The main purpose of sit-ups is to strengthen the "stomach" muscles by challenging the abdominal group: the rectus abdominus muscles, or "abs" (two thin strips of muscle that extend from the breastbone to the pelvis), and the three layers of muscles that flank the abs. This might seem to be a simple order to fill, but it's not.
Sit Up and Avoid Pitfalls : Great care and excellent technique are required to strengthen the abdominal muscles with sit-ups. To be effective, sit-ups must pull the torso upward from a lying position toward the knees using only the abdominal group. Often, however, other, more powerful, muscles (those that flex the legs and hips) do much of the work. This is especially true with straight-leg sit-ups.
Bending the knees during sit-ups helps neutralize the action of the hip flexors and makes the abdominal muscles work more. Even so, the abdominal group tends to be involved only in the initial phase of the sit-up, after which the hip flexors take over. In addition, doing sit-ups rapidly and with momentum, knees bent or not, does not work the abdominal group very much. That's why raising slowly only part way works the abdominal muscles best.
Sit-ups also can be hazardous to your lower back, especially when using the straight-leg variety, which arches the back and may create overextension and strain. Twisting (right elbow to left knee and vice versa) at the top of the sit-up movement is not only useless, it places tremendous rotational stress on the lower back that can lead to injury. When doing sit-ups, never push through back pain. Stop immediately at even the slightest twinge in the lower back.
Absolutely Excellent Sit-Ups : TV is full of infomercials for abdominal exercise machines that promise to strengthen abs and trim the waist in no time. While some of these gadgets may help you use better form and get more out of your sit-ups, they won't perform miracles. In fact, you can achieve similar benefits on your own simply by knowing how to perform sit-ups properly:
Lie on your back on a padded surface, bending your knees to about 90° with your feet flat on the floor. Don't anchor your feet, because doing so will bring leg and hip flexor muscles into the action.
Choose the position of your hands and arms according to your abdominal strength. The closer your hands are to your head, the more difficult sit-ups become. As a beginner, rest your hands at your sides. When you get strong, you can cross your arms across your chest. Eventually, cross your arms behind your head with each hand on the opposite shoulder if you're able.
Don't, however, interlace your fingers behind your head. When you do, you tend to pull on your head, which can stress the neck and cause injury. Pulling on your head also makes the abdominal muscles work less.
Start each movement slowly, as if you are in slow motion.
Focus on using your abdominal muscles only. Close your eyes and visualize the abdominal muscles tensing and shortening like slow-moving cables through a pulley that draws your shoulders and head off the floor.
Exhale while the abdominal muscles contract and pull you upward. This will suck the muscles inward, ensuring involvement of the deeper muscles. Inhaling may cause your abdomen to protrude, leading to overarching and strain of the lower back.
Stop about halfway to the upright position about 6 to 12 inches off the floor and tense your abdominal muscles. Hold this position briefly, then lower slowly to the floor. As the abdominal muscles begin to tire, you may not be able to rise to midway, but go as high as you can.
Upon returning to the starting point, touch the floor lightly with your upper back and head, keeping the abdominal muscles tense, then begin the next movement.
If you find that sit-ups are too demanding, try doing only the curl-down phase. Assume a sitting position by pushing yourself upward with your arms. Slowly lower to the floor, keeping your abdominal muscles tensed. Return to the up position and repeat.
Don't overdo it. One set of 5 properly executed sit-ups or curl downs is enough at first. Add 1 sit-up each workout until you reach 15, then add more sets. When you can do three sets of 15, change hand positions to add resistance.
Sit-Ups: No Cure for Ab Flab : Strengthening the abs will not remove fat from the waistline. There is no such thing as spot reduction, because muscles do not fuel exercise by using the fat that surrounds them. Instead, during exercise the body tends to mobilize fat from storage depots throughout the body, so the fat used as fuel during sit-ups may come from the legs, back, face, or other areas. To remove body fat, you must burn calories, and lots of them. The abdominal muscle group is relatively small, and the number of calories expended during a bout of sit-ups is minimal. A brisk walk or jog will expend more calories than hundreds of sit-ups. Remember: This information is not intended as a substitute for medical treatment. Before starting an exercise program, consult a physician.