Sit-Ups: This might be the most controversial exercise on the list. Sit-ups are the Holy Grail of abdominal strengthening exercises. Fitness tests include this exercise. Many people start their morning with 100 sit ups. Gym equipment and other machines focused on abs sell like crazy (just watch some late night infomercials).
I tell people all the time that we need to work on their core and abdominal strength, so why is this an exercise I hate? Basically, I like to do more stabilization exercises in the neutral position rather than doing a lot of flexion like occurs with sit ups. Most people spend the majority of the day slumped in bad posture working at a computer leading to a flexed spine. Rather than reinforce that position with a sit-up, I prefer to teach people to get into a neutral spine and engage the abdominals there with exercises like pelvic tilts or planks.
Try these exercises instead:
Lie on back with knees bent and feet flat on floor. Tighten abdominals and tilt pelvis backward (back flat against floor).
Perform 3 sets of 10 reps.
Begin in a push-up position supporting yourself with elbows rather than hands. Tighten abdominals making sure there is no arch in the back or mid-section.
Hold 30 seconds, repeat 3 times.