A jack-knife is an abdominal exercise designed to work the abdominal muscles, principally the lower abs. There are a lot of variations of jackknife exercises which allow people of different ability and ages to work their abdominal muscles.

  • Bodyweight
  • Compound

It can be performed on an exercise ball, the floor, the exercise bench or on an exercise mat. It is a toughie, so don’t lose heart if it proves too difficult to do over a couple of reps in the start.

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Suggested Equipment

Benefits of Jackknife Exercise

  • It is a real test for the abs and thus helps build strong abs.
  • Jack-knife exercise is a great, low-impact practice for strengthening the muscle of the abdomen. These are also beneficial to achieving a six pack quickly.
  • Jackknife is challenging for the muscles of the back and improves posture. The lower back also gets benefit from the maneuver, and one can find himself becoming more flexible after weeks of jack-knifing.
  • They increase the flexibility and strength of the entire torso; externally and internally.

Muscles Worked

  • Rectus abdomens
  • Obliques
  • Chest
  • Calves
  • Thighs,
  • Gluteus

How to Do a Jackknife

  1. Lay flat on your back, with your legs fully extended and your feet together. 
  2. Extend your arms, bringing them towards your head, so your biceps are next to your ears.   
  3. Hold a lightweight plate in your hands, or if you’re a beginner just use body-weight. 
  4. Now, raise your legs off the floor and bring your knees towards your chest, whilst simultaneously bringing your arms up and towards your knees.
  5. Remember to keep your arms straight. 
  6. Exhale, as you contract your abdominal muscles and go into a tuck position. 
  7. At this point, your head and upper shoulders should rise slightly off the floor.
  8. When the weight plate is hovering above your knees, hold this position for a moment.
  9. Then, start to slowly lower your arms and legs back down, inhale as you go. 
  10. As you extend don’t let your hands or feet touch the floor, instead, hove them a few centimetres above. 
  11. Your head and shoulders should stay raised.
  12. The jackknife is a core exercise that mainly targets your upper and lower abdominal muscles, including your rectus abdominis and obliques.
  13. To make this exercise more challenging try keeping your legs straight throughout the entire exercise.