8 : 4/ Muscles 147 



the conversion of chemical energy to mechanical work. These heat 

 measurements are based essentially on temperature measurements. 

 They are difficult because the maximum temperature rise associated with 

 a muscle twitch is only 0.003 °C, and the heat is developed very rapidly. 

 A. V. Hill refined his techniques to the point that he could resolve a few 

 million ths of a degree change in a few milliseconds. 



Hill's experiments showed there were three different types of heat 

 production occurring during muscular contraction. The first, called 

 resting heat, is associated with metabolism in the resting muscle. The 

 second type of heat production, initial heat, accompanies actual con- 

 traction and relaxation. The third general type is called recovery heat; 

 it is liberated for 20-30 min following activity. 



The resting heat is an indication of continuous metabolism in the 

 muscle. It can be altered by stretching the muscle as well as by changes 

 in ionic strength in the surrounding fluids. It is not a constant or simple 

 quantity. 



When a muscle contracts and then relaxes, the second type of heat pro- 

 duction overrides the resting heat production. This initial heat con- 

 sists of several components. While the muscle contracts, it develops a 

 "maintenance heat" which starts just after the spike potential passes and 

 continues until relaxation. Some of this maintenance heat is actually 

 produced before contraction occurs. There is, in addition, a "heat of 

 shortening." Under isotonic conditions when the muscle lengthens, a 

 heat of relaxation is measured equal to the work done by the load. 



These heat changes attracted the interest of many investigators. 

 However, they are difficult to interpret. There is no simple relationship 

 between the work done and the extra heat produced. The reasons for 

 the rise in heat production before contraction and the dependence of 

 resting heat on muscle length are still not understood. This basic lack 

 of understanding emphasizes the incompleteness of current molecular 

 models of muscular activity. 



4. Muscle Chemistry 



In the previous section, the various physical changes accompanying 

 muscular contraction were presented. These all involve molecular 

 changes and the conversion of chemical free energy to other forms of 

 energy. Accordingly, it is appropriate to examine the chemical con- 

 stituents of muscle. These include the types of molecules active during 

 contraction and relaxation. The chemical transformations necessary 

 for energy production are also indicated. 



There are more water molecules within the muscle, and indeed within 



