454 



NA TURE 



[September 30, 1922 



physique is not confined to these war figures. Sir 

 George Newman, in his valuable and interesting Report 

 on Preventive Medicine, has directed attention to the 

 enormous amount of time which is annually lost through 

 sickness. The minimum average amounted to 

 14,295,724 weeks (or a period upwards of 270,000 years) 

 oi sickness per annum, and this figure did not include 

 absence from work due to maternity benefit, sanatorium 

 treal ment, or absence for less than four days per patient. 

 This is the evidence of the National Health Insurance. 

 The design of the organism which has to stand the 

 strain is not at fault. It is an organism which, in the 

 language of the engineer, is abundantly supplied with 

 factors of safety, and has an over-all high factor of 

 safety. The body is not designed merely to perform 

 the minimum amount of work or to stand the minimum 

 strain ; there is always a reserve. The perfect co- 

 ordination of the different parts of the organism is re- 

 quired, because the human being is capable of intense 

 muscular exertion for short periods. The intensity of 

 the work is, as a general rule, inversely proportional to 

 the length of time during which it must be carried out. 

 II. in the human organism, we were concerned merely 

 with the co-ordinated action of a series of effectors, 

 with the capacity of a certain group of muscles to 

 perform a given amount of work, the solution of the 

 problem would be relatively simple. But we are 

 dealing with a living organism, capable not only of 

 doing work, but of repairing the worn-out parts, as 

 and when required. Further, we are dealing with 

 an organism which varies not only in its capacity to 

 perform work, but in its " will to work." We are 

 dealing with a subtle organism which has a whole series 

 of protective mechanisms at its command, an organism 

 which can be fatigued and rendered useless, as a work- 

 ing unit, by an amount of work on a particular day 

 which on another day it can perform with the utmost 

 ease and without apparent fatigue. 



The efficiency of a man is not merely dependent on 

 the amount of work which can be performed by his 

 muscles ; the circulatory, respiratory, and nervous 

 systems are of equal importance, and all are intimately 

 related. In spite of the many and varied stresses and 

 strains to which the organism is subjected in the course 

 of life, as the result of the many factors of safety, unless 

 the overloading is excessive, too frequent or too long 

 continued, the organism, so long as it remains physio- 

 logical, is practically unaffected by ordinary hard 

 work. 



If we turn now to the consideration of the factors 

 which influence the efficiency, both in the mechanical 

 and the industrial sense, we find that the main con- 

 trolling factor is undoubtedly the condition known as 

 fatigue. Fatigue is a word just as frequently used as 

 efficiency, and yet it is almost impossible to give an 

 accurate definition of the term. Generally speaking, 

 it is to be regarded as the antithesis of efficiency. 



The study of the metabolism has given little or no clue 

 so far to the real nature of fatigue. Benedict and I 

 carried out a certain amount of experimental work on 

 this phase of the question. Our results show that the 

 subject may be on the very verge of absolute collapse, 

 and yet, so far as the metabolic determination goes, 

 there is no very marked evidence of diminished effi- 

 ciency in a mechanical sense. In an experiment with 



M. A. M., win), in the post-absorptive state, rode on a 

 bicycle ergometer for nearly four and a half hours until 

 on the verge of collapse, doing 208.000 kilogrammetres 

 of external work during the time, the metabolism was 

 determined six times during the riding period with the 

 following result : 



It will be noted, as might be expected, that there is 

 some slowing of the rate at which the work is done, but 

 the diminution in the net efficiency, in spite of the fact 

 that the subject admitted he was completely done at 

 the conclusion of the last determination, is not striking. 



Obviously, then, the capacity to carry on is limited 

 by the genesis of fatigue. But it is equally obvious in 

 practice that a man may be engaged in strenuous labour 

 for many hours without acute signs of impending 

 exhaustion. How is this condition attained ? There 

 are at least four factors which, to my mind, play pre- 

 dominant roles in the attainment of maximum efficiency, 

 namely, the rate of the performance of work, the 

 amount of rest offered or taken by the subject, the 

 rhythm with which the work is performed, and the 

 work habits developed by the worker. Although I 

 shall attempt to examine each of these factors separ- 

 ately, it is not to be inferred that they can really be 

 considered as independent phenomena. As a matter 

 of fact, they are all intimately related, and usually 

 merge into one another. 



Of these four factors probably most attention has 

 been devoted to the rate or speed at which work is 

 carried out. Benedict and I found, for example, 

 working with a carefully calibrated bicycle ergometer, 

 that there was a very close connexion between the 

 speed at which work was done and the mechanical 

 efficiency. There was a very definite falling off 

 with increased speed, as the following table shows. 

 Unfortunately it was impossible to get our subject 

 to pedal slower than 70 revolutions per minute. 



We found further that if the amount of effective 

 muscular work done was kept constant, that the 

 efficiency fell with an increase of speed. Thus with 

 effective work equivalent to 1-95 calories performed 



NO. 



2761, VOL. I 10] 



