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