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.