Decrease in stiffness during shortening in calcium activated skinned muscle fibers

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Abstract
Single fibers from frog sartorius or semitendinosus muscle were mechanically skinned and activated in ATP salt solution containing 10 μM Ca2+ (7°C). After development of an isometric contraction, fibers were released at constant speed (0.03–2.4s−1). During ramp shortening, stiffness was determined from the slope of the tension-length diagram obtained during superimposed quick stretches. Both force and stiffness decreased, as the ramp shortening proceeded and approached a steady value after about 60 ms. An increase in speed of shortening caused a decrease in fiber tension and stiffness and an increase in the ratio of stiffness to tension, suggesting a decrease in both the number of attached crossbridges and in the average force per crossbridge.