Changes in twitch tension induced by quick stretch 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 recording and for stimulation through the cut sciatic nerve. A study was made on changes of twitch tension induced by quick stretch, stress relaxation and slow stretch. Quick stretch applied immediately before the twitch response produces changes of twitch tension similar to those obtained by slow stretch. Stress relaxation after quick stretch is accompanied by twitch tension greater than that obtainable after slow stretch at all lengths of the muscle observed. The amount of increase of twitch tension induced by stress relaxation is large below the optimum length and small beyond the optimum length of the muscle. When stress relaxation is virtually complete, this increase of twitch tension disappears, the magnitude of the twitch being thereafter equal to that obtainable by slow stretch. It is suggested that the increase is due to a dynamic state of the length-resting tension relationship during stress relaxation when length is increasing and tension is decreasing. Disappearance of the increase of twitch tension when length-resting tension equilibrium is established after stress relaxation supports this suggestion.