168 GROWTH 



close analysis reveals that physiological activity destroys proto- 

 plasm. The fatigue of an athletic contest, or of a day's labor in 

 the shop or the fields, is for the moment destructive of growth. 

 Often more tissue is destroyed than is rebuilt after the activity 

 occurs. The question then, may well be raised, what is the true 

 relation of activity to growth. 



We have already expressed the law that physiological activity 

 of living protoplasm favors its further growth and develop- 

 ment. The very act of protoplasm necessary to liberate energy 

 stimulates the complex processes of metabolism which later pro- 

 duce further growth of the tissue reacting. This is the chief un- 

 derlying principle of physical training. But like all such so- 

 called laws of biology the law holds true only within a limited 

 range of activity. That range cannot be expressed in general 

 terms but must be determined both as to intensity and as to 

 duration of activity in adaptation to the physical capacity of 

 each individual. 



Reactions to short and intensive physical effort are coexten- 

 sive with the reactions of the different physiological systems of 

 the body. The respiratory, circulatory, cutaneous, nervous, mus- 

 cular, alimentary, and endocrine systems all take part. Each of 

 these responds by more intensive catabolic changes and aug- 

 mented energy output. In the after-period the "compensatory 

 reactions," or augmented activities, continue for a time and 

 metabolism is correspondingly favored. Not only will there be 

 full repair but growth will be actually accelerated. This is the 

 true stimulating effect of exercise. The beneficial effects extend 

 throughout the body. The muscles are stronger and larger, the 

 nerve reactions are more vigorous, secretions are better, the cir- 

 culation is more tonic and the heart stronger, nutrition is ac- 

 celerated and growth is stimulated. These responses are more 

 delicately balanced in the young. It must be the keen insight of 

 the trained arbiter of our athletic activities that determines the 

 intensity, the duration, the repetitions, and the correlations with 

 age with the degree of strength and development already es- 



