Delay of twitch relaxation induced by stress and stress relaxation

¹ Department of Physiology, University of Louisville School of Medicine, Louisville, Kentucky

The triceps surae of male rats was prepared in situ for isometric and isotonic recording and for stimulation through the cut sciatic nerve. Relaxation in isometric twitches is markedly delayed in muscle showing high resting tension. The time course of the delayed twitch relaxation is similar to that of stress relaxation in quickly stretched resting muscle. Delay of twitch relaxation is exaggerated in experiments with quick release and foreloading. The findings led to the following conclusions: a) the parallel elastic elements, in muscle with resting tension, lose stress during shortening of contractile elements and regain stress during relaxation of contractile elements. b) The exaggerated delay of twitch relaxation, produced in quick release experiments, is due to increased stress and stress relaxation made possible by enhanced release from stress in the preceding contraction phase of the twitch. c) Stress relaxation in parallel elastic elements accounts for the observed delay of twitch relaxation in muscle with resting tension.