48 CONTRACTILITY 



say, with the area of longitudinal surfaces within the 

 muscle. This indicates that contraction is dependent upon 

 change in tension between two surfaces, probably between 

 sarcostyles and sarcoplasm. The development of lactic 

 acid may be the factor determining the change of tension. 



But surface tension is not the only property influenced 

 by contraction. A fatigued muscle has a higher osmotic 

 pressure than resting muscle. Upon this fact is based 

 a theory which attributes contraction to an aggrega- 

 tion of colloid particles, with consequent hberation of 

 electrolytes. The increased concentration of these causes, 

 by osmosis, a flow of water in a particular direction. 



We may thus sum up what we have said above. Muscular 

 activity consists of two alternating phases : (1) ^ fhase 

 of contraction which, though associated with the formation 

 of lactic acid, is essentially a physical process involving 

 surface and osmotic phenomena. No gaseous metabohsm 

 is involved. (2) A 'phase of recovery consisting in the 

 restoration of a state of high potential. It is in this phase 

 that oxygen is used. 



The Mechanical Efficiency of Contraction 



It has been found that the energy of tension may have 

 an efficiency of 100 per cent. But in the phase of recovery 

 an amount of heat is produced equal to that produced in 

 the phase of contraction. This reduces the efficiency of 

 the whole contractile process to 50 per cent. In sustained 

 contraction, or tetanus, the efficiency becomes very much 

 diminished ; the form of contraction is therefore an important 

 factor. The average efficiency of contraction has been 

 estimated at about 25 per cent. 



