What Connection Is There between Overwork and Excretion? 



Getting Tired^ When you "chin" yourself on a bar four, five, or six 

 times, until you can do no more, this does not mean that you will never be 

 able to chin yourself again. After resting awhile, perhaps a day or an hour, 

 or perhaps only ten or fifteen minutes, you can chin yourself again as well 

 as at first. What happens in the first place to make you stop? Or what 

 happens during the rest to enable you to do the work again? As soon as 

 work commences, waste substances begin to accumulate in the cells. The 

 wastes are formed faster than they are carried away. The result is a "poi- 

 soning" of the protoplasm of the working cells. 



When muscles are working slowly, the glucose fuel is oxidized, first into 

 lactic acid, then into water and carbon dioxide. When muscles work very 

 rapidly, as in running, lactic acid formed in the first stages of oxidation 

 accumulates in the cells and is but slowly removed by the blood. Since the 

 lactic acid results from using oxygen faster than it is supplied by the blood 

 and lungs, it is said to represent an "oxygen debt". During rest this "oxygen 

 debt" is quickly repaid by an increased rate of respiration and circulation 

 (see page 193). In the meantime the lactic acid interferes with the opera- 

 tion of the muscles and in effect "poisons" nerves and other tissues. When 

 hard work is sustained for any considerable time, we say that the muscle is 

 fatigued. Some of this lactic acid is distributed by the blood to other tissues 

 of the body, and tissues which have not been active become "fatigued". 



Fatigue May Be General We have all been taught that "a change of 

 work is the best kind of rest." To a certain extent this is true. When I am 

 reading a difficult book and begin to doze over it, I am not too tired to play 

 a game of tennis or even to read exciting fiction. But past a certain point, 

 fatigue affects the whole body; getting tired from study unfits one for 

 muscular work or play. Thus records made on the ergograph by any person 

 will show great variation, according to the condition of the body. A record 

 made early in the morning will differ from one made at the close of a game 

 of chess (see illustration, p. 224). From these and similar experiments we 

 have learned that exhausting physical work tires the brain and the sense 

 organs. And we have learned that severe mental work tires the whole body. 



We cannot conclude, however, that hard work is to be avoided. On the 

 contrary, hard work is useful physiologically, as well as otherwise. It stimu- 

 lates the many metabolic processes and so helps to keep the body in good 

 condition. We can use knowledge about fatigue to organize our work in 

 more effective ways. By planning carefully, by adjusting the rate of work, 

 and by arranging alternate periods of work and relaxation we can do much 

 to reduce fatigue. 



iSee No. 5, p. 226. 

 222 



