Evidence for two distinct cross-bridge populations in tetanized frog muscle fibers stretched with moderate velocities

Measurement of muscle fiber stiffness during stretch with two continuous different velocities in skeletal muscle fibers

Ca2+ activated single glycerinated muscle fibers were stretched by 3% of its slack length Lo with moderate velocities ranging from 0.015 to 0.15Lo, and muscle fiber stiffness during and after stretch was measured by applying sinusoidal vibrations (peak-to-peak amplitude, 0.1% of Lo, 2kHz). During a single stretch, the fiber was first stretched by 1.5% with a velocity, and then it was further stretched by 1.5% with another velocity. It is suggested that, during stretch, a considerable fraction of cross-bridges slow down their cycling rate and increase their force-generating ability.