It is in the joints of human body that movements of the bones take place. The movement itself is caused by the pull of sheets and cords of very tough tissue(组织) called muscle. Muscle tissue has the special ability to shorten itself so that the bone on which it pulls has to move. When muscle tissue shortens, it also bunches up (凸起). Muscle tissue covers the body in sheets, and bands that lie between the skin and the skeleton. The bones are the framework of the body, but the muscles fill out the body shape. Most muscles extend from one bone to another. When the muscle between the bones shortens, one bone has to move. The point where the muscle is fastened to the unmoving bone is called the origin of the muscle, whereas the point where the muscle is fastened to the bone that is to be moved is called the insertion. Sometimes the muscle is not attached directly to the bone but to a tough, non-stretchable cord, or tendon that is attached to the bone. Muscles do not push they can only pull. To bend the arm at the elbow the muscle at the front of the upper arm has to shorten and bunch up. To unbend the arm, other muscles in the back of the arm have to shorten. These two sets of muscles--the front and back--are said to act in opposition to each other. When one set is working, the other set is usually relaxed. But there are times when both of them work. Sometimes muscles are called upon to do more than simply pull in one direction. They may have to perform. a turning motion. To be able to do this, the muscle must be attached to the bone at an angle. By pulling, the muscle can cause the bone to pivot. According to the passage, movement occurs when a muscle______.