Structure of a muscle

A single muscle is fat in the middle and tapers towards the ends. The middle part, which gets fatter when the muscle contracts, is called the belly of the muscle. If you contract your biceps muscle in your upper you may feel it getting fatter in the middle. You may also notice that the biceps is attached at its top end to bones in your shoulder while at the bottom it is attached to bones in your lower arm.

Notice that the bones at only one end move when you contract the biceps. This end of the muscle is called the insertion. The other end of the muscle, the origin, is attached to the bone that moves the least (see diagram 35).