Jack knife sit-ups Lie flat on your back with arms overhead and legs straight. Lift both arms and legs simultaneously, bring toes towards hands, and then slowly go back down.

Place your hands by your temples, with your elbows pointing out to the side. Lift your left knee, and at the same time, bend ...

The main difference between sit-ups and crunches is the range of motion involved in each exercise. Sit-ups require you to lift all the way up, while crunches require a slight lift off the ground.