Lie on the floor with the legs extended straight and your hands at your head where you would place them for a sit-up or crunch.
 
Simultaneously perform a sit-up with the trunk and bend one knee to lift it up to meet your elbow at the top of the sit-up while keeping the other leg extended straight in its original position.
 
Return to an extended body position without allowing the heels to touch the floor in between reps, keeping the abs crunched to maintain a hollow position, and lift the other knee for the next rep.
 
Purpose
The single-leg 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. It trains more control over the pelvis-spine relationship, and can help balance out asymmetry in hip flexor ability.
 
Programming
The single-leg jack knife should be performed for sets of 10-30+ and will generally be more of a stamina exercise with higher reps.
 
Variations
The single-leg jack knife can be done with a pause at the top and/or bottom, and with rotation of the trunk.