Jackknife Sit Up
Auto DetectedA 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
Training Style
Setup Requirements
Requires Rack
No
Requires Bench
No
Requires Spotter
No
Space Needed
Medium
Noise Level
Low
Muscle Breakdown
View Muscle MapAbs
10/10Rectus Abdominis, Transverse Abdominis
Hip Flexors
8/10Iliopsoas
Obliques
6/10External 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.
- Engage your core and simultaneously lift your torso and straight legs off the floor.
- Reach your hands toward your feet, meeting at the highest point to form a V-shape.
- Hold the peak contraction briefly while maintaining straight limbs.
- Slowly and with control, lower both the legs and torso back toward the floor.
- 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.
Similar Exercises
Jackknife Sit Up
Body Weight
Abs
Sit-Up
Body Weight
Hip Flexors
Decline Sit Up
Decline Bench, Body Weight
Abs
V-Sit
Body Weight
Abs
Dumbbell V-Sit
Dumbbells, Body Weight
Abs, Hip Flexors
Stability Ball V-Up
Stability Ball, Body Weight
Abs
Stability Ball Stir the Pot
Body Weight, Stability Ball
Abs
Pull-Up Bar Hanging Leg Raise
Body Weight, Pull-up Bar
Abs
Stability Ball Crunch
Stability Ball, Body Weight
Abs
Stability Ball Deadbug
Stability Ball, Body Weight
Abs
Master your movement.
Experience automatic exercise detection and rep counting - powered by the motion of your Apple Watch.
subscribe to our newsletter
ProductContact Us
hello@trainfitness.aiFind Us
130 Spadina Avenue, Toronto,
Ontario, M5V 0H4, Canada
©2025 All Rights Reserved
