Sit-ups are the basic exercise that is used to strengthen the abdominal muscles. They involve laying flat and raising one's torso from the ground. Compared to abdominal crunches, sit-ups exercise many other core muscles.
People who are interested in fitness are usually keen on developing their abdominal muscles. The crunch exercise isolates the main abdominal muscle, the rectus abdominus. Sit-ups exercise many other muscles, including the hip flexors, the iliopsoas, the tensor fasciae latae, the rectus femoris, the sartorius, and to a very minor degree, the obliques. Sit-ups must be performed regularly at high volumes to develop strong abdominal muscles.