Good muscle tone improves coordination. When muscles are slightly contracted, they can react more rapidly if and when greater exertion is necessary. Muscles with poor tone are usually soft and flabby, but exercise will improve muscle tone.

There are two general types of exercise: isotonic and isometric. In isotonic exercise, muscles contract and bring about movement. Jogging, swimming, and weight lifting are examples. Isotonic exercise improves muscle tone, muscle strength, and, if done repetitively against great resistance (as in weight lifting), muscle size. This type of exercise also improves cardiovascular and respiratory efficiency, because movement exerts demands on the heart and respiratory muscles. If done for 30 minutes or longer, such exercise may be called aerobic, because it strengthens the heart and respiratory muscles as well as the muscles attached to the skeleton.

Isotonic contractions are of two kinds, concentric or eccentric. A concentric contraction is the shortening of a muscle as it exerts force. An eccentric contraction is the lengthening of a muscle as it still exerts force. Imagine lifting a book straight up (or try it); the triceps brachii contracts and shortens to straighten the elbow and raise the book, a concentric contraction. Now imagine slowly lowering the book. The triceps brachii is still contracting even as it is lengthening, exerting force to oppose gravity (which would make the book drop quickly). This is an eccentric contraction.

Isometric exercise involves contraction without movement. If you put your palms together and push one hand against the other, you can feel your arm muscles contracting. If both hands push equally, there will be no movement; this is isometric contraction. Such exercises will increase muscle tone and muscle strength but are not considered aerobic. When the body is moving, the brain receives sensory information about this movement from the joints involved, and responds with reflexes that increase heart rate and respiration. Without movement, the brain does not get this sensory information, and heart rate and breathing do not increase nearly as much as they would during an equally strenuous isotonic exercise.

Many of our actions involve both isotonic and isometric contractions. Pulling open a door requires isotonic contractions of arm muscles, but if the door is then held open for someone else, those contractions become isometric. Picking up a pencil is isotonic; holding it in your hand is isometric. Walking uphill involves concentric isotonic contractions, and may be quite strenuous. Walking downhill seems easier, but is no less complex. The eccentric isotonic contractions involved make each step a precisely aimed and controlled fall against gravity. Without such control (which we do not have to think about) a downhill walk would quickly become a roll. These various kinds of contractions are needed for even the simplest activities.

Anabolic steroids are synthetic drugs very similar in structure and action to the male hormone testosterone. Normal secretion of testosterone, beginning in males at puberty, increases muscle size and is the reason men usually have larger muscles than do women.

Some athletes, both male and female, both amateur and professional, take anabolic steroids to build muscle mass and to increase muscle strength. There is no doubt that the use of anabolic steroids will increase muscle size, but there are hazards, some of them very serious. Side effects of such self-medication include liver damage, kidney damage, disruption of reproductive cycles, and mental changes such as irritability and aggressiveness.

Female athletes may develop increased growth of facial and body hair and may become sterile as a result of the effects of a male hormone on their own hormonal cycles.

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