FATIGUE OF MUSCLE. 579 



which is stationary during the latent stage, but in molecular processes that are 

 responsible also for the process of negative variation in the current. 



FATIGUE OF MUSCLE. 



The term fatigue is applied to that condition of diminished functional 

 capacity in which the muscle is placed as a result of prolonged activity. 

 This condition is recognized during life by a peculiar sensory perception 

 localized in the muscles. In the intact body the fatigued muscle is 

 capable of recovery, as is also the excised muscle to a slight degree. A 

 muscle is more readily fatigued than its motor nerve. 



The cause of fatigue is the accumulation in the muscular tissue of the 

 products of metabolism, fatigue-bodies, that are formed as a result of mus- 

 cular activity. Among these products are : phosphoric acid, free or com- 

 bined in acid salts; acid potassium phosphate; glycerin-phosphoric 

 acid ( ?) ; and carbon dioxid. The accuracy of the foregoing explanation is 

 indicated by the fact that the fatigued muscle becomes again more capa- 

 ble of activity if the substances named are washed away by the passage 

 of a normal solution (0.6 per cent.) of sodium chlorid or of a weak solu- 

 tion of sodium carbonate through the blood-vessels of the muscle. The 

 consumption of oxygen on the part of the active muscle also promotes 

 fatigue; for the passage of arterial (but not venous) blood through the 

 vessels removes the fatigue by replacing substances that have been con- 

 sumed by the muscle. Conversely, a muscle that is capable of activity 

 may be rapidly fatigued by the injection of dilute phosphoric acid, acid 

 potassium phosphate, or dissolved meat-extract into its vessels. An 

 animal may be fatigued also by the transfusion of blood from a com- 

 pletely fatigued animal. A muscle fatigued by work absorbs less oxy- 

 gen while in this condition, and it also generates only a small additional 

 amount of acid and of carbon dioxid. The activity that gave rise to 

 fatigue has thus induced considerable metabolic activity in the muscle. 



The fatigued muscle requires a stronger stimulus than the fresh 

 muscle in order to perform the same amount of work, that is, to lift a 

 weight the same distance. The fatigued muscle is no longer able to raise 

 heavy weights; its absolute muscular energy is therefore diminished. 

 If the muscle is loaded with the same weight throughout the experiment, 

 and if the stimulus is a maximal one (strong induced opening shock), 

 then, from one contraction to the other, the height of the lift steadily 

 diminishes by a constant fraction of the shortening. The fatigue-trac- 

 ing is, thus, a straight line. The more rapidly the contractions follow 

 one another, the more marked is this diminution in the height of 

 the lift, and conversely. The excised muscle becomes fatigued to the 

 point of exhaustion after a certain number of contractions. Under 

 such circumstances it is a matter of indifference whether the stimuli 

 follow one another in rapid or in slow succession. Analogous conditions 

 are also observed in connection with submaximal stimuli. 



The fatigued muscle requires, further, a longer period of time for its 

 contraction, which, therefore, takes place more sluggishly. Finally, 

 the period of latent stimulation is also lengthened in a state of fatigue. 

 The fatigued muscle is said to be more extensible. 



If the muscle is loaded with a weight so heavy that it cannot be 

 lifted at all when contraction takes place, the muscle, nevertheless, be- 

 comes fatigued, and, indeed, in a still higher degree than if it were able 



