Attenuation of muscle spindle firing with artificially increased series compliance during stretch of relaxed muscle

Abstract

Muscle spindles relay vital mechanosensory information for movement and posture, but muscle spindle feedback is coupled to skeletal motion by a compliant tendon. Little is known about the effects of tendon compliance on muscle spindle feedback during movement, and the complex firing of muscle spindles make these effects difficult to predict. Our goal was to investigate changes in muscle spindle firing using added series elastic elements (SEEs) to mimic a more compliant tendon, and to characterize the accompanying changes in firing with respect to muscle-tendon unit (MTU) and muscle fascicle displacements (recorded via sonomicrometry). Sinusoidal, ramp-hold-release, and triangular stretches were analyzed to examine potential changes in muscle spindle instantaneous firing rates (IFRs) in locomotor-and perturbation-like stretches as well as history dependence. Added SEEs effectively reduced overall MTU stiffness and generally reduced muscle spindle firing rates, but the effect differed across stretch types. During sinusoidal stretches, peak firing rates were reduced and IFR was strongly correlated with fascicle velocity. During ramp stretches, SEEs reduced the dynamic and static responses of the spindle during lengthening but had no effect on initial bursts at the onset of stretch. Notably, IFR was negatively related to fascicle displacement during the hold phase. During triangular stretches, SEEs reduced the mean IFR during the first and second stretches, affecting the history dependence of mean IFR. Overall, these results demonstrate that tendon compliance may attenuate muscle spindle feedback during movement, but these changes cannot be fully explained by reduced muscle fascicle length and velocity. New FindingsO_ST_ABSWhat is the central question of the study?C_ST_ABSLittle is known about the effects of tendon compliance on muscle spindle function. We asked whether increasing the series compliance the muscle-tendon unit muscle spindle Ia responses to stretch. We also test the relationship between muscle spindle firing rates and muscle fascicle biomechanics. What is the main finding and its importance?Muscle spindle firing was generally attenuated with added series compliance, with the exception of the initial burst at the onset of stretch. Overall, the changes depended upon stretch profiles, and could not be fully explained by muscle fascicle length and velocity.