The crunch exercise primarily works the rectus abdominis, transversus abdominis and oblique muscles. The rectus abdominis muscles form the six-pack and are located perpendicularly along the front of the stomach. These muscles are part of the core, which supports bending and standing up.
The crunch is an anaerobic, strength-training exercise that does not contribute to weight loss; however, it strengthens the core and leads to improved balance and posture. Crunches are most effective when performed slowly. The oblique muscles run perpendicularly along the sides of the torso. They support bending and twisting, and they help the digestive process by compressing the abdominal contents.