Although the exercise term "sit-up" is often used interchangeably with "crunches," there's a slight difference. While both crunches and sit-ups start in the same laying-down position, sit-ups involve lifting your entire torso up into a semi-seated position. In contrast, crunches simply require you to lift your shoulders and upper back off the ground. While the sit-up offers many health and fitness benefits, talk to your doctor before trying this exercise, especially if you have a history of back or neck pain.