SECTION IV 



THE EFFECT OF MUSCULAR WORK ON 

 METABOLISM 



WHEN .an animal performs muscular work its total output of energy must be 

 increased, and this must take place at the expense either of an increased 

 intake of food or an increased destruction of the tissues of the body. For 

 many years a theory put forward by Liebig received universal acceptance 

 among physiologists. According to this view the food-stuffs could be 

 divided into two classes, namely, the proteins, which were the plastic food 

 substances and were concerned in the building up of tissues, and, secondly, 

 the fats and carbohydrates, which took no part in the building of the tissues, 

 but were oxidised for supplying heat to the body. Muscular work was sup- 

 posed to be attended with a breakdown of the muscular tissue, and therefore 

 to be performed at the expense of an increased protein metabolism, which had 

 to be made good by adding to the protein intake in the food. If this view 

 were correct one would expect to find a great increase in the nitrogenous 

 metabolism of the body with every increase in muscular work. Such an 

 increase has, in fact, been found by Pfliiger in dogs which were nourished on 

 a purely protein diet. In these animals the sole source of energy to the 

 organism was protein, and therefore any additional call on the energies of 

 the body must be associated with an increased intake of food or an increased 

 loss of material from the body in the shape of protein. In an animal which 

 is enjoying a normal mixed diet, or has a store of carbonaceous material in its 

 tissues in the shape of fat, there is no increase of nitrogenous metabolism 

 during muscular work which would correspond in any way to the amount 

 of work done. This was shown long ago by Voit in experiments on the dog. 

 The following Table represents the nitrogenous metabolism, i.e. the amount 

 of muscular material in the shape of protein metabolised during the day, 

 under varying conditions of rest and activity, during starvation and after 

 food : 



DOG I 

 Food 



Flesh metabolism 

 Grm. 

 164 

 167 

 149 

 637 



Condition of dog 



Rest 



Work 



Rest 



