The ends of most skeletal muscles are joined to bones by a tough, flexible connective tissue called tendon. This end of the muscle is called the origin. The other end, called the insertion, is attached to a bone that moves when the muscle contracts. When a person stands erect, many skeletal muscles contract to make the body rigid. The skeletal muscles also can make one part of the body move while another part stays stiff.
Skeletal muscles act both ways because they work in pairs.
One muscle of each pair is called the flexor. It bends a joint and brings a limb closer to the body. The other muscle, the extensor, does the opposite. For example, the biceps muscle in the front of the upper arm is a flexor. When this muscle contracts, the elbow bends and the forearm and hand move toward the shoulder.