Here's a past MD Fitness client performing ab crunches with a sledge hammer. You can specify the part of a muscle an exercise targets by the direction of the movement. For example, regular crunches range of motion is confined to the top region of the body so only the upper part of the abdominals are activated. Reverse crunches target the lower abdominals because it's range of motion is confined to the lowest parts of the body. However, this exercises's range of motion targets upper, lower and the obliques (waist).