Jackknife Sit-Up
Auto DetectedBodyweight core exercise that targets rectus abdominis, hip flexors, and obliques for strength and stability; used in conditioning and calisthenics to build abdominal definition.
About Exercise
Equipment
Difficulty
3/5 • Intermediate
Primary Muscle Groups
Abs, Hip Flexors
Secondary Muscles
Quads, Glutes
Popularity Score
7
Goals
Training Style
Setup Requirements
Requires Rack
No
Requires Bench
No
Requires Spotter
No
Space Needed
Small
Noise Level
Low
Muscle Breakdown
View Muscle MapAbs
10/10Rectus Abdominis, Transverse Abdominis
Hip Flexors
8/10Iliopsoas
Obliques
7/10External Obliques, Internal Obliques
Quads
5/10Rectus Femoris
Glutes
3/10Glute Max
Programming
Typical Rep Range
8-20 reps
Rest Between Sets
30-60 seconds
How to Perform
Lie flat on your back on a mat with legs extended straight and together, arms straight overhead close to ears, lower back neutral.
- Engage core and exhale as you simultaneously lift upper body and straight legs off the floor.
- Continue lifting until hands reach toward toes, forming a V shape with body.
- Pause briefly at top, squeezing abs.
- Inhale and slowly lower arms and legs back toward starting position with control.
- Keep feet and hands hovering slightly above floor to maintain tension.
- Repeat for reps.
Coaching Tips
Form Cues
- Keep legs straight.
- Reach hands to toes.
- Squeeze abs at top.
- Control the descent.
- Maintain neutral spine.
Breathing
Exhale during the lift to engage core; inhale as you lower back to start.
Tempo
2-1-2
Range of Motion
From flat on back to V shape with legs at 35-45 degrees from floor and torso raised to meet.
Safety
Safety Notes
- Avoid if acute lower back pain.
- Keep neck neutral to prevent strain.
- Engage core to protect spine.
- Start with bent-knee variation if beginner.
Spotting
No spotting needed for bodyweight exercise; perform in open space.
Common Mistakes
- Using momentum instead of core.
- Arching lower back.
- Tucking chin to chest.
- Letting legs bend excessively.
- Dropping limbs fully between reps.
When to Avoid
- Lower back injuries
- Neck strain
- Hip flexor issues
Flexibility Needed
- Adequate spinal flexion
- Hip flexion range to 90 degrees
Build Up First
- Basic plank hold
- Controlled sit-up form
Also known as
V-Up, Jackknife Crunch
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