Muscle Activation During An Abdominal Crunch

The abdominal crunch is a popular exercise, especially for those who are trying to develop "six pack" abs. Not surprisingly, researchers have found that crunches primarily target the rectus abdominis muscle, which is responsible for that six-pack appearance; crunches also activate several other muscles of the abdominal wall.

Rectus Abdominis

The rectus abdominis muscle runs vertically down the front of the abdomen. At the upper end it attaches to the sternum and the cartilages of ribs 5 through 7, and at the lower end to the pubis. It is divided horizontally by several tendinous intersections. When it contracts, as in a crunch, it flexes the spine, causing the muscle fibers between the tendinous intersections to bulge out and creating the appearance of a six pack in people with low body fat.

Upper and Lower Rectus Abdominis

Electromyography studies have shown that the rectus abdominis is the muscle most strongly activated in a crunch. Contrary to a popular belief, it appears that crunches target both the upper and lower fibers of the rectus abdominis. Although there are differences in the activity between the upper and lower fibers with different versions of crunches, they are small, and it does not appear possible to completely isolate one section or the other.

The oblique muscles

The abdominal obliques, which comprise the internal and external obliques, are active in rotating the trunk and in side bending. If they contract on both the right and left sides together, they also flex the spine. Electromyography studies have found that they are active in abdominal crunches, although not to the degree that the rectus abdominis is. Abdominal exercises which incorporate a twist would more strongly activate the obliques.

Other muscles

The fourth muscle of the abdominal wall, the transversus abdominis is also active in crunches. Additionally, there is a small amount of activation of the rectus femoris muscle, one of the hip flexors. However, the activity of the hip flexors is much smaller in crunches than in some other abdominal exercises such as situps, making crunches likely to be safer for those with low back pain.