254 CARNEGIE INSTITUTION OF WASHINGTON. 



mechanical efficiency of the human body as a machine. The subjects were 

 all young men in perfect health, four of whom were unaccustomed to bicycle 

 riding and the fifth a specially well-trained professional bicyclist with whom 

 the larger portion of the experiments were made. All the experiments were 

 carried out in the post-absorptive state, i. e., at least 12 hours after the last 

 food had been taken. The investigation extended over a period of several 

 months and involved several hundred experiments. 



It was conclusively demonstrated that during severe muscular work there 

 is a distinct alteration in the character of the materials burned in the body, 

 as the evidence indicated a selective combustion of carbohydrate material, 

 although the experiments do not point to an exclusive combustion of carbo- 

 hydrate during muscular work. 



The major portion of the experiments was devoted to a study of the 

 relat'onship between the total heat output and the effective external mus- 

 cular work, thereby giving information in regard to the mechanical efficiency 

 of the man. The net efficiency {E) of the body would be represented by 



w 



E= YT, in which W is the external work expressed in calories and C the 



total caloric output of the body. With small amounts of work, W is small 

 and the efficiency is low. If the heat output necessary for maintenance is 

 deducted from C, the efficiency obviously becomes greater. The needs for 

 maintenance increase directly, depending upon whether the subject is lying 

 quietly in bed, sitting upright on the ergometer doing no work, or rotating 

 the pedals with no resistance or with varying intensity of resistance. Each 

 stage of activity represents a different base-line, and by superimposing more 

 severe work upon the various base-lines the increments in the total heat 

 production and in the work done could be determined and the ratio between 

 them, i. e., the mechanical efficiency, readily computed. An extensive 

 discussion of these various base-Unes and of what is meant by "gross" and 

 "net" efficiency, together with the careful computation of the maximum 

 efficiency, leads up to an interesting discussion as to the analysis of the 

 chemical and thermal processes involved in severe muscular work. 



Besides the two main themes, the report considers numerous other impor- 

 tant questions relative to the muscular work of man, such as the effect of 

 muscular work on the pulse-rate, the body temperature, the mechanics of 

 respiration, and the drafts upon the body-material, the maximum working 

 capacity of man, and particularly the after-effects of work. A certain 

 amount of evidence was available for a comparison of results obtained with 

 trained and untrained individuals. 



Under certain conditions, particularly when the subject is riding with a 

 moderately severe load and immediately begins riding with a very severe 

 load, it is possible to superimpose a load upon the human body so that 40 

 per cent of the increment in the total heat output may be in the form of 

 effective external muscular work transmitted to a machine, such as the 

 bicycle ergometer. No indication was given of the possibility of "over- 

 loading" the human machine so far as mechanical efficiency was concerned. 

 Obviously the power of human endurance is limited, but an interesting 

 point is the fact that the professional bicyclist performed all of the experi- 

 ments without food and that on at least one day he did an amount of work 

 equal to a "century" run over ordinary roads. This experiment gave the 

 data for computations in regard to the probable amount of available gly- 

 cogen in the human body. 



The report is accompanied by an extensive review of the earlier literature 

 with an analysis of the results obtained by former investigators. 



