Sit ups strengthen your belly muscles, provided that you know how to do them. Lie on your back with your knees bent with your soles on the floor. Place your hands on your chest and slowly raise your head off the ground. Then, slowly raise your shoulders off the ground to a height of about ten inches and then lower your shoulders. Repeat ten times. Rest a minute and then do two more sets of ten.
When this exercise feels easy, you can take a light weight and wrap it in a towel and hold it in your hands behind your neck or on your chest. As you become stronger, increase the weight. Keep your knees bent to so you will use your belly muscles instead of the your iliopsoas muscles. Using the iliopsoas muscles can increase the curve in your lower back to cause back pain. Sitting up all the way up also uses your iliopsoas muscle instead of your belly muscles.