Jackknife Sit-Up
The Jackknife Sit-Up is a dynamic bodyweight core exercise that targets the entire abdominal wall by simultaneously lifting the torso and legs into a V-shape. This movement builds explosive core strength and improves coordination between the upper and lower body.
Because the Jackknife Sit-Up places high demand on both the core and hip flexors, our AI analyzes your rep speed and consistency to detect fatigue before your form degrades. By tracking your RPE and daily recovery metrics like HRV, the app can determine if your core is recovered enough for this intense flexion movement or if you should swap it for a more stable regression to protect your lower back. Additionally, if you have previously logged hip pain, the system will automatically recommend alternative exercises that isolate the abs without aggravating the hip flexors.
Form Cues
- Lie flat with arms extended overhead and legs straight
- Exhale and lift your torso and legs simultaneously to meet in the middle
- Reach your hands toward your shins or feet at the peak
- Balance briefly on your glutes at the top of the movement
- Lower your body slowly back to the floor with control
- Don't swing your arms aggressively to create momentum
- Don't allow your lower back to arch excessively as you lower your legs
- Don't strain your neck by leading with your chin
- Don't let your heels slam into the floor between reps
Common Mistakes
- Using momentum instead of muscle tension
- Arching the lower back during descent
- Bending knees excessively
- Pulling on the neck
- Incomplete range of motion
Muscles Worked
This exercise is a powerhouse for the rectus abdominis, effectively engaging both the upper and lower abdominal fibers through a simultaneous folding motion. The hip flexors serve as strong secondary movers to assist in lifting the legs, requiring the deep core stabilizers to work overtime to protect the lumbar spine.
Primary
Secondary
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