87. Rest for the Muscles. Rest is necessary for the tissues, that they may repair the losses sustained by work; that is, a period of rest must alternate with a period of activity. Even the heart, beating ceaselessly, has its periods of absolute rest to alternate with those of work. A steam-engine is always slowly, but surely, losing its fitness for work. At last it stops from the need of repair. Unlike the engine, the body is constantly renewing itself and undergoing continual repair. Were it not for this power to repair and renew its various tissues, the body would soon be worn out.
This repair is really a renovation of the structure. Rest and work are relative terms, directly opposed to each other. Work quickens the pulse and the respiration, while rest slows both. During sleep the voluntary muscles are relaxed, and those of organic life work with less energy. The pulse and the respiration are less frequent, and the temperature lower than when awake. Hence sleep, “tired Nature’s sweet restorer,” may be regarded as a complete rest.
The periods of rest should vary with the kind of exercise. Thus exercise which produces breathlessness requires frequent but short rests. The trained runner, finding his respiration embarrassed, stops a moment to regain his breath. Exercises of endurance cause fatigue less quickly than those of speed, but require longer rest. Thus a man not used to long distances may walk a number of hours without stopping, but while fatigue is slow to result, it is also slow to disappear. Hence a lengthy period of rest is necessary before he is able to renew his journey.
88. Amount of Physical Exercise Required. The amount of physical exercise that can be safely performed by each person, is a most important and practical question. No rule can be laid down, for what one person bears well, may prove very injurious to another. To a certain extent, each must be guided by his own judgment. If, after taking exercise, we feel fatigued and irritable, are subject to headache and sleeplessness, or find it difficult to apply the mind to its work, it is plain that we have been taxing our strength unduly, and the warnings should be heeded.
Age is an important factor in the problem, as a young man may do with ease and safety, what might be injurious to an older person. In youth, when the body is making its most active development, the judicious use of games, sports, and gymnastics is most beneficial. In advanced life, both the power and the inclination for exercise fail, but even then effort should be made to take a certain reasonable amount of exercise.
Abundant evidence shows that physical development is most active from thirteen to seventeen years of age; this manifests itself clearly by increase in weight. Hence this period of life is of great consequence. If at this age a boy or girl is subjected to undue physical strain, the development may suffer, the growth be retarded, and the foundation laid for future ill health.
Fig. 43.—Student exercising in the School Gymnasium on the Rowing Machine. (From a photograph.)
The proper amount of exercise must vary greatly with circumstances. It may be laid down as a fairly safe rule, that a person of average height and weight, engaged in study or in any indoor or sedentary occupation, should take an amount of exercise equivalent to walking five or six miles a day. Growing children, as a rule, take more exercise than this, while most men working indoors take far less, and many women take less exercise than men. Exercise may be varied in many ways, the more the better; but for the most part it should always be taken in the open air.