Voluntary movements are believed to undergo preparation before they are executed. Preparatory activity can benefit reaction time and the quality of planned movements, but the neural mechanisms at work during preparation are unclear. For example, there are no overt changes in muscle force during preparation. Here, using an instructed-delay manual task, we demonstrate a decrease in human muscle afferent activity (primary spindles) when preparing to reach targets in directions associated with stretch of the spindle-bearing muscle. This goal-dependent modulation of proprioceptors began early after target onset but was markedly stronger at the latter parts of the preparatory period. Moreover, whole-arm perturbations during reach preparation revealed a modulation of stretch reflex gains (shoulder and upper arm muscles) that reflected the observed changes in spindle activity. We suggest that one function of central preparatory activity is to tune muscle stiffness according to task goals via the independent control of muscle spindle sensors.

人们认为，自愿运动在执行之前要经过准备。准备活动可以提高反应时间和计划运动的质量，但在准备过程中起作用的神经机制尚不清楚。例如，在准备过程中，肌肉力量没有明显的变化。在这里，使用指令延迟手动任务，我们展示了在准备到达与主轴轴承肌肉拉伸相关的方向上的目标时，人类肌肉传入活动（主要纺锤体）的减少。这种依赖于目标的本体感受器调节在目标开始后早期开始，但在准备期的后期明显更强。而且，伸展准备期间的全臂扰动揭示了牵张反射增益（肩部和上臂肌肉）的调节，这反映了观察到的纺锤体活动的变化。我们建议中枢准备活动的一项功能是根据任务目标通过肌梭传感器的独立控制来调整肌肉僵硬。