 8–15. Stop if you feel lower-back pain and regress to an easier variation.

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
Equipment: None (bodyweight only)

Progressions and Regressions

None

Frequently Asked Questions

What are the benefits of the Jack Knife?

The Jack Knife improves core strength, hip flexor coordination, and abdominal endurance while promoting body awareness and spinal control. It develops both static and dynamic core stability useful for posture and athletic movements without equipment.

What are common mistakes when doing the Jack Knife?

Common mistakes include using momentum instead of core contraction, arching the lower back, bending the knees, and dropping limbs too quickly. Control each rep, maintain a neutral spine, and keep legs as straight as possible to reduce injury risk.

How can I progress or regress the Jack Knife?

To progress, increase reps, slow tempo, add light ankle weights, or try V-ups and hanging leg raises. To regress, bend the knees, perform single-leg raises, or reduce range of motion. Prioritize form before adding load or volume.