WORK PRODUCTION 557 



Series II : 



Hill climbing without practice 38.83 grams 



Hill climbing after 14 days' practice . . . . 31.00 grams 



That the less use of accessory muscles is not the only cause 

 of this increase in efficiency is indicated by experiments upon 

 convalescents, which have shown that the gradual strengthening 

 of the muscles results in a more economical performance of 

 their work, largely independent of any special training for a 

 particular kind of work. 



Conversely, fatigue has been shown by numerous observers 

 to materially increase the relative amount of metabolism per 

 unit of work. Schnyder l summarizes the matter in the state- 

 ment that it is not the work itself but the muscular effort re- 

 quired which determines the amount of metabolism, a conclu- 

 sion which seems to have anticipated Hill's results 2 regarding 

 the relation of muscular tension to metabolism. 



662. Intensity of Work. It has already been shown (649) 

 that the gross efficiency of the body tends to increase with the 

 intensity of work, i.e., with the number of units of work per- 

 formed in a unit of time, for the reason that the proportion 

 of the total energy expended which is devoted to useful work 

 increases. On the other hand, common observation tends to 

 show that this can be true only within limits, and that excessive 

 work is performed uneconomically. 



The intensity of the work may be increased by increasing 

 either the speed, the load moved, or the angle of ascent. It 

 would be anticipated, therefore, that an undue increase of any 

 one of these factors would result in a diminished net efficiency. 



663. Influence of speed. That great speed in horizontal 

 locomotion involves a largely increased expenditure of energy is 

 evident. The race horse or the track athlete traveling a mile 

 at top speed obviously metabolizes vastly more energy than 

 one traveling the same distance at a moderate rate. 



In the case of the horse, Zuntz and Hagemann's results on 

 horizontal locomotion at a walk (652) show an increased net 

 expenditure of energy per kilogram weight and meter distance 

 with increased speed, while locomotion at a trot showed no 

 distinct increase up to a speed of about 7^ miles an hour. 



1 Ztschr. Biol., 33 (1896), 289. 2 Jour. Physiol. (London), 42 (19"), i. 



