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

1. The length changes of a central segment of a frog muscle fibre were measured during and after a quick shortening was applied to the end of the fibre, by attaching two markers an using a spot follower apparatus. In this way it was shown that the stiffness of tetanized single frog fibres as mounted in our apparatus was located predominantly in the sarcomeres, and that the ends were comparatively stiff. 2. The stiffness of tetanized frog single fibres at 0 degrees was measured by applying a small 4 kHz sinusoidal length change, and measuring the resultant tension change. This was done during the first few milliseconds after a quick release, and while the fibre was shortening at constant velocity. 3. 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 cross‐bridges. 4. The stiffness during the 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. 5. The slope of the stiffness‐force plot during the fast tension transient was consistently and significantly less than the slope of the 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 non‐linear end compliance. 6. 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.