MECHANICAL EFFICIENCY OF THE HUMAN BODY 
139 
with the load, with the heaviest loads there appeared definite indications of 
a decrease in efficiency. Since with practically all the heavy loads the rate 
of speed was high, the possible relationship between efficiency and load nat- 
urally becomes a relationship between efficiency and speed. 
From the curves in fig. 5 it can be seen that to obtain 1.565 calories of 
effective muscular work at 70 revolutions per minute, it is necessary that the 
subject give off 7.61 calories of total heat, thus showing a gross efficiency 
of 20.6 per cent. Furthermore, in order to produce 2.425 calories of external 
muscular work at 130 revolutions, the subject must actually give off 15.04 
calories of heat, corresponding to a gross efficiency of 16.1 per cent. The 
values for the gross efficiencies at the different speeds have been computed 
and are presented in table 124. Since in computing the net efficiency, the 
value used for the base-line, i. e., that obtained with the subject lying quietly 
on a couch, is constant and applicable to the whole curve irrespective of 
speed, it is obvious that the values for the net efficiency would be similarly 
affected by the speed. 
Table 124. — Gross efficiency of subject M.A.M. at varying speeds vrith 
a current of 1.5 amperes. (See Fig. 5.) 
Revolu- 
Gross 
Revolu- 
Gross 
tions per 
effi- 
j tions per 
effi- 
minute. 
ciency. 
minute. 
ciency. 
p. ct. 
p. ct. 
70 
20.6 
110 
17.6 
80 
20.0 
120 
16.9 
90 
19.2 
130 
16.1 
100 
18.4 
When we consider the variations in the output of work with increased 
speed, further relationships between the curve for the total heat output and 
that for the effective muscular work are found which indicate the influence 
of speed. From the upper curve it is seen that the output of heat is constant 
per 10 revolutions; on the other hand, the increase in the effective muscular 
work performed is not constant for each 10 revolutions, but there is a distinct 
falling off. If, therefore, we divide the increase in the external muscular work 
between any two points on the curve by the increase in the total heat output 
corresponding to the same two points, we get an efficiency based upon in- 
creasing speed, the degree of magnetization, i. e., the load, being the same. 
For instance, in changing from 70 to 80 revolutions per minute, there is an 
increase in the effective muscular work equivalent to 0.205 calories. Under 
these conditions there is an increase in the total heat output of 1.24 calories. 
Dividing the increase in the heat output due to the muscular work (0.205 
calories) by the increase in the total heat output (1.24 calories), we find an 
efficiency for the increased amount of work performed of 16.53 per cent. 
Computations of a similar nature have been made for the various increases 
in speed and the results are given in table 125. 
It is thus evident that at the higher speeds with the same degree of mag- 
netization there is a much larger heat output for the same amount of external 
muscular work performed, and consequently the efficiency decreases greatly 
as the speed increases, the optimum efficiency being at the lowest rates of 
speed, namely, about 70 revolutions. In perhaps no other table in connection 
