The jack knife is one of my favourite exercises. It targets the lower abs. It can be done anywhere and is a great exercise to do between sets of other exercise.
Step 1:
Sit on the end of the bench legs extended knees straight with heels an inch off the ground. Grab the underneath of the bench for stability keeping shoulders and hips square.
Step 2:
Bring knees towards your chest with your knees and ankles together.
Step 3:
Bring knees up as far as you can. Then extend legs back to starting position.
