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 Jackknife Sit-Ups place high demand on the hip flexors, Iridium scans your 7-day workout history to ensure recent leg training hasn't critically fatigued this secondary muscle group. The AI also tracks your rep performance relative to body weight to calculate progressive overload, adjusting daily volume targets to keep you near your RPE limit.
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
Get Personalized Coaching for Jackknife Sit-Up
Don't guess your way through weights or workouts. Download Iridium for automatic, AI-powered coaching that adapts to your recovery and goals.