FATIGUE OF VOLUNTARY MUSCULAR CONTRACTION 561 



A typical tracing, figure 326, shows that as the strength of the stimulus is 

 increased the amplitude of the contractions quickly mounts from the minimal 

 to a maximal, after which all further increase in the strength of the stimulus 

 produces contractions of practically the same height. The first perceptible 

 contraction is called the minimal contraction, the strength of the current 

 which produced it is a minimal stimulus for that preparation. The contrac- 

 tions of the greatest amount are called maximal contractions. The weakest 

 stimulus which produces a maximal contraction is called the maximal stimu- 

 lus, and all stronger stimuli supramaximal. 



FIG. 352. The Type of Contractions given by the Gastrocnemius of the Frog to a 

 series of stimuli occurring at regular recurrent intervals. (Taskinen.) 



7. The Effect of Fatigue on the Amplitude of a Series of Simple 

 Muscle Contractions. #, Arrange the recording apparatus and set the 

 induction coil for single stimuli. Adjust the recording lever of the muscle 

 to a smoked-paper kymograph and set the speed of the kymograph to 

 revolve at the rate of i mm. per second. 



b. Prepare a gastocnemius muscle for direct stimulation and mount it in 

 a moist chamber. 



r. Now stimulate the muscle with the make induction (short-circuiting 

 the break) once every two seconds. The contractions will be recorded as 

 vertical marks on the drum in regular order, at a distance of 2 mm. apart, 

 hence very slight changes in amplitude are readily detected. The contrac- 

 tions gradually increase in height for the first ten or twenty contractions, the 

 phenomenon of treppe, then run for from fifty to one hundred contractions 

 of practically uniform amplitude, after which there is a gradual but sharp 

 decrease known as fatigue. Repeat on the second gastrocnemius. 



d. Repeat the experiment after ten minutes' rest. The former varia- 

 tions occur now very rapidly, indicating that the fatigue effects are only 

 partially recovered from. 



8. Fatigue of Voluntary Muscular Contraction, Demonstration. 

 The human voluntary muscles are used to demonstrate this experiment. 

 Use a Mosso's ergograph, or any one of its numerous modifications. If the 

 original form is used, then the muscle should be loaded with about 3 kilos, 

 and contractions once a second recorded until the muscle can no longer lift 

 the load. The load may have to be adjusted to the individual, but should be 

 chosen so that exhaustion will be obtained with about thirty contractions. 



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