MECHANICAL EFFICIENCY OF THE HUMAN BODY 121 
that is quite inconsistent with the theory that there were large compensatory 
movements. To anyone who watched the sprocket-chain during all of these 
tests it was, however, very obvious that it was impossible for this subject 
to sit quietly upon the motor-driven machine and allow his legs to rotate 
freely without attempting to stop them or to increase the speed of the machine. 
Likewise, in the no-load experiments without the motor, it was impossible 
for him to maintain an even rate, and there was unquestionably a tendency 
to "back-pedal" at times. 
GROSS AND NET EFFICIENCY OF SUBJECTS LN RESPIRATION EXPERIMENTS 
WITH THE BICYCLE ERGOMETER. 
In conformity with the custom of practically all of the earlier writers, 
we have computed the gross efficiency of our subjects, using simply the rela- 
tionship between the heat equivalent of external muscular work and the total 
heat output; the results are given in table 116. The values for the net 
efficiency are likewise given, together with the data for the oxygen intake 
and heat output per minute during both the work and the rest periods, with 
the heat equivalent of muscular work. 
The values for the different factors have been arranged, first, in the order 
of subjects, and second, on the basis of ascending values for the heat equiva- 
lent of external muscular work. The reason for this particular arrangement 
will appear in examining the figures in the table, since the gross efficiency 
percentages as here shown indicate invariably an increasing gross efficiency 
with an increased load. Thus, in the experiment with E. P. C, in which the 
load was very light and the effective work equivalent to but 0.44 calorie 
per minute, the gross efficiency was 11.3 per cent. When the load was ap- 
proximately double, and the effective work 0.93 calorie, the efficiency was 
increased to 16.1 per cent, while with a load approximately triple, and the 
effective work 1.41 calories, the efficiency was 19.9 per cent. 
The reason for this increase in the gross efficiency with an increasing 
load is easily seen when we consider the fact that a considerable amount of 
heat is required to maintain the body over and above that used for effective 
muscular work. When the subject lies quietly upon a couch, the heat output 
is only that required for body maintenance, this being an approximately 
constant value in all experiments. With a fight load the maintenance re- 
quirement forms a large proportion of the total energy output, so that the 
deduction of the resting metabolism has a pronounced effect. As the load 
is increased, however, there is no increase in the maintenance requirement, 
since it is an approximately constant value; consequently, with increasing 
loads, the extra heat output is due solely to the external muscular activity, 
and the percentage for the gross efficiency is accordingly increased. As an 
illustration, a comparison may be made with a boiler in a boiler-house where 
sufficient steam must be maintained to keep the boiler and the boiler-room 
warm. If in addition a small draft is made upon the supply of steam for 
running a small engine, the output of mechanical energy from the engine 
bears a relatively small proportion to the total heat output from the boiler, 
and the efficiency of the plant as a whole is very low. With increasing drafts 
upon the supply of steam, the percentage of the basal requirement for keeping 
the boilers and the engine-room warm becomes less and less, and the percen- 
tage of gross efficiency is correspondingly increased. 
