Smart Workout Diary

Bench Jackknife Sit-Up

The bench jackknife sit-up is a bodyweight abdominal exercise performed while seated on a bench. It combines trunk flexion with bringing the knees toward the chest, primarily training the rectus abdominis and hip flexors. It can support core strength, abdominal control, and bodyweight conditioning.

Demonstration

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

Instructions

  1. Set up on a stable bench: Sit near the front edge and lightly hold the sides of the bench for support.
  2. Start extended: Lean your torso back slightly and extend your legs forward without locking the knees. Keep your chest lifted and spine long.
  3. Bring the body together: Exhale as you draw both knees toward your chest and curl your torso toward them using your abdominal muscles.
  4. Control the return: Inhale as you slowly extend the legs and lean back to the starting position, keeping your feet from dropping abruptly.
  5. Repeat smoothly: Use a controlled range of motion and stop if you cannot keep the bench stable or maintain a comfortable lower-back position.
Muscle Group
Abs
Other Muscle Groups
Hip Flexors
Type
Strength
Mechanics
Compound
Equipment
Bench, Body Only
Difficulty
Intermediate

Technique tips

  1. Move from the abs: Think about drawing the ribs and pelvis toward each other rather than swinging the legs.
  2. Avoid momentum: Slow the lowering phase; swinging reduces abdominal work and can make the bench less stable.
  3. Adjust the range: Keep the knees more bent or do not lean as far back if you cannot control your lower back.
  4. Breathe steadily: Exhale during the tuck and inhale during the extension without holding your breath.
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