446 THE FUNCTIONS OF CROSS-STRIATED MUSCLES 



thereby produced. Experiments on exsected frog's muscles indicate that there 

 is a direct influence on the contractile substance (Huge). 



Fatigue of one group of muscles exercises an unmistakable influence on 

 other muscles e. g., fatigue of the legs hastens fatigue of the arms ; but mus- 

 cular training reduces such effects. 



It has been shown also that purely mental work hastens muscular fatigue 

 to a very great extent. It might be supposed that this part of fatigue is purely 

 central; but the matter is not so simple. The same result is obtained where an 

 artificial stimulus is applied to the median nerve or directly to the flexor mus- 

 cles of a person fatigued by mental work. 



Finally, by other experiments which cannot be described here, Mosso has 

 shown that while the mechanical work performed by a muscle decreases as 

 fatigue comes on, the nervous effort and the intensity of the processes which 

 call forth the contractions progressively increase. By a method especially 

 adapted to the purpose, it may even be shown that the nervous mechanism 

 is being greatly strained before there is any sign of fatigue in the external 

 work done. This, as Treves remarks, would explain the fact that athletes 

 not infrequently are attacked by severe neurasthenic pains. 



An increase in the output of CO 2 and in the consumption of O 2 is another 

 characteristic of fatigued muscle ; that is, the utilization of energy becomes more 

 and more unfavorable with the progress of fatigue. 



What has been said here concerning fatigue and recovery applies especially 

 to the skeletal muscles. Other muscles fatigue much more slowly and require 

 a much shorter time of recovery in order to remain permanently in functional 

 condition. The best example of this is the heart, which throughout life works 

 uninterruptedly with rest periods of only about 0.4 second. That the smooth 

 muscles also are capable of long-continued work is shown by the tonus which 

 they maintain in the arterial walls. 



In view of the facts presented here it becomes a matter of interest to 

 inquire under what circumstances the greatest amount of muscular work can 

 be performed. This question cannot be answered fully at present, but we have 

 some facts bearing on the subject which are of considerable interest. When 

 one does the same amount of external work in two sets of experiments which 

 differ only in the circumstance that the load and the distance through which 

 it is lifted vary in reverse order (e. g., 20 kg. X 0.3 m. and 30 kg. X 0.2 m.) 

 it is found that the fatigue comes on much more rapidly with the heavier load 

 (Stupin). The conclusion is that the size of the load and not the absolute 

 amount of muscular work done determines how long the movement can be 

 continued. But if the load is too small the muscle evidently cannot accom- 

 plish much work. We may say, therefore, in general that the greatest quantity 

 of work can be done when the load is of medium size. 



This medium load may be found (Treves) by choosing a weight with which 

 a person while perfectly fresh can do the greatest absolute amount of work and 

 observing the record until the contractions exhibit evidence of fatigue. If now 

 the weight be diminished so that the contractions of the same extent can con- 

 tinue, and so on, a weight will finally be found with which the person can 

 continue the work at the same rhythm indefinitely. 



