170 SECTIONAL ADDRESSES, 
optimum. In an eight-oar boat the recovery takes almost as long as the 
stroke, both occupying about one second. It is of interest how practical 
experience has gradually evolved a speed of movement which is almost 
exactly what a physiologist might have predicted as the most efficient. 
At a stroke of about 32 per minute the mechanical efficiency is apparently 
near its maximum. An enormous amount of work has to be done in pro- 
pelling a boat at speeds like 10 to 12 miles per hour. According to Hender- 
son, each member of the crew of an eight-oar boat must exert about 0.6 
of a horse-power. Clearly if this enormous amount of external work 
is to be done it must be accomplished by working under efficient conditions : 
those conditions necessitate a stroke of a particular frequency; only 
when the race is very short is it permissible, in order to obtain a greater 
output, to work less efficiently by adopting a more rapid stroke. The 
stroke may rise to 40 per minute for a short distance: in such an effort 
the oxygen debt is accumulating rapidly and exhaustion will soon set in. 
The amount of work, moreover, will not be proportionately greater, 
probably only slightly greater, than at the lower frequency. The con- 
ditions which determine the speed of movement, the ‘ viscous-elastic ’ 
properties of muscle, are what ultimately decide the length of the oars 
and the speed of movement in a racing-boat. It is interesting to find— 
as, of course, was really obvious—how closely athletics is mixed with 
physiology. 
Wastefulness of High Speeds. 
This last discussion leads us to the question of what determines the 
great wastefulness of the higher speeds. Why, returning to fig. 2, does 
a speed of 280 steps per minute require 24 litres of oxygen per minute, 
while a speed of 240 steps per minute requires only 8 litres of oxygen ? 
The answer depends upon the variation of external work with speed of 
muscular movement. In a series of recent papers it has been shown 
that in a niaximal muscular movement the external work decreases in 
a linear manner as the speed of shortening increases. At sufliciently 
high speeds of shortening no external work at all can be performed. 
In most of these athletic exercises, apart from the case of rowing, a large 
proportion of the mechanical work is used in overcoming the viscous 
resistance of the muscles themselves. At high speeds of running only a 
small fraction of the mechanical energy of the muscles is available to 
propel the body, once the initial inertia has been overcome. The speed 
of shortening is so rapid that little external work can be done. The work 
is absorbed by internal friction, or by those molecular changes which, 
when the muscle is shortening rapidly, cause its tension to fall off. When 
working against an external resistance, as in rowing, there is an optimum 
speed. If an effort is to be long continued it must be made at a speed 
not far from the optimum. When, however, the whole of the resistance 
to movement is internal, as in running, there is no optimum speed: the 
expense of the movement increases continually as the speed goes up ; 
the faster we move, the greater relatively the price: our footsteps are 
dogged by the viscous-elastic properties of muscle, which prevent us from 
moving too fast, which save us from breaking ourselves while we are 
attempting to break a record. 
