There are three hamstring muscles, located at the back of the thigh. They flex the leg on the thigh, such as when one kneels. They also extend the hip whenever one, for example, sits in a chair. Hamstring injuries are probably the most common muscle injury among runners. Maintaining flexibility and strengthening the muscle helps to prevent injury. Hamstring muscles are also prone to reinjury.

The platysma muscle in the side of the neck is probably the most variable. It can cover the whole region in some people while in others it is straplike or in a few situations it is missing completely.