558 PHYSIOLOGY. 



If the height be expressed in feet and the weight in pounds, then 

 the work performed is measured in units of foot-pounds. Likewise, 

 should the height be measured in meters and the weight in grams, 

 then the work done is expressed in grammeters. 



In studying the heights of contraction in a loaded muscle it is 

 found that the heights of lift continuously diminish, but the actual 

 work done by the muscle increases rapidly and then more slowly until 

 it reaches its maximum with a load of 200 grams. After that point 

 the work done slowly decreases and then more rapidly until it receives 

 a load of 700 grams, when the muscle is unable to contract. 



Dynamometer. The common, clinical form of dynamometer is 

 much used to determine the absolute force of certain muscles. The 

 instrument is very useful to determine the difference in grip between 

 the two hands in cases of paralysis. The patient grasps the instru- 

 ment in his hand and squeezes upon it; the power exerted is regis- 

 tered in kilograms. 



Muscles are Most Perfect Machines. They take the best ad- 

 vantage of the fuel supplied to them and give in return a very high 

 percentage of energy in the form of work. They, by legitimate exer- 

 cise, increase in strength and power so that they progressively per- 

 form more work. 



The steam engine, to which muscles are frequently compared, is 

 inferior in every respect. The best-made steam engine shows as 

 work only about 12 per cent, of the total energy supplied to it by the 

 oxidation of the coal, while about 88 per cent, is transformed into 

 heat. Muscle transforms 25 per cent, of its energy into work and 75 

 per cent, into heat to warm itself. 



The proportion of work to heat is not a fixed one. If you gradu- 

 ally increase the stimulus, both work and heat increase ; but the heat- 

 production is increased more rapidly and reaches its maximum sooner. 

 Heat-production decreases more rapidly than the amount of work pro- 

 duced, when the muscle is exhausted. When the muscle is loaded 

 so it cannot contract, or an unweighted muscle is made to contract, 

 no work is produced and all the energy is converted into heat. 



Fatigue. Fatigue is due to a chemical and physiological altera- 

 tion of the muscles. It is characterized by a pain, more or less acute, 

 localized in the muscles. The alterations in the muscles fatigued 

 are due to an accumulation of toxic products of the metabolism of 

 the muscle. Sarcolactic acid is one of these fatigue-products, and 

 when applied to a muscle it causes a state of exhaustion. Whatever 

 the fatigue-products may be, a muscle exhausted by a series of con- 



