MUSCULAR CONTRACTION 911 



Treppe and Fatigue. Applying these facts to the phenomenon of 

 treppe and fatigue, we see that the continued use of a muscle will be 

 attended with an increase in the hydrogen-ion concentration as the re- 

 sult of the liberation of lactic acid in the muscle substance. For all tis- 

 sues there is an optimal concentration of hydrogen-ions at which their 

 activities are carried out to the best advantage. The initial change in the 

 hydrogen-ion concentration which accompanies the first few contractions 

 brings the muscle into a more favorable condition arid treppe results. 

 On further production of lactic acid the optimal condition is exceeded 

 and the muscle fibers become successively less and less able to perform their 

 work. If the muscle is supplied with an adequate circulation a point is 

 soon reached at which the excess of lactic acid is restored as fast as it 

 is formed, by virtue of the oxygen supplied by the blood, and beyond 

 this point fatigue does not proceed further, successive contractions fol- 

 lowing one another for a long time with undiminished force. The mus- 

 cle is then said to have reached a fatigue level in Avhich the constructive 

 processes during the rest between contractions just balance the destruc- 

 tive processes during activity. If the circulation is inadequate, or lack- 

 ing, as it is in an isolated nerve muscle preparation, the increase in 

 the hydrogen-ion concentration goes on as the result of the accumula- 

 tion of lactic acid until the contraction of the muscle becomes impossible. 



The fatigue of muscle is consequently due to the accumulation of lac- 

 tic acid and probably other waste products in the muscle substance, 

 and in protracted exertion to the using up of the materials which upon 

 oxidation restore the energy lost in the act of contraction and replace 

 the lactic acid in the condition in which it existed in the rested muscle. 



Under normal circumstances skeletal muscle is protected from the 

 development of any harmful degree of fatigue. This is because its ac- 

 tivity is initiated through the nervous system, parts of which become 

 fatigued long before this condition comes on in the muscles. We have 

 already seen that the nerve fiber is unfatigable. The synapses, on 

 the other hand, and the myoneural junction are easily fatigued and 

 cease to conduct excitations to the muscles long before the muscle it- 

 self becomes too fatigued to respond to direct stimulation. A further 

 protection against fatigue in protracted muscular activity is afforded 

 by the fact that the threshold of excitation of the individual muscle 

 fibers is raised by repeated stimulation. It is quite possible that as 

 the result of this tendency each fiber may fail to respond to the nerve 

 impulses reaching it as soon as it becomes fatigued, and consequently 

 it has an opportunity to rest, while other fibers in the muscle carry on 

 the work in hand. The falling off in the height of the successive con- 

 tractions of a muscle which is being fatigued is probably due not only to 

 the reduction of the degree of contraction of which each fiber is capable, 



