Smart Workout Diary

Jackknife Sit-Up

The jackknife sit-up is a bodyweight abdominal exercise in which the arms and legs rise together to form a V-shaped position. It primarily trains the rectus abdominis while also involving the hip flexors, supporting core strength, trunk control, and abdominal endurance.

Demonstration

Jackknife Sit-Up — demonstration 1
Jackknife Sit-Up — demonstration 2

Instructions

  1. Set up: Lie face up on a mat with your legs straight, feet together, and arms extended overhead.
  2. Brace: Gently tighten your abdomen, keep your lower back controlled against the mat, and look toward the ceiling.
  3. Lift together: Exhale as you raise your straight legs and upper body at the same time, swinging your arms forward toward your feet.
  4. Reach the top: Balance briefly in a V-shaped position, reaching toward your shins or toes without forcing your neck forward.
  5. Return slowly: Inhale and lower your torso, arms, and legs together under control. Stop the range before your lower back arches excessively, then repeat.
Muscle Group
Abs
Other Muscle Groups
Hip Flexors
Type
Calisthenics
Mechanics
Compound
Equipment
Body Only
Difficulty
Intermediate

Technique tips

  1. Move as one unit: Start each repetition by bracing the abdomen rather than throwing the arms or legs upward.
  2. Control the lowering phase: A slow descent keeps tension on the abdominals and helps prevent excessive lumbar arching.
  3. Use an appropriate range: Reach only as far as you can while keeping the movement smooth and your neck relaxed.
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