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July 18, 2024

Abdominal Exercise: Jack Knives

Jack Knives Exercise Example VideoPrimary Muscle

Lower and Upper Abs

Secondary Muscles

Obliques - Hip Flexors

Equipment Needed

Flat Bench

Mechanics Type


Proper Exercise Technique

Sit across a flat bench with your upperbody leaning back and your legs fully extended out in front of you. Place your hands on the front of the bench to secure your body in place. With your lower back rounded, bring your upper body forward and simultaneously curl your legs in until your knees are touching your chest. Squeeze your abs in this position and then return back to the starting position.

Make sure to keep your lower back rounded as you perform your reps rather than arched. It is also very important to keep the tension on your abs throughout the entire exercise by not allowing yourself to rest at any point and by utilizing a complete range of motion.

Jack Knives Exercise - Start Position

Jack Knives exercise

Jack Knives Exercise - Finish Position

Jack Knives exercise

Jack Knives Exercise Video


Jack Knives Exercise Abdominal Video



Return to the Exercise Database - Exercise Video Guide

More Abdominal Exercises

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