Sit-ups are a low-impact exercise that help strengthen various sets of muscles. According to Fitness Magazine, Pilates roll-ups are the most beneficial type of sit-ups. This exercise is 30 percent more effective at strengthening abdominal muscles than bent-knee sit-ups. However, if your goal is to strengthen abdominal, head, neck and shoulder muscles, the American Council on Exercise suggests bent-knee sit ups are best. Consult with a physician before starting new exercises.
Researchers at Auburn University studied Pilates roll-ups and found them to be the best sit-ups for the six-pack area, or rectus abdominus muscles. The roll-up engages more muscles than the traditional crunch. To do a roll-up, lie on your back with your legs together and flat on the ground. Raise your arms straight overhead. Bring your arms forward and slowly raise your shoulders and upper back off the floor while reaching for your toes. Slowly return to the original position.
Bent-knee sit-ups (or crunches) target your transverse abdominus and rectus abdominus (also known as ab) muscles. Oblique muscles, which stabilize other sets of muscles, also benefit. To perform crunches, lie on your back with your feet flat on the floor and knees bent upward and together. Clasp your hands behind your head with your elbows on the floor. Use your abdominal muscles to lift your shoulders and upper back off the ground, bring your knees toward your elbows, and hold this position for a few seconds. Lower your back and shoulders slowly back to the mat.
Regular sit-ups are most helpful if you have lower back pain and need to strengthen core and back muscles. To perform a regular sit-up, lie flat on your back with your legs extended outward and hands clasped behind your head. Use your abdominal and core muscles to lift your shoulders, upper and lower back off the ground, bringing your elbows as close to your knees as possible. Slowly return to the original position.
If you have persistent back pain when you perform crunches, sit-ups or roll-ups, consider using a stability ball. Sit-ups of all types rely on use of hip flexor muscles, which attach to the lower back. If these muscles are overused, lower back pain can result. If you perform sit-ups on a stability ball, the ball allows for the natural spine curvature and helps the hip flexor muscles relax. A ball also provides a more comfortable surface for your spine than a hard floor or even an exercise mat.