284 REPORTS ON THE STATE OF SCIENCE. — 1915. 



Section 1. — The Nature and Causes of Fatigue. 



If we define fatigue in general as a ' diminution of the capacity for 

 work which follows excess of work or lack of rest, and which is 

 recognised on the subjective side by a characteristic malaise,' we at 

 one and the same time put forward its most familiar symptom and its 

 main external cause. 



This ' subjective sensation of malaise ' is found, according to Dr. 

 McDougall's address to the British Association in 1908, as a local sen- 

 Bation of fatigue particularly in the muscles, as a general feeling of 

 tiredness or limpness, and also as the experience of sleepiness. But 

 such sensations are no more than symptoms of the diminution of work- 

 ing-capacity and not always even that. As Dr. Eivers has stated: 

 ' A distinction must be made between the sense of fatigue — the sensa- 

 tions which supervene during the performance of work, and the lowered 

 capacity for work executed. These conditions, which may be spoken 

 of as subjective and objective fatigue respectively, do not always run 

 parallel courses. In the performance of mental work especially, decided 

 sensations of fatigue may be experienced when the objective record 

 shows that increasing and not decreasing amounts of work are being 

 done; and there may be complete absence of any sensations of fatigue 

 when the objective record shows that the work is falling off in quantity, 

 or quality, or in both. ' This insistence on the distinction between sub- 

 jective and objective fatigue, however, does not imply that the one has 

 no influence on the other. As Weber points out (see Index of Docu- 

 ments D8), ' This psychically conditioned fatigue is by no means without 

 its influence on working capacity . . . and in the long run it can 

 undoubtedly cause an unfavourable general disposition which will ulti- 

 mately find a physical expression. ' 



If excess of work or lack of rest figures in our definition as the 

 antecedent to fatigue it must not make us overlook the physiological 

 modus operandi of fatigue as distinguished from those quite external 

 causes. During the last twenty years it has been found, in fact, that 

 muscular fatigue is caused by the accumulation of the poisonous pro- 

 ducts of activity and the exhaustion or diminished supply of the sub- 

 stances necessary for the continuance of activity, ' and,' wrote Pro- 

 fessor Lee in 1910, * there is every reason to believe that the main 

 principles of muscular fatigue are demonstrable in the other tissues and 

 organs of the body — that in them also fatigue is characterised, physi- 

 cally, by a diminution of working power and, chemically, by both the 

 destruction of energy-yielding substances and the appearance of toxic 

 metabolic products. ' But, as Dr. Rivers has pointed out, * however 

 satisfactory these (physiological) definitions may be ideally, their appli- 

 cation is wholly impracticable in the present state of our knowledge, 

 even in the present case of the fatigue of isolated muscle and still more 

 so in the case of general bodily fatigue or of mental fatigue. ' 



In studying fatigue from the economic standpoint it is of course 

 the objective diminution of working capacity and the external causes 

 found in industry that will be the prime consideration. Subjective 

 fatigue will be of importance just so far as it influences objective fatigue. 

 Physiological or ' internal ' phenomena will gain importance as inter- 



