I've had people come to me with the complaint that they've done hundreds of crunches, and can not understand why their belly is not hard and flat like it should be.
This is because there are four different areas that you need to target in order to be sure of a flat stomach, and crunches only work on one. The very best exercises I have found to strengthen and tone your mid-section are the series of Pilates exercises that target your core.
Well-known for deep abdominal and back training work, Pilates is a total-body-workout that emphasizes natural spine curves. Consistent use of Pilates training increases significantly lean muscle tissue and bone density, while also doing more for your body than conventional weight training. It does not isolate muscle groups, but instead trains the whole body with each exercise while combining moves that target the arms and legs with the trunk and aligning your muscles with the natural spinal curves to make your daily movements more supportive of a healthy back. These exercises help to make your range of motion and flexibility better, easing stress and strain on your joints.
As you work on your core exercises, you will "feel the burn" in your abdomen, and your sides, and your lower abdomen. This, of course, means that you are using muscles you have not in a long time and you are training those muscles for strength. Of course, lean muscle mass will also burn off any excess fat on your body much more quickly.