According to the University of the Western Cape, skeletal muscles work in pairs so that they can alternately pull on a bone to achieve movement. In other words, when a human wants to bend his elbow, he must relax his triceps muscle, while contracting his biceps. Such muscle groups are called antagonistic muscles, because they pull in opposite directions.
When a muscle is stimulated to move part of the body’s skeleton, it contracts, pulling the bone along with it. The muscle’s antagonist must simultaneously relax and elongate so that its partner can contract. As the University of the Western Cape explains, muscles do not impart any force when they relax and elongate, so the muscle that is doing the pulling causes the movement. When a muscle contracts, it become shorter and thicker. This can be seen when people flex their muscles.
These antagonistic muscle groups use the skeleton as a lever. Levers are classified by the different locations of the load, applied effort and fulcrum. The body features all three types of lever. According to the University of the Western Cape, the muscles that move the head forward and back are an example of a first class lever. Second class levers, in which the load lies between the load and the effort, are found in the feet. Third class levers, which apply effort between the load and the fulcrum, are found in the arms and legs.