114 



PHYSIOLOGICAL TRIGGERS 



ORIGIN OF THE SELF-EXCITED VIBRATION OF FIBRILLAR MUSCLE 



The extensive data obtained by Hill and his co-workers have been inter- 

 preted to show that muscle is a two-component system consisting of an un- 

 damped series elastic element and a contractile element. The elastic element 

 cannot be studied in the presence of the contractile element unless special 

 precautions are taken. Either the shortening of the contractile element must 

 be calculated and subtracted from the muscle shortening, or the muscle short- 

 ening must be so rapid that the contractile element does not have lime to 

 shorten. The elastic force component of the vibrating system might be identi- 

 fied with the action of this elastic element. In fibrillar muscle the elastic 

 element is quite stiff, requiring roughly a force of 2.5 kilograms to stretch it 

 I centimeter. If the line EF of figure iC is the tension-length relation of the 

 pure elastic element, deviations from these tensions are due to the presence of 

 the contractile element, which must generate the sustaining force by shorten- 

 ing and lengthening. The change in length of the contractile element in the 

 cycle is somehow controlled by the motion or the tension changes during the 



cycle. 



The changes in length of the elastic element EE and the contractile element 

 CE might occur in the following manner. Referring to figure iC, from F to G 

 the ('E is shortening while the EE is lengthening to reach its maximum value 

 when the tension is maximum at G. From G to H both EE and CE are short- 

 ening, CE reaching its shortest length at H. The CE lengthens from H to E 

 while the EE is shortening, and so the tension falls more rapidly than the 

 pure elastic tension, the slope of the line EF. At E the rate of shortening of 

 the EE equals the rate of lengthening of the CE and so the muscle reaches its 

 shortest length. From E to I the CE lengthens more rapidly than the EE 

 shortens, so the tension falls and the muscle lengthens. At I the EE reaches its 

 shortest length and begins to lengthen and so from I to H' both the CE and 

 the EE are lengthening. The CE begins to shorten at H' while the EE is 

 lengthening and at F the rates are equal and the muscle attains its greatest 

 length. 



The EE shortens and lengthens in the cycle in phase with muscle tension 

 (the greater the tension the longer the spring) and so lags behind muscle 

 length by the phase angle. The shortening and lengthening cycle of the CE 

 precedes muscle length by 90° and so is ahead of the EE by 90° -f <f. The 

 amount of shortening of the CE determines the phase angle between tension 

 and length by setting the value of the sustaining force. If the CE does not go 

 through a symmetrical cycle such as the one discussed above, the loops will 

 be irregular. Because the loading of the muscle is properly controlled during 

 flight, the loops are more symmetrical and larger than those obtained with the 

 isolated preparation. 



