join muscles to bone. They grow into the periosteum, which is the outer covering of the bone. When the muscle , the tendon pulls on the bone it is attached to and moves the bone. Muscles have an origin and an insertion. The is where the tendon of a muscle is anchored to a fixed point on the skeleton. The is where the tendon is attached to the bone which moves. For example, the bicep's insertion is in the radius.

When the contracts, the arm is flexed at the elbow because the radius bone is pulled closer to the shoulder. Voluntary muscles work in pairs called antagonistic systems. The antagonistic muscle to the biceps is the triceps. When the bicep contracts, the triceps and vice versa.

Did you know?
The bicep muscles are usually associated with the arms. However, any muscle that has two points of origin is called a bicep. There are biceps in the arms, but also in the thigh. Just as biceps have two points of origin, triceps have three points of origin.