Lie on the floor with the legs extended straight and your hands at the sides of your head. Simultaneously perform a sit-up and lift your knees to bring them and your elbows together. Pause in this top position before returning to an extended body position without allowing the heels to touch the floor, and pause there before beginning the next rep.  


Purpose
The jack knife provides some additional challenge and variation to the sit-up, trains both the abs and hip flexors, and is good to maintain continuous tension in the abs. Including pauses increases the demand on trunk/hip control and adds isometric stabilization of the trunk.
 
Programming
Perform sets of 10-30+ reps.