108 THE HUMAN BODY 



as an expression for any form of energy. If we speak of any engine 

 as able to furnish a certain number of calories we mean that if all 

 the energy were to appear as heat that many calories would be 

 liberated. As a matter of fact much of the energy may actually 

 take other forms, as it does in the case of the contracting muscle. 



When the energy is manifested as mechanical work it is meas- 

 ured in terms of the work done. Since the simplest form of work 

 is probably the raising of weights against the resistance of gravity 

 the units of work are based on weight and height. Thus the 

 fool-pound is the amount of energy involved in raising a weight of 

 one pound to a height of one foot. A calorie is approximately 

 equivalent to 3 foot-pounds. In the metric system the unit of 

 mechanical energy is the gram-centimeter. In round numbers 

 41,000 gram-centimeters represent the same amount of energy as 1 

 calorie. 



The Energy Output of Muscle. Studies of the mechanical 

 energy developed by selected groups of muscles in the Body can 

 be made directly . An excellent method is by means of a station- 

 ary bicycle. The wheel can be made to revolve against a measured 

 resistance, and thus the work done can be readily determined. As 

 already stated, muscles in the Body work very inefficiently. This 

 is in part due to the less favorable conditions of energy liberation 

 in the physiological tetanus as compared with the simple twitch, 

 but more because of the mechanical disadvantages at which 

 muscles work. They pull at the short arms of levers, and the 

 direction of their pull is usually oblique (Chap. VIII). The energy 

 output of an isolated muscle is also easy to determine, although 

 to be certain that the utmost possible has been obtained is not so 

 simple. The muscle operates by forcible shortening. That means 

 that the muscle pulls upon the weight to which it is attached. If 

 the tension developed is greater than the weight the latter is lifted 

 and work is done. If the tension is insufficient to raise the weight, 

 there is no manifestation of mechanical energy. All the energy 

 liberated takes the form of heat. It has been found that in a 

 given muscle under given conditions the tension developed is fairly 

 constant and directly proportional to the total energy manifested 

 in the contraction; whereas the actual mechanical work done de- 

 pends, as we have already seen (p. 98) on various factors, such as 

 the relation of the load to the absolute strength of the muscle. If 



