CHAPTER VI. 
THE INFLUENCE OF MUSCULAR EXERTION UPON 
METABOLISM. 
Ir is a matter of common experience that muscular exertion 
results in a very marked increase in the vital activities of the body. 
The rate of circulation and respiration is greatly quickened and the 
increased metabolism in the organism is shown by the loss of weight 
and by the increased demand for food to make good the destruction 
of tissue. Indeed, no other factor even approaches muscular exer- 
tion in the extent to which it increases the metabolic activities of 
the body. 
We have now to. consider in some detail the nature of muscular 
exertion and the precise character of its effects upon metabolism. 
§ 1. General Features of Muscular Activity. 
Muscular Contraction. 
The work of the muscles is accomplished by contracting, and a 
brief consideration of some of the more prominent general features 
of muscular contraction will conduce to an intelligent study of the 
main subject of the chapter. It will be possible here to consider 
this phase of the subject only in its most general outline, and the 
reader is referred to works on physiology for details. 
When a suitable stimulus, which in the living animal is usually 
a nerve stimulus, is applied to a.muscle it contracts; that is, it 
tends to grow shorter and thicker. This change is brought about 
by a shortening and thickening of the individual fibers of which 
the muscle is built up. A single stimulus, such, for example, 
as that caused by the making or breaking of an electric circuit, 
gives rise to what is known as a simple muscular contraction. If 
such a stimulus is repeated with sufficient frequency it produces a 
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