38 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



theories of muscle contraction. Second. Some electrical energy 

 is developed during the contraction. The means of detecting 

 and measuring this energy will be described in a subsequent chapter. 

 Considered quantitatively, the amount is small. Third. Work 

 is done if the muscle is allowed to shorten during the contraction. 

 By work is meant external or useful work that is, the muscle lifts 

 a weight or overcomes an opposing resistance. If a muscle con- 

 tracts against a weight too heavy to be lifted, or a resistance too 

 strong to be overcome, it does no external work, although, of 

 course, much energy is liberated as heat or, as it is sometimes called, 

 internal work. The work done by a muscle during contraction is 

 measured in the usual mechanical units, by the product of the 

 load into the lift. That is, if a muscle lifts a weight of 40 grams 

 to a height of 10 millimeters, the work done is 40 X 10 = 400 

 gram-millimeters, or 0.4 grammeter. We can in calculations 

 convert external work into heat or internal work by making use 

 of the ascertained mechanical equivalent of heat, according to 

 which 1 calorie = 426.5 grammeters of work. The work, 0.4 

 grammeter, supposed to be done in the above experiment, would be 

 equivalent, therefore, to 0.4 -=- 426, or about 0.001 of a calorie. 



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 



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 llquivalent Efficiency. 



200 gm. 25.6 microcalories. 2905 gm.-mm. 6.83 microcalories. ' = 26+ %. 



Experiments have shown that the efficiency of different muscles 

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

 * Fick, "Pfliiger's Archiv," 16, 58, 1878. 



