Jackknife Sit Up

Auto-detected exercise indicatorAuto Detected
Intermediate
Home Friendly

A dynamic bodyweight core exercise where the torso and legs lift simultaneously to meet in a V-shape, highly engaging the entire abdominal musculature for improved stability and strength.

About Exercise

Equipment

Body Weight

Difficulty

3/5 • Intermediate

Primary Muscle Groups

Abs, Hip Flexors

Popularity Score

7

Goals

Hypertrophy
Stability
Endurance

Training Style

Calisthenics
Functional Training

Setup Requirements

Requires Rack

No

Requires Bench

No

Requires Spotter

No

Space Needed

Medium

Noise Level

Low

Muscle Breakdown

Abs

10/10

Rectus Abdominis, Transverse Abdominis

Hip Flexors

8/10

Iliopsoas

Obliques

6/10

External Obliques, Internal Obliques

Programming

Typical Rep Range

10-20 reps

Rest Between Sets

45-90 seconds • Shorter rest for endurance, longer for strength focus.

How to Perform

Lie supine on the floor with legs straight and arms extended fully overhead. Ensure your lower back is pressed flat against the floor to establish core engagement.

  1. Engage your core and simultaneously lift your torso and straight legs off the floor.
  2. Reach your hands toward your feet, meeting at the highest point to form a V-shape.
  3. Hold the peak contraction briefly while maintaining straight limbs.
  4. Slowly and with control, lower both the legs and torso back toward the floor.
  5. Return to the starting position without allowing the legs or arms to fully rest on the ground.

Coaching Tips

Form Cues

  • Form a rigid V-shape
  • Keep legs straight
  • Control the descent
  • Exhale on effort

Breathing

Exhale forcefully as you crunch up and reach the peak V position, then inhale slowly as you control the lowering phase.

Tempo

2-0-1

Range of Motion

The movement spans from a fully extended supine position to the maximal stable V-shape, where only the glutes contact the ground.

Safety

Safety Notes

  • If straight legs cause low back strain, bend the knees slightly (performing a tuck V-Up).
  • Focus on smooth, controlled motion rather than speed.
  • Stop immediately if sharp pain is felt in the hips or lower back.

Spotting

Not recommended; this is a bodyweight exercise. Scale down the range of motion if struggling.

Common Mistakes

  • Rounding the lower back excessively
  • Dropping the legs too fast
  • Using momentum to sit up
  • Failing to meet at the center

When to Avoid

  • Acute low back pain or injury
  • Severe hip flexor tightness
  • Recent abdominal surgery

Flexibility Needed

  • Adequate spinal flexion
  • Good hamstring flexibility

Build Up First

  • Competent standard Sit Up
  • Ability to perform a static hollow hold

Also known as

V-Up, V-Sit Up, Cocoons, Jackknife Crunch

Found this helpful?

Share your thoughts or help us improve this guide.

Hero Image

Master your movement.

Experience automatic exercise detection and rep counting - powered by the motion of your Apple Watch.

We use cookies to enhance your browsing experience, serve personalized content, and analyze our traffic. By clicking 'Accept All', you consent to our use of cookies.