The purpose of doing curl-ups and sit-ups is to strengthen the abdominal muscles, particularly the rectus abdominis. Having strong abdominal muscles helps you maintain good posture, with your back and pelvis properly aligned. This makes your movements more efficient and can keep your lower back healthy and pain-free. While sit-ups used to be the norm in physical fitness assessments and workouts, curl-ups have become increasingly more prevalent due to their effectiveness for strengthening the abdominal muscles and their reduced potential for injury.
For both sit-ups and curl-ups, you begin in the same position: lying on your back with your knees bent and feet flat on the floor. You have several choices for the placement of your arms and hands. You can cross your arms over your chest, stretch them along the floor, grasp your fingers behind your head or place your palms on your thighs. For a curl-up, you slightly tuck your chin to your chest and curl your head, shoulders and upper back off the floor. Once you have curled your shoulder blades off the floor, you roll back down to the starting position. Your lower back remains in contact with the floor throughout the exercise. In a sit-up, you curl your entire back off of the floor so that you end in a seated position before rolling back down.
For both sit-ups and curl-ups, you bring your torso closer to your thighs. This action is created by hip flexion, and a variety of muscles can be used to produce this movement. Ideally, you want to use your abdominal muscles -- the obliques, rectus abdominis and transverse abdominis -- rather than your hip flexors -- your psoas and your rectus femoris. When you use an extended range of motion, such as in a full sit-up, your hip flexors become more involved in producing the movement.
When you use your hip flexors to do sit-ups, your pelvis tilts. This compresses the discs in your lower back and, over time, can lead to injury. Curl-ups, on the other hand, do not significantly compress the discs in your lower back.
A final consideration with respect to both curl-ups and sit-ups is whether or not your feet are held in fixed position. In some physical assessment protocols, someone is responsible for holding your feet on the floor. When your feet are fixed, however, your abdominal muscles work less, and your hip flexors work more, even when you do curl-ups. Since the purpose of these exercises is to strengthen and build endurance in the abdominal muscles, you should avoid fixing your feet under a sofa or other object when doing these exercises on your own.