60 VETERINARY PHYSIOLOGY 



The following experiment will illustrate the influence of 

 increasing the load on the work-doing power of a muscle 



Load ill Space through which Work in Gram. 



Grams. lifted in mm. mm. 



x 4-5 0-0 



20 x 3-0 60-0 



40 x 2-37 94-8 



60 x 2-00 120-0 



80 x 175 140 ; 



100 x 1-2 120-0 



It will be seen that increasing the load at first increases the 

 amount of work done, but that after a certain weight is reached, 

 it diminishes it. There is, therefore, for every muscle, so far 

 as its working power is concerned, an optimum load. 



In studying the amount of work a muscle or set of muscles 

 can do, the element of time must always be considered. 

 Obviously contracting muscles will do more work in an hour 

 than in a minute. Hence in trying to form any idea of the 

 amount of work a muscle can do, this must be expressed in 

 work units per unit of bulk and per unit of time. The work 

 unit generally employed is the kilogram-metre the work 

 required to raise one kilogram to the height of one metre 

 against the force of gravity. 



Various instruments Ergometers have been devised for 

 measuring the amount of work done by various groups of 

 muscles under different conditions. 



The average working capacity of skeletal muscle may be 

 estimated as follows : A labourer who raises 130,000 kilos 

 through one metre during his eight hours of work does a good 

 average day's work. His muscles weigh about 25 kilos, and 

 thus each gram of his muscle will do 5 kilogram-metres per 

 diem, or '06 gram metres per second. 



When required, much larger amounts of work can be done 

 for short periods. It has been calculated that in the sprint of 

 & 100 yards race, work is done at something like 2 gram metres 

 per second, about thirty times the rate at which a labourer's 

 muscles work. But to increase the rate at which work is done 

 requires an increase in the expenditure of the energy-yielding 

 materials in greater proportion than the increased work just 



