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.

[Illustration:  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.

89.  Time for Exercise.  It is not prudent to do hard work or take severe exercise, just before or just after a full meal.  The best time is one or two hours after a meal.  Vigorous exercise while the stomach is busily digesting food, may prove injurious, and is apt to result sooner or later in dyspepsia.  On the other hand, severe exercise should not be taken on an empty stomach.  Those who do much work or study before breakfast, should first take a light lunch, just enough to prevent any faint feeling.  With this precaution, there is no better time for moderate exercise than the early morning.

In the case of children, physical exercises should not be undertaken when they are overtired or hungry.  Neither is it judicious for adults to take vigorous exercise in the evening, after a long and arduous day’s work.

90.  Walking, Running, and Jumping.  Walking is generally regarded as the simplest and most convenient mode of taking exercise.  Man is essentially a walking animal.  When taken with a special object in view, it is the best and most pleasant of all physical activities.  It is suited for individuals of all ages and occupations, and for residents of every climate.  The child, the athlete, and the aged are all able to indulge in this simple and effective means of keeping the body in health.

In walking, the muscles of the entire body are brought into action, and the movements of breathing and the circulation of the blood are increased.  The body should be erect, the chest thrown out, the head and shoulders held back, and the stride long and elastic.  It is an excellent custom to add to the usefulness of this fine exercise, by deep, voluntary inhalations of pure air.