VOLUNTARY MUSCLE. 33 



FATIGUE. M^ 



It is well known that excessive WEBSRl SBSBB 

 use of a muscle reduces its efficiency. j^SSSSl rW^JrltinB 

 In fact all phases of contraction are 

 affected. A careful study of the sub- 

 ject shows the following changes. 

 Repeated contraction at first causes an 

 increase in response, called the "stair- 

 case" or "treppe" effect. A maximum 

 is soon reached and persists for a time. 

 Then the contractions gradually de- 

 crease in magnitude until a point is 

 reached where the muscle fails to re- 

 spond (Fig. 9). In the latter stage 



of fatigue the muscle may fail to relax nnMiiiiHiiiMiiiifc jiii 

 entirely between twitches so that it ^^^K'r/iIWi !K**iBim 

 persists in a state of "contracture." 



It has been shown that certain 

 substances are produced during 

 muscular activity such as carbon- 

 dioxide, mono-potassium phosphate 

 and para-lactic acid. If these sub- 

 stances are injected into a fresh 

 muscle the first effect is to augment 

 the contractile response. Injection 

 of larger amounts reverses the effect 

 so that the response is reduced. In 



other words it is possible to induce in MD i|BIIDS*SW^ 

 a fresh muscle the various stages of HHf !HlSBiiJHsl 

 excitability and fatigue by means of 

 the waste substances produced in BBl IIISSIIB^^SB 

 muscle, the effect depending upon HiiilliUll 

 the quantity of these substances. 

 These observations lead one to con- 

 clude that the accumulation of waste IkVXV 



^k'vNt^H^SiKl ^ u 



substances in a muscle is a large BBHfci T' iBSit Wl 





factor in producing these conditions. 

 This is further substantiated by the 

 fact that increasing the circulation to 



1 1 



