MUSCLE 



245 



as the speed in walking rose above 78 metres per minute, 

 the rate of expenditure of energy per unit of distance covered 

 was increased. This is confirmed by experiments done 

 on man. It has been found that in marching, the optimum 

 rate — the rate at which the most work can be done on the 

 least expenditure of energy — is something over 3 miles an 

 hour. To force the pace above this leads to a dispro- 

 portionate demand for energy and is less economical. 



Fig. 128, showing the extent of combustion in muscle 

 measured by the COg produced, illustrates this. 



Lift 



Work done 



4"7 kgm. 



per unit of time. 



Rapid succession of stimuli. 



Lift 



Work done 



6 kgm. 



per unit of time. 



Slow succession of stimuli. 



Fig. 127. — To show the influence of varying the rate of stimulation 

 on the work done per unit of time. 



The efficiency of a muscle as a machine thus depends 

 upon the — 



1. Load lifted. 



2. Rate at which work is done. 



Hence the efficiency of a machine is often expressed in 

 terms of Horse Power — the unit being 76 kgm. per sec. 



Measurement of work— 



1, In the isolated onuscle — 



(a) The work done by the single isolated muscle of the frog 

 in a single twitch is got by multiplying the weight lifted by 

 the extent of shortening of the muscle {Practical Physiology). 



(h) If the work during a series of contractions is to be 

 measured, some means of adding the effect of each contrac- 

 tion to its predecessor must be employed, as is done by 

 the work-collector of Fick {Practical Physiology). 



