ABSTRACT
There are well over 500 muscles in the body constituting close to 50% of the weight and 50% of the metabolic activity in the body. They are found as three different types: skeletal muscles attached to the bones, cardiac muscle found in the heart, and smooth muscle found in the internal organs and the walls of the blood vessels. Only skeletal muscles will be discussed in detail here because of their relevance to motion. Skeletal muscles are attached to the bones on either side of a joint (see Figure 3.1) by the tendons discussed previously and have the property of actively contracting and shortening, and in doing so, moving the bones. However, because muscle is soft tissue, the reverse action of active lengthening is not possible and a second set of muscles is required to bring the limb back to its original position. Thus, one or several muscles, termed agonists or prime movers, act as the primary activators of motion. An opposing set of muscles (typically on the opposite side of the joint) are termed antagonists, counteract the agonists and oppose the motion. Typically, one set of muscles is active, while the opposite set is relaxed. For example, during elbow Ÿexion, the biceps or brachioradialis is the agonist (and also a Ÿexor) while the triceps is the antagonist and also an extensor. However, during elbow extension, the triceps becomes the agonist (but is still an extensor), while the biceps becomes the antagonist. The muscle attachments to bones are also given speci£c names. The origin is the most proximal attachment, or the one nearest the trunk, while the insertion is the most distal or distant attachment from the trunk.
