The skeleton is a complex set of levers that can be pulled in many different directions by contracting or relaxing skeletal muscles. Most muscles extend from one bone to another and cross at least one joint. One end of a skeletal muscle, the origin, attaches to a bone that remains relatively stationary when the muscle contracts. The other end of the muscle, the insertion, attaches to another bone that will undergo the greatest movement when the muscle contracts. When a muscle contracts, its insertion is pulled toward its origin. The origin is generally closer to the midline of the body and the insertion is farther away.