Situps are a classic calisthenic exercise that strengthens, firms and tightens stomach muscles. While situps are effective for toning the internal core muscles, they're not a high calorie-burning exercise, and they do not have the ability to "spot reduce" stomach fat. If you're looking to lose belly weight in addition to tightening up your core and abdominal muscles, more vigorous aerobic work is necessary.
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According to CNN.com, situps are worthwhile because they work several of the body's 29 core muscles as opposed to simply isolating the upper and lower abdominals. Situps force the midsection to lift the upper body and require constant contraction in the activated muscles for controlled lowering as well.
One exercise that's similar to the situp is the crunch. If you're not strong enough to do slow, controlled situps without straining your neck or jerking the movements, a crunch is a viable alternative. Instead of sitting up all the way, simply contract your abs and raise your torso several inches off the ground while keeping your eyes on the ceiling, your chin off your chest and your fingertips behind your ears. Standing movements, such as upright pike crunches and knee crunches, also help flatten the belly but don't require you to recline and offer less chance of back strain.
You'll need to do enough cardio to put your body into a caloric deficit if you want to lose weight. Situps don't burn as many calories as aerobic exercises, which raise the heart rate and keep it elevated for the duration of the activity. While Harvard Health reports that 60 minutes of jogging burns close to 596 calories for a 155 pound person, HealthStatus.com reports that an hour of situps will burn only about 325 calories. Situps will tone your stomach muscles no matter how much you weigh, but those tightened muscles won't be visible if you have belly fat that you need to lose first.
There's no reason to choose between aerobics and situps. Since both offer such significant benefits, it's best to combine them, as the American Council on Exercise recommends, and add flexibility as the third element of a complete routine. For the best tummy-flattening results, do other core exercises as well as situps. Consider the plank, crunch variations, the quadruped and activities with an aerobic element, such as mountain climbers and plank jacks. Finally, speak with your doctor while establishing a workout routine to get personalized, professional advice that's appropriate to your physical condition and fitness level.