MUSCULAR ACTIVITY 101 



cance as a phase of fatigue, contracture is important as proving 

 that the relaxation of muscle is not a mere passive falling back 

 from the contracted state, for if relaxation were just that the rate 

 of the falling back should be the same at one time as at another. 

 Since the relaxation rate varies, becoming slower at the beginning 

 of fatigue, there must be a definite relaxation process and we are 

 bound, in our analysis of the muscle machine to take the relaxa- 

 tion process into account as well as the contraction process itself. 



If the recurrent stimulation of the isolated muscle is continued 

 beyond the phase of contracture there soon develops the phase 

 which accords more fully with our ordinary conception of fatigue. 

 The muscle contracts more and more feebly until finally it refuses 

 to respond at all to stimulation. After this stage is reached a 

 rest of a few minutes often suffices to restore the muscle to a con- 

 dition in which it will show a considerable degree of activity, al- 

 though usually not so much as it exhibited when fresh. 



The Nature of Fatigue. We can understand why muscles be- 

 come fatigued if .we recall the fact that the muscle is a chemical 

 engine. The* energy for contraction is furnished by chemical 

 transformations within the muscle whereby substances containing 

 large amounts of energy enter combinations of less energy value, 

 and liberate the surplus energy for the use of the muscle. These 

 resulting compounds, of low energy value, are waste products. 

 Their relation to the muscle is that of the ashes to the furnace. 

 Unless they are gotten rid of they interfere with further activity. 

 This hampering of a chemical process by its own products is a well- 

 known principle of chemistry. To avoid the effect the products 

 should be removed as fast as they are formed. In the body the 

 agency for removing waste products from the muscles is the blood- 

 stream flowing through them. Under ordinary circumstances 

 this is sufficient, so that muscular fatigue is not commonly felt. 

 In isolated muscles, however, there is no stream of blood. The 

 only way in which waste products can be gotten rid of is by the 

 slow process of dialysis from the fibers into the surrounding lymph. 

 Fatigue comes on rather quickly, therefore, in isolated muscles 

 that are stimulated repeatedly. 



The reader should be cautioned at this point against attempting 

 to apply the description of fatigue just given to all his own expe- 

 riences of weariness. The fatigue of isolated muscles is muscular 



