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Jackknife Sit-Up

The Jackknife Sit-Up primarily targets your Abdominals. It requires bodyweight and is best suited to beginner lifters.

Beginner level · Equipment: Bodyweight · Compound · Pull

Primary muscles: Abdominals

How to perform Jackknife Sit-Up

  1. Step 1. Lie flat on the floor (or exercise mat) on your back with your arms extended straight back behind your head and your legs extended also. This will be your starting position.
  2. Step 2. As you exhale, bend at the waist while simultaneously raising your legs and arms to meet in a jackknife position. Tip: The legs should be extended and lifted at approximately a 35-45 degree angle from the floor and the arms should be extended and parallel to your legs. The upper torso should be off the floor.
  3. Step 3. While inhaling, lower your arms and legs back to the starting position.
  4. Step 4. Repeat for the recommended amount of repetitions.

Frequently asked questions

What is the proper technique for a jackknife sit-up?

Lie flat with arms extended overhead, then simultaneously lift your torso and legs to meet in a V-shape. Focus on keeping your limbs straight and exhaling as you reach for your toes.

Which muscles are primarily targeted during this exercise?

The jackknife sit-up primarily targets the rectus abdominis. It also engages the deep transverse abdominis and hip flexors to stabilize the movement and lift the legs.

What are the most common mistakes to avoid?

Avoid using momentum or swinging your arms to pull yourself up. Ensure the lower back does not arch excessively during the eccentric phase to maintain constant tension on the core.

How can beginners modify the jackknife sit-up?

Beginners can perform tucked knee-ins by bending the knees toward the chest instead of keeping the legs straight. This reduces the lever length and lessens the strain on the hip flexors.

How should I program jackknife sit-ups into my routine?

Perform 3 to 4 sets of 10 to 15 controlled repetitions. Since it is a bodyweight movement, it can be integrated into a core circuit two to three times per week.

Sample sets & reps for Jackknife Sit-Up

  • Beginner endurance: 3 sets × 8-12 reps, 60-90s rest. RPE 6-7, leave 3 reps in reserve.
  • Hypertrophy: 4 sets × 12-20 reps, 60s rest. RPE 8, last set to failure.
  • Conditioning: 3-5 sets × AMRAP for 30-45s, 30-60s rest. Steady pace, controlled tempo.

Pick the scheme that matches your goal. Beginners should start at the lower end of every range; progress weight only when you hit the upper end of the rep range with one or two reps in reserve.

Related Abdominals exercises

  • Ab Crunch Machine — machine
  • Ab Roller — other
  • Advanced Kettlebell Windmill — kettlebell
  • Barbell Ab Rollout — barbell
  • Barbell Ab Rollout - On Knees — barbell

See all Abdominals exercises →

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