Stiffness of active smooth muscle during forced elongation

The stiffness of isometrically contracting mesotubarium superius and ovarian ligament smooth muscle from estrous female rabbits was measured continuously by using sinusoidal length perturbations (at 80 Hz, less than 15 microns peak to peak). Muscles were stimulated with alternating current fields, and all records were digitized using a microcomputer system. Phase-angle data were used to resolve computed stiffness into elastic and viscous components. Stiffness measurements were continued during long ramp-type stretches (up to 25% of muscle length) delivered as soon as force was maximal. To use the period of isometric tension development as a standard for comparison, the expected stiffness was computed during the long stretch. Stiffness was reduced in approximate proportion to the ramp stretch rate, and the reduction was confined largely to the elastic component. Cooling the muscle increased the stiffness deviation at a given stretch rate. It is proposed that the long stretch detaches cross bridges that can reattach to new sites as myofilaments shear past one another. At higher shearing speeds, less time is available for reattachment and stiffness is further reduced.