THE PHYSIOLOGY OF MUSCULAR WORK 201- 



men and animals do not eat too much, if they perform an adequate 

 amount of work. 



The increased consumption of food which is associated with 

 muscular work generally involves an increase of protein, carbo- 

 hydrate, and fat, as shown in the dietaries already given. There 

 is need of more protein, even although it is agreed that the destruc- 

 tion of that substance in the body 'is not increased by muscular 

 work : there is a retention of nitrogen during muscular work, and 

 this nitrogen appears to be stored or built up in the body as protein. 

 Bornstein l found that he retained in eighteen days an amount of 

 nitrogen which would correspond to 800 grms. of muscle. Muscles 

 develop during work, and thus the protein of the tissues is increased. 



For the supply of energy during muscular work the carbohydrates 

 and fats are increased. These foods stuffs can replace one another, 

 for fat is formed from carbohydrate in the body, and there is evi- 

 dence to show that fat may give rise to carbohydrate. The relative 

 amounts of these articles of food selected by different men appear 

 to depend upon individual tastes and the comparative cost. Fat 

 is dearer than carbohydrate, but it is a concentrated food. 



The energy values of the different foods are most conveniently 

 expressed in terms of the heat which they yield on combustion. 

 One gramme of dry protein or of carbohydrate yields 4'1 calories, 

 and an equal weight of fat yields 9-3 calories. The protein does 

 not undergo complete combustion in the body ; for this reason 

 the physiological value given above is less than the physical value 

 as determined by the calorimeter. For the supply of the energy 

 needed during the performance of muscular work the different 

 food substances can be substituted according to their equivalent 

 quantities ; thus 100 grammes of fat, 211 grammes of protein, and 

 230 grammes of carbohydrate are physiologically isodynamic. The 

 muscles obtain energy from each kind of food, but the carbohydrates 

 and fats are the ones most readily used. The relative quantities 

 will depend upon the diet ; carbohydrates form a large proportion 

 of the food consumed by most working men. 



A man absorbs per kilogramme of his body weight and per 

 hour about 0'29 grm. of oxygen and discharges about 0'33 grm. 

 of carbon dioxide, when he is at rest ; directly he performs work 

 the respiratory exchange increases, thus a walk at the rate of about 

 three miles an hour will raise the values four or five times. If 



1 See Cohnheim, Ergelmisse der Physiol., Zweiter Jahrgang, Abtheiluug 1, s. 621. 



