The jack knife crunch is a more demanding variation of the classic crunch, and an easier variation of the jack knife.
 
Lie on your back and bend your knees slightly and lift your feet off the floor—the feet will stay off the floor throughout a set. Perform a crunch to curl the shoulders up and toward the hips while simultaneously bringing the knees slightly toward the shoulders—the motion of the knees should be of a similar distance to that of the shoulders.
 
Maintain continuous tension in the abs to keep the upper back off the floor during the movement.
 
Purpose
The jack knife crunch is a good combination of continuous ab tension like an isometric hold and minimal motion for both the abs and hip flexors. It can be used for a low-demand ab exercise to reduce difficulty or to allow very high repetitions, and it can be used in training preparation to get the trunk activated without excessive fatigue.
 
Programming
The jack knife crunch is a perfect candidate for “max” reps—don’t take sets to actual failure, but you can push it. This is a good exercise for training preparation, or in addition to heavier, lower-rep or more difficult ab work in a given workout or in a circuit.