Exercise guide · Intermediate · Compound

Jackknife Sit-Up: How to Do It & Muscles Worked

The jackknife sit-up is a bodyweight exercise targeting the abdominal muscles. It involves simultaneously lifting the legs and upper body to meet in the middle, engaging the core and hip flexors.

Primary muscles
Abs
Secondary muscles
Quads
Equipment
Body Weight
Level · type
Intermediate · Compound
Demonstration of the jackknife sit-up
Front view: muscles worked by the jackknife sit-up. Primary: abs. Secondary: quads.
Back view: muscles worked by the jackknife sit-up. Primary: abs. Secondary: quads.
Primary Secondary

How to do the jackknife sit-up

  1. Step 1: Lie flat on your back with your legs extended and your arms overhead.
  2. Step 2: Engage your core and lift your legs and upper body simultaneously, reaching your hands towards your toes.
  3. Step 3: Pause for a moment at the top, then slowly lower your legs and upper body back down to the starting position.
  4. Step 4: Repeat for the desired number of repetitions.

Sets, reps and programming

Suggested sets, reps and rest for the jackknife sit-up by training goal
Goal Sets Reps Rest Effort
Strength 3–5 3–6 3–5 min 1–3 reps in reserve
Muscle growth 3–4 6–12 2–3 min 1–2 reps in reserve
Muscular endurance 2–3 12–20 1–2 min 1–3 reps in reserve

Ranges follow the ACSM position stand on progression in resistance training. Muscle grows across a wide range of loads when sets end close to failure (Schoenfeld et al., 2017), longer rests between heavy sets support strength and size (Schoenfeld et al., 2016), and stopping a rep or two short of failure works about as well as training to failure (Grgic et al., 2022). Estimate your working weights with the strength calculators.

Jackknife Sit-Up alternatives

More exercises that train the abs as their main target:

More abs exercises

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Sources

  1. American College of Sports Medicine. Progression models in resistance training for healthy adults. Med Sci Sports Exerc. 2009;41(3):687-708.
  2. Schoenfeld BJ, Grgic J, Ogborn D, Krieger JW. Strength and hypertrophy adaptations between low- vs. high-load resistance training. J Strength Cond Res. 2017;31(12):3508-3523.
  3. Schoenfeld BJ, Pope ZK, Benik FM, et al. Longer interset rest periods enhance muscle strength and hypertrophy in resistance-trained men. J Strength Cond Res. 2016;30(7):1805-1812.
  4. Grgic J, Schoenfeld BJ, Orazem J, Sabol F. Effects of resistance training performed to repetition failure or non-failure on muscular strength and hypertrophy: a systematic review and meta-analysis. J Sport Health Sci. 2022;11(2):202-211.

Step-by-step instructions and demonstration media are from the ExerciseDB exercise dataset.

This guide is general fitness information, not medical advice. Stop if an exercise causes pain and speak to a doctor or physiotherapist about injuries or health conditions before starting a new programme.