MUSCULAR ACTIVITY 107 



one kind of energy (chemical) into another kind (mechanical) its 

 energy relationships can be studied in the same manner as in other 

 sorts of engines. We are familiar with various classes of these. A 

 steam power plant and an automobile fall in the same group with 

 muscles in that they are devices for transforming chemical energy 

 (oxidation of 'fuel) into mechanical. A hydro-electric installation 

 converts the mechanical energy of the water-fall into electrical 

 energy. An ordinary dry cell is an engine for the conversion of 

 chemical energy into electrical, and a motor for the conversion of 

 electrical energy into mechanical. All these engines have in 

 common the feature of relative inefficiency. When we speak of 

 the efficiency of an engine we mean the ratio of the amount of 

 useful energy it gives out to the total amount it uses up. No en- 

 gine is 100 per cent efficient; in none can all the energy put in be 

 recovered in available form. There is always a fraction of the 

 total energy which manifests itself in the form of heat. The hot 

 flue gases from the furnace, the hot bearings on the locomotive, 

 the hot water in the cooling system of the automobile; all these 

 signify energy which from the standpoint of the machine is wasted. 

 Muscles share with other engines this feature of inefficiency. As 

 used in the Body the muscles are only about 20 per cent efficient. 

 That is, in order to do a given amount of muscular work we must 

 burn in our bodies enough fuel to give a total energy output five 

 times as great. The balance, of 80 per cent, takes the form of 

 heat, and explains why we find ourselves so warm after vigorous 

 exercise. Isolated muscles under favorable circumstances may 

 show an efficiency of nearly 50 per cent. 



Energy Units. In order to be able to discuss energy relation- 

 ships intelligently we need to have some means of designating 

 definite amounts. The form of energy into which all other forms 

 tend to convert themselves is, as we have seen, heat. A convenient 

 energy unit, then, is the heat unit. The amount of heat required 

 to raise the temperature of 1 gram (~^ oz.) of water 1 degree 

 centigrade (strictly from zero to 1) is taken as the unit. This is 

 known as the calorie. For convenience when large amounts of 

 heat are involved a second unit just one thousand times as great 

 is also used. This is called the kilocalorie or simply the large 

 Calorie, distinguished from the small calorie by the use of the 

 capital initial. Although the calorie is strictly a heat unit it serves 



