Sit-Ups vs. Crunches Which Is Better?

Traditional sit-ups emphasize sitting up rather than merely pulling your sternum down to meet your pelvis. The psoas muscles run from the lower back to the front of the thighs.  This muscle action is to pull the thighs closer to the torso.  This action is the major component of sitting up. Because
of this, when you are doing sit ups your psoas muscles are the primary muscles being engaged and not your abdominal muscles. So sit-ups are a pretty useless exercise if you want to get your abs in shape. Instead, use crunches, because they directly work your abs.

How To Do Crunches The Right Way

Lie flat on your back with your knees bent and your feet together flat on the floor and about 10-15 inches from your buttocks.  Your hands should either be crossed on your chest, by your side, or cupped behind your ears. Without moving your lower body, curl your upper torso up and in toward your knees, until your shoulder blades are as high off the ground as you can get them. Only your shoulder blades should lift--not your back.  As you come to the highest point tighten and flex your abdominals for a brief second. Slowly lower yourself back to the starting position. Repeat for reps.

Note: Remember crunches only strengthen and tone the abdominal muscles;  you need a lowered calorie intake to burn off the fat, so the abs then show through clearly.