determined mainly by the kind of joint it is associated with and the way the muscle is attached to the joint. At one end of some muscles are long white tendons that attach the muscles to bone. The point of fixed attachment of a muscle to bone is called the origin. The more flexible attachments, especially attachments to a movable bone, are termed insertions. Muscles seldom act alone; they usually work in groups held together by sheets of a white fibrous tissue called fascia. Muscles make up about one-half of the total body weight. Their main functions are threefold:  Providing movement—including internal functions such as peristalsis (rhythmic waves of muscular contraction within the intestines).  Maintaining body posture—through muscle tone, as in the muscles of the head, neck and shoulders, which keep the head up.  Providing heat—through chemical changes that take place during muscle activity, such as exercise that warms the body. In addition, muscles are involved in such essential bodily functions as respiration, blood circulation, digestion, and other functions such as speaking and seeing. MUSCLE CONTRACTION Muscle tissue has a highly developed ability to contract. Contractibility enables a muscle to become shorter or thicker, and this ability, along with interaction with other muscles, produces movement of internal and external body parts. Muscle contraction in a tissue or organ produces motion and provides power and speed for body activity. A contracting muscle is referred to as a prime mover. A muscle that is relaxing while a prime mover is contracting is called the antagonist. STIMULUS FOR CONTRACTION All muscles respond to stimulus. This property is called excitability or irritability. The mechanical muscular action of shortening or thickening (also called contraction) is activated by a stimulus sent through a motor nerve. All muscles are linked to nerve fibers that carry messages from the central nervous system. CONTRACTION AND RECOVERY The chemical action of muscle fibers consists of two stages, contraction and recovery. In the contraction stage, two protein substances (actin and myosin) react to provide energy through the breakdown of glycogen into lactic acid. In the recovery stage, oxygen reacts with lactic acid to release carbon dioxide and water. MUSCLE FATIGUE When a muscle contracts, it produces chemical waste products (carbon dioxide, lactic acid, and acid phosphate) which make the muscle more irritable. If contraction is continued, the muscle will cramp and refuse to move. This condition is known as fatigue. If it is carried too far, the muscle cells will not recover and permanent damage will result. Muscles, therefore, need rest to allow the blood to carry away the waste materials and bring in fresh glucose, oxygen, and protein to restore the muscle protoplasm and the energy that was used. TONICITY Tonicity, or muscular tone, is a continual state of partial contraction that gives muscles a certain firmness. Isometric muscle contraction occurs when the muscle is stimulated and shortens, but no movement occurs, as when a person tenses his or her muscles against an immovable object. Isotonic muscle contraction occurs when the muscle is stimulated. The muscle shortens and movement occurs. An example would be lifting an object. EXTENSIBILITY AND ELASTICITY Muscles are also capable of stretching when force is applied (extensibility) and regaining their original form when that force is removed (elasticity). MAINTENANCE OF MUSCLE TISSUE During exercise, massage, or ordinary activities, the blood supply of muscles is increased. This additional blood brings in fresh nutritional material, carries away waste products more rapidly, and enables the muscles to build up and restore their efficiency and tone. The importance of exercise for normal muscle activity is clear, but excessive muscle strain is damaging. For example, if a gasoline motor stands 1-18