When Ca2+ activated skeletal muscle fibers are stretched with a moderate velocity, the force developed above the level of isometric force starts to decay after a sudden reduction of stretch velocity. To investigate behavior of the cross bridges to reduction of stretch velocity, we measured muscle fiber stiffness. Glycerinated single fibers from the rabbit psoas muscle were isometrically attached (Lo: 2.4um) between force transducer and servo motor. Sinuoidal lenght change of 0.1% p-p Lo at 2 kHz was applied to the fibers. When isometric force reached the steady state, the fibers were stretched 1.5%Lo at the velocity of 0.35 Lo/s and then further stretched 1.5% Lo at 0.035Lo/s. Muscle fiber stiffness and force were simultaneously recorded. The force increased by the first stretch decreased transiently with reduction of stretch velocity, and increased slowly, and after the completion of stretch it caused stress relaxation. On the other hand, muscle fiber stiffness started to decrease by the first stretch and then increased in the late phase pf the first stretch, and it increases slowly during the following slow stretch, after completion of stretch it settled in the level lower than the stiffness level under isometric force. Although number of attached cross bridge decreases during the first stretch from the above results, it is found that when stretch velocity decrease suddenly, large change in number of attached cross bridges is not seen even if it generates comparatively large force by lock-in cross bridges. [J Physiol Sci. 2007;57 Suppl:S93]
