lever /lē″vər, lev″ər/ [L, levare, to lift up] , any one of the numerous bones and associated joints of the body that act as a simple machine so that force applied to one end of the bone tends to rotate the bone in the direction opposite from that of the applied force. The muscles of the body produce the forces that move the levers. The basic components of a lever are the fulcrum, the force arm, and the weight arm. A first-class lever, such as the joint between the base of the skull and the first cervical vertebra, has a fulcrum between the weight and the applied force. The body contains few second-class levers, which have the weight between the fulcrum and the force. A third-class lever, such as the forearm and elbow, has the force between the fulcrum and the weight. The body uses its third-class levers for speed and its first-class levers for either force or speed, depending on the force applied to the weight arm.
