ED WARD (;. BOETTIGER II3 



lo shorten rai)i(lly, o. i 0.2 millimeiers, the tension falls to, or nearly to, zero 

 and then slowly returns to a level characteristic of the shorter length. This 

 behavior is apparently a property of all muscles and of the glycerinated psoas 

 preparation as well. In fibrillar muscle, however, as shown in ligure 3Z) follow- 

 ing the rapid shortening (quick release), the tension continues to fall even 

 though the muscle length is not changing. This fall in tension has not been 

 demonstrated in any other muscle. A still more dramatic difference in be- 

 havior is seen when fibrillar muscle is restretched. The full length is attained 

 with a return of only one-third to one-half the initial tension, lig. 3/I I'\)llow- 

 ing the com])leli()n of the stretch, the tension rapidly rises and may even ex- 

 ceed momentarily the level at the time of release. To show that this behavior 

 is not due to some property of the recording system, the changes in length 

 were repeated with the unstimulated muscle stretched passively to the same 

 tension found in the active muscle at the moment of release. The result is 

 shown in figure 7,C. In this control experiment tension and length are in phase. 



There is an optimum speed of stretch for the muscle which is related to the 

 normal operation frequency of the system. In the experiment shown in figure 

 7,B there is no break in the curve of tension rise to indicate the end of the 

 stretch. This stretch had a duration of 5 milliseconds. For a full cycle at this 

 rate the period would be 10 milliseconds and the frequency 100 per second, 

 which is the characteristic wing beat frequency in these bumble bees. If the 

 stretch is slower, the rise of tension is slower, a greater portion of the tension 

 rise occurring during the change in length. With more rapid stretches a definite 

 notch appears in the tension record at the end of the stretch. An extreme case 

 is seen in figure 3/I where the stretch was accomplished in 12 milliseconds. 

 The tension rises during the stretch to values near or above the isometric 

 tension, and then falls as quickly almost to the level at which the stretch 

 started. The subsequent rise in tension was erratic, as though some damage 

 had been done to the muscle. 



As Pringle (12) has pointed out, there are no adequate experimental results 

 to show changes in tension in a muscle restretched after a rapid shortening. 

 Attempts by us to repeat the above experiments on comparable non-tibrillar 

 muscle have not been fully satisfactory. However, the flight muscle of a moth 

 showed a raj)id rise in tension during restretching, the tension overshooting 

 the isometric level and then slowly returning to this level. As this behavior is 

 quite different from that observed in fibrillar muscle, one may conclude that 

 fibrillar muscle is unique in its response to release and restretch. Thus, in any 

 movement the tension changes lag behind the length changes. When the 

 movement is suddenly stopped, the tension change continues until it again 

 comes into phase with length. So after a rapid shortening the tension falls, 

 and after a rapid lengthening the tension rises. 



