Isometric relaxation of striated muscle

¹ National Institute of Neurological Diseases and Blindness, National Institutes of Health, Bethesda, Maryland

Isometric relaxation was studied in rat peroneal muscle after supramaximal stimulation of the muscle along its entire length. There appear to be three phases of relaxation. First is a period of slow tension decay which becomes more prominent between twitches and full tetani, but then seems independent of tension and fatigue. This phase might represent retardation of relaxation. After an inflection, the relaxation curves decay exponentially. This second phase is unchanged by several experimental conditions; at 36 C, the exponential decay factor has a mean value of 55 (with time in sec). When the muscle is greatly stretched, graphical analysis reveals another inflection and a slower, third phase of relaxation. With this exception, stretching and fatigue have little or no effect on the time course of isometric relaxation. Analysis of the second and third phases suggests passive restoration of a stretched viscous elastic system.