112 



PHYSIOLOGICAL TRIGGERS 



fibrillar muscle made to shorten and lengthen at various frequencies by an 

 external agent. Unfortunately, the movement was not sinusodal due to me- 

 chanical limitations, and so the loops found were quite irregular. At lower 

 frequencies the tension was greater on lengthening and smaller on shortening, 

 the cycle absorbing energy. With increasing frequency the phases gradually 

 reversed so higher tensions occurred during the shortening phase, the muscle 



|i25 m»«c.-i| 



\j.f^ 



Fig. 3. Effect of quick release and quick stretch on tension of the isometrically tetanized 

 fibrillar muscle preparation. A, 1-2 msec, stretch; B, a 5-msec. stretch; C, control unstimulated 

 muscle stretched to same tension as in B\ D, delayed restretch showing loss of tension at end 

 of release. 



now doing work against the driving force. This shift amounts to a change in 

 the phase angle from positive to negative. The crossover point did not occur 

 at the same frequency in all preparations, and with the crude experimental 

 set-up available, quantitative analysis was not possible. 



One can further investigate the shortening and lengthening phases by releas- 

 ing and restretching a tetanized ])reparation in isometric contraction. The 

 results of such experiments are shown in figure 3. When the muscle is allowed 



