GENERAL MUSCLE PHYSIOLOGY 193 



muscle is 3 kg for I sq. cm. of cross-section, of the human 

 muscle it is 10 kg. 



The work done by an active muscle is zero if the load is 

 zero or if the load is so great that the muscle can no longer 

 raise it. Between these two extremes the amount of work 

 done increases with the increasing load up to a certain 

 maximum, beyond which it decreases. 



The raising of the load to a height equal to the extent of 

 the contraction of the muscle is not the greatest amount of 

 work capable of being performed by the muscle. The 

 muscle does more work when 



(1) The load is not raised, but is thrown upward; it can 

 then rise higher than the corresponding contraction of the 

 muscle. 



(2) When the contracting muscle after it has raised the 

 load is gradually unloaded. Then the muscle shortens more 

 and performs new work by raising the lessened load. Many 

 muscles in the human body, because of the relation of their 

 joints, work according to this advantageous principle of 

 unloading. 



In addition to the performing of real work, the muscles 

 also perform the function of keeping raised weights sus- 

 pended and of holding the individual parts of the body 

 together. This also takes place with expenditure of energy 

 by the tension of the muscles. 



An adult man can perform, in eight hours, a work of 

 about 300,000 kilogrammetres. 



B. The formation of heat by active muscles. Of the 

 energy set free by an active muscle at most only one-fourth 

 is used for the performance of work, the remainder being 

 transformed into heat. 



The muscles work much more economically than the steam- 

 engine, for, in the best-constructed steam-engine, only one-tenth 

 of the energy set free by the burning of the coal is used in doing 

 work. 



When no external work is done, all the transformed force 

 appears as heat; in this case we can calculate the extent of 



