62 HUMAN PHYSIOLOGY 



One factor requires special consideration, namely, the 

 Load. 



We have already seen that as the load is increased the 

 extent of contraction is diminished. 



The following experiment will illustrate the influence of 

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



Work in Gram, 

 mm. 



O'O 



60-0 

 94-8 

 120-0 

 140-0 

 120-0 



It will be seen that increasing the load at first increases 

 the amount of work done, jut after a certain weight is 

 reacrjecr. diminishes it. There~is, therefore, for^very 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 con- 

 sidered. 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 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 0*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 a 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 pro- 



