Jack Knives

How to do the Jack Knives

This exercise can be performed with your knees bent or kept straight. Lie on the ground (beginner) or a bench (advanced) with your lower body not on the bench. Complete a crunch and bring your knees and shoulders together. This exercise takes a lot of balance and is great for any athletes.

Spotter Information:
Encourage away.

Note: If you feel you are starting to lose your balance, put your foot down. A bench allows for more stabilizer muscles to be activated to maintain your balance.

These are the muscles , equipment and categories of this exercise.

  • PRIMARY: Abdominal
  • Lower Abdominal
  • Belly
  • Stomach
  • Hip Flexors
  • Calisthenics

Averages for MyFit Users

The table below represents data from over 200,000 exercise logs and gives us an average for this exercise.

Average Weight LiftedAverage Reps One Rep Max Average
10.6 lbs16.7 20.7 lbs


What is a good starting weight?


Based on an average weight lifted of 10.6 lbs for all MyFit users we suggest you start at 50% of that weight:

Try 5 lbs and aim for 12-15 reps.




Alternatives