Levers

Connections between joints, called "synovial joints," are fulcrums, the bones they connect are levers, and the muscles attached to them apply force (or resistance). An example of a first-class lever is the joint between the skull and the atlas vertebrae of the spine: the spine is the fulcrum across which muscles lift the head. An example of a second-class lever is the Achilles tendon, pushing or pulling across the heel of the foot. A third-class lever might be the elbow joint: when lifting a book, the elbow joint is the fulcrum across which the biceps muscle performs the work.