38 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



it constitutes a part of the heat used to maintain the body tempera-, 

 ture, as is explained in Chapter LI. The mechanical work done by 

 a muscle in lifting a load is usuallj^ expressed in gravitational units, 

 as gram meters or gram millimeters, given by the product of the 

 load into the lift. If a muscle hfts a load of 40 grams to a height 

 of 10 millimeters the work done is 40 X 10 = 400 gram millimeters 

 or 0.4 grarn meter. The mechanical work or mechanical energy 

 can be expressed in terms of heat by the use of the so-called 

 mechanical equivalent of heat according to which 1 calorie = 426.5 

 gram meters. Mechanical work of 0.4 gram meter is equivalent 

 in heat to about 0.001 calorie (w)- When a muscle contracts 

 against a resistance which it cannot overcome (ivsometric contrac- 

 tion) no mechanical work is done. The portion of liberated energy 

 which would have been used to shorten the muscle and lift the 

 weight now serves to put the muscle into a state of greater elastic 

 tension. When the stimulus ceases and the muscle relaxes this 

 energy of tension is converted into heat or, we may say, is lost as 

 heat.* 



The Efficiency of the Muscle as a Work Machine. — In any 

 engine constructed to perform external work it is desirable to know 

 what efficiency the engine exhibits, that is to say, what per cent, of 

 the total energy given to the engine or developed in the engine may 

 be obtained as external or mechanical work. If we represent by Q 

 the total energy developed and by W the energy, expressed in the 

 same unit, obtained as work, then the efficiency E is given by the 

 formula 



-I- 



Numerous experiments to determine this value have been made 

 upon the isolated muscles of frogs. These muscles can be taken 

 from the body, be made to contract under different loads by arti- 

 ficial stimuli, and the work done and the heat developed can both 

 be measured with considerable accuracy. The earlier experiments 

 of this kind indicated that the frog's muscle under optimum condi- 

 tions may exhibit an efficiency as high as 25 to 30 per cent., as is 

 shown by the following example taken from Fick's investigations :* 



Load. Heat Developed. Mechanical Work. Thermal Equivalent Efficiency. 



200 gm. 25.6 miorocalories. 2905 gm. -mm. 6.83 microoalories. ^256 "" ^^"'" ^''' 



Experiments have shown that the efficiency of different muscles 

 in the same animal varies or may vary, and likewise the efficiency 



* Hill and Hartree, loc. cit. 



fFick, "Pfliiger's Archiv," 16, 58, 1878. 



