254 THE PHYSIOLOGY OF MUSCULAR WORK 



ducts such as lactic acid may be formed and may injuriously affect 

 the activity of the muscle fibres or their nerve endings. It may 

 be, as Hough ( 51 ) and Hill ( 52 ) have suggested, that the muscular 

 soreness experienced during work and for some hours afterwards 

 is due to the waste products of activity, such as lactic acid. 



There is another kind of muscular soreness which is not ex- 

 perienced at all during the time of exercise, but is felt the next 

 day as an uncomfortable or even painful stiffness which gradually 

 disappears during movement, but may be noticeable for two or 

 three days. This form of soreness Hough thinks may be due to 

 ruptures within the muscle fibres. It is difficult to obtain evidence 

 upon this point, and it would appear more reasonable to attribute 

 it to similar causes to those given for the first kind of muscular 

 soreness. 



The onset of fatigue, both local and general, can be delayed 

 or largely prevented by progressive training. This may be due 

 to the fact that the body by practice performs work more econo- 

 mically and adjusts more readily the circulation of the blood to 

 meet the general and local needs of nutrition. It may be that 

 the body acquires an immunity to its waste products or increases 

 its capacity for oxidising or otherwise rendering them inert. The 

 " receptive substance " between the nerve and the muscle may 

 be an important element in muscular fatigue. 



Weichardt ( 53 ) maintains that there is a toxin which causes 

 fatigue, that this toxic substance is not lactic acid, and that the 

 living body produces an antagonistic substance, an antitoxin. 

 The injection of blood, serum, or other fluids containing the toxin 

 is said to produce in an animal all the signs of fatigue, unless it 

 has been previously protected by a dose of the antitoxin. These 

 observations need confirmation, but in support of the view other 

 arguments have been advanced. The toxicity of the urine is 

 increased by muscular exercise and the sweat secreted during 

 hard work is toxic, whereas that poured out by a man at rest but 

 exposed to a heated and moist atmosphere is free from harmful 

 substances. It is well known that an untrained man experiences 

 the day after hard and prolonged exercise a general feeling of 

 lassitude and disorder ; this has been attributed to an auto-intoxi- 

 cation or poisoning by the products of the unusual muscular activity. 



Ryffel has found that even after moderate exercise lactic acid 

 can be detected in the urine and sweat, but further observations 



