Muscular System
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Muscle Action
9.4 Muscle fibres respond to nerve impulses by contracting. When enough nerve impulses are received, the entire muscle contracts and pulls any attached tissue along with it, resulting in movement of that tissue. Skeletal muscles often work in pairs to bend and straighten joints. The biceps and triceps muscles move the arms at the elbow. Contracting the biceps muscle pulls upward on one of the bones of the lower arm, causing the arm to bend. When the biceps relaxes and the triceps contracts, the arms is pulled in the opposite direction. Even the simplest body movements involve the coordination of many muscles. For example, it takes 13 muscles to smile and 34 muscles to frown.
Common Muscle Problems
9.5 Considering that muscles are used every time you move, it is not surprising that muscle injuries are common. Although regular exercise preceded by a proper warm-up reduces the risk of muscle injury, it can still occur – especially when a person ignores his or her limitations. Charley horses and pulled hamstrings are common jogging-related injuries. A charley horse is a painful contusion, or bruising, of the muscle and is frequently accompanied by a tear in muscle fibres. A pulled hamstring is an overextension or strain of one of the muscles in the back or the thigh. Inflammation of the flexor muscles of the front portion of the lower leg causes another common condition, known as shin splints. A muscular cramp is a prolonged and painful involuntary muscular contraction. A spasm is a brief, involuntary contraction. The exact cause of muscle cramps and spasms is unknown. Experts suggest that they may result from several causes: insufficient oxygen, calcium, or magnesium; fatigue; or various drugs. Tendinitis is the inflammation of the connective tissue surrounding a tendon.
Why does Muscular Fatigue develop?
9.6 The causes of muscular fatigue are not well understood. One explanation is that when muscles contract they produce lactic acid. Lactic acid creates an environment in which muscles are not able to respond to stimulation. However, new research indicates that there are other contributing factors, such as depletion of stored energy and muscle glycogen, inadequate blood flow to the muscles, and low oxygen levels. Well-conditioned athletes experience less muscle fatigue. It is known that they accumulate less lactic acid in their muscles and that conditioning actually increases the capillary density of the skeletal muscles, increasing blood flow.
