The Ball Jackknife is a fun exercise that really works your core, since you need to constantly stabilize yourself during the movement.
Step 1: Start by getting into a push up position, but with the tops of your feet resting on a stability ball. Arms should be straight (and will remain like that through the whole movement.)
Step 2: Curl your knees in to your chest, rolling the ball in with you. You will control the ball’s movement with your toes.
Return to the starting position by pushing your feet back. Repeat.
Ball Jack Knife video:
See more exercise guides in The Bodyweight Exercise Collection.
Levi Bloom is an experienced endurance athlete who has been training and competing for over 17 years. A former Cat 1 road and mountain bike racer (professional class on the regional circuit), he is now a cycling coach (USA Cycling Level 3 Certified) and sports nutrition coach (Precision Nutrition Level 1 Certified).
Stability ball jack knifes are great for getting more out of the rectus abdominis, much better than regular crunches.