Tension and Extensibility Changes in Muscle Suddenly Stretched During the Twitch Response

Abstract

Changes in twitch tension and changes in extensibility in the triceps surae of the rat and in the sartorius of the frog were studied by application of quick stretch in the resting and in the active states of the muscles. Effects of quick stretch were similar in frog muscle and rat muscle at 25[degree]C. The relation of developed twitch tension to extensibility was not altered by cooling in rat muscle. The tension induced by small amts. of stretch (about 5% of the muscle length) brings about changes of twitch tension that are essentially the same as those brought about by changes of resting tension seen in a length-tension diagram. Under these conditions quick stretch produces an increase of twitch tension when the resting length of the muscle is shorter than that required for a maximal twitch. Quick stretch produces a decrease of twitch tension when the resting length of the muscle is equal to or longer than that required for a max. twitch. The combined tension of a twitch and a large amt. of quick stretch (9-17% of the muscle length) minus the tension of a stretch alone greatly exceeds max. twitch tension if the stretch is applied at lengths shorter than that required for a max. twitch. This difference is usually less than a max. twitch, if stretch is applied at lengths equal to or greater than that required for a max. twitch. It is suggested that the tension in excess of max. twitch tension, produced by large amts. of quick stretch in muscle allowed to shorten, is due primarily to increased passive resistance to stretch induced in elastic components in series with the contracting components. The effect of twitch contraction on extensibility of muscle is dependent upon the resting length. When muscle is allowed to shorten, extensibility is much smaller in the contracting muscle than in the resting muscle at the same length. When the resting length is equal to or greater than the length required for a max. twitch, the extensibility is about equal in contracting and in resting muscle at the same length. Extensibility at the peak of a max. twitch is greater than extensibility in resting muscle at the same tension.