70 PHYSICAL ASPECTS OF MUSCLES 



proaching 100%. But, if the energy-producing system is to produce the 

 most actual power, the system must be designed to work near 50% 

 efficiency. You may find further discussion of this point in relation to 

 biological systems in the paper whose reference is given at the end of this 

 section. 



This digression allows us to conceive of a classification of muscles into 

 two types: those which exert large forces through small distances, and 

 others which exert small forces through large distances. Roughly speak- 

 ing, these types correspond to what is actually found in nature. Typical 

 striated muscles, such as facial, arm and leg, and heart muscles, contract 

 very little, but exert large forces. Smooth muscles, such as many involun- 

 tary muscles, e.g., bladder muscles, contract appreciably, and exert cor- 

 respondingly smaller forces over longer time periods. 



Other than for measuring the external work done, the variables in 

 Hill's equation do not afford much scope for investigation of muscle 

 action problems. However, estimates of the energies involved in the 

 various processes may be obtained from heat measurements. The sensi- 

 tivity of the temperature measurements involved is fairly great but is 

 well within the scope of modern technology, which has no trouble measur- 

 ing temperature changes of millionths of a degree. When the temperature 

 changes in muscles are determined before, during, and after a muscle 

 contraction, it is found that there are three main stages of heat produc- 

 tion. 



(a) Resting heat. This is the energy expended in just keeping the 

 muscles ready to perform. The metabolism within the muscle has the 

 function of keeping the metabolic machinery in readiness for action in 

 the extended position so that it can contract when the stimulus reaches it. 



(b) Initial heat. This energy appears to be associated with the con- 

 traction process, being composed of two chief parts: the heat of develop- 

 ing the tension in the muscle, and the heat associated with the actual 

 contraction itself. 



(c) Recovery heat. This, as the name implies, is associated with the 

 return of the muscle to its pre-contracted state, after which the resting 

 heat keeps the muscles in readiness. The recovery heat includes the 

 buildup of the molecules responsible for the quick availability of energy 

 which characterizes the muscle action. In this case, much of the quick- 

 energy activity is effected through the molecules of ATP and of creatine 

 phosphate, the former of which is a general energy storage battery for 

 the metabolic machinery of cells, the latter of which is an energy storer 

 quite specific for the needs of muscles. 



Some of the points mentioned above may be directly observed. It was 

 first shown by Szent-Gyorgy that if an excised muscle is placed in 

 glycerol, many small molecules are dissolved out of the muscle, leaving 



