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.


Workouts Plans with Jack Knives

Purpose Type Level
Sport Specific Advanced
Sport Specific Advanced
Sport Specific Intermediate
Strength Intermediate
Muscle Specific Advanced
Sport Specific Intermediate
Strength Intermediate

Make a Workout

Crafting a workout for the gym is extremely important. Consideration should be maintained throughout your designing process. If you just want to find one to use we offer plenty here. If you want to make your own you can start now.

Tracking Your Weights

There are lots of easy ways to track your sets and reps. One easy way is through a little book that you can take with you to the gym. This book has progress charts, workouts and lots of tools you can use to always make sure you are improving.