On the sit-ups, work all areas of the abdomen (lower, middle, and upper), but don’t go overboard. It is important to strengthen the abdominals because they help prevent lower-back problems and support the diaphragm.
However, overdevelopment of your abdominal region may restrict your ability to breathe during intense exercise. An effective breath comes from expanding your abdomen, not from raising your shoulders. If your abdominals are overdeveloped, you cannot get a relaxed, deep breath. My rule of thumb is no more than five minutes of moderate abdominal work twice a week.