Variation of muscle stiffness with tension during tension transients and constant velocity shortening in the frog.

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Abstract
The length changes of a central segment of a frog [R. temporaria] muscle fiber were measured during and after a quick shortening was applied to the end of the fiber by attaching 2 markers and using a spot follower apparatus. In this way it was shown that the stiffness of tetanized single frog fibers as mounted in the apparatus was located predominantly in the sarcomeres and the ends were comparatively stiff. The stiffness of tetanized frog single fibers at 0.degree. C was measured by applying a small 4 kHz sinusoidal length change and measuring the resultant tension change. This was done during the 1st few milliseconds after a quick release and while the fiber was shortening at constant velocity. The stiffness during the fast tension transient after a quick release was always less than the stiffness before release, supporting the idea that the fast recovery is not due to attachment of extra crossbridges. The stiffness during steady shortening was always less than when isometric. A line fitted to this stiffness-tension plot, when extended, intercepted the stiffness axis at less than half the isometric value. The slope of the stiffness-force plot during the fast tension transient was consistently and significantly less than the slope of stiffness-force plot during steady shortening, further supporting the conclusion that only a small part of the decrease seen during shortening could be due to nonlinear end compliance. Possible ways of reconciling these results with recent reports of x-ray diffraction suggesting little if any change in the position of myosin heads during steady shortening are discussed.