4. The amount of exercise must be regulated by individual fitness. A chain is no stronger than its weakest link. The muscles may be stronger than the heart or lungs, and the latter may be fatally injured by an amount of exercise which the muscles can well bear. Hence the importance of ascertaining the condition of the vital organs before entering on a course of training.

Another important bearing of this rule is in relation to the exercise of growing boys and girls. When we remember that a boy at school will sometimes grow six to eight inches in a year, it is evident that much energy is being expended in this direction, and that excessive gymnastic exercise can only do harm. Between the ages of fifteen and seventeen there is usually the greatest amount of physical development, and if there is great muscular strain at this period, growth is interfered with, and the power of resistance to disease may be seriously lowered.

5. Every part of the body ought to be exercised. This is done spontaneously by the infant. Every muscle of his body acts in sheer delight. The evils of exercise confined to particular groups of muscles have been already described. Lawn tennis is very valuable as affording exercise for both limb and trunk muscles.

6. Exercise should not be taken immediately after meals, as thus digestion is interfered with.

7. Exercise should be taken, as far as possible, in the open air. A small amount of exercise out of doors is much more invigorating than a large amount indoors.

The Forms of Exercise taken may be divided into recreative and educational, though both of course may be recreative under many circumstances.

The primarily recreative exercises, such as rowing, cricket, football, tennis, hockey, will, it may be hoped, be never replaced by educational gymnastics, though the latter possess a high value. The recreative influence as well as the influence on the power of self-control of such games as cricket and football render them of national importance.

Educational gymnastics can be applied to exercise the muscles of any part of the body, and can be exactly graduated to individual requirements. Singing, speaking, and reading aloud, are forms of muscular exercise very much neglected, and they are particularly important, as the lungs and voice are by these means greatly strengthened, and rendered much less liable to the inroads of disease.

Professor Haughton has shown that the work done by a man walking on a level surface at the rate of three miles an hour is equivalent to raising his own weight, plus the weight he carries through 1 ∕ 20 of the distance walked.