Your core muscles include those in your lower back, hips, pelvis and abdomen. The purpose of situp exercises is to strengthen these muscles and help them work effectively in tandem with one another. Stronger core muscles are supposed to help alleviate or even avoid back pain. They should also decrease muscle spasms, protecting your back against injury. Situps are not an effective way to achieve these goals.
Situps are effective at one thing: strengthening your core muscles. Stronger core muscles help stabilize your body as you move through the world. This means you'll be less likely to trip, which can help protect against injuries and falls when you play sports. Stronger core muscles also mean that you can maintain better posture because it won't strain your back to stand and sit up straight.
Even though they strengthen, situps don't tone your core. So if you have excess fat around your abdomen, situps won't do anything to get rid of it. For that, you'll need to add cardiovascular exercise and eat fewer calories. Situps can also put too much of your body weight onto your fragile spinal discs. According to biomechanics professor Stuart McGill of the University of Waterloo, this can cause disc herniation, in which a disc presses on a nerve root and actually causes pain instead of reducing it.
Sometimes situps can be outright dangerous. They should not harm the fetus during the first trimester, or 13 weeks, of pregnancy. But after that, avoid lying on your back at all if you are pregnant, as it can lower your blood pressure and prevent the needed blood flow to the fetus. Abstain from situps if you have any bone conditions like osteoporosis, which makes the bones brittle and susceptible to injury. You will be risking compression fractures if you attempt situps.
The University of Rochester Medical Center says you can reduce the risk of injury by keeping your feet, hips and knees aligned. Keep your knees bent and feet flat on the ground but not locked down by a heavy piece of furniture or bar. Remember to rely on your abs to do the work instead of putting stress on your neck or shoulders. Situps are also safer once you've already done some strengthening as well. Finally, breathe deeply, and never push to the point of pain.