Is the Muscle Spindle a Length Sensor? A Model of Extrafusal Muscle Length Control Based on Intrafusal Muscle Force

Muscle spindles are described as encoding muscle length and velocity. However a study showed that, under certain conditions, muscle spindle afferents respond to force-related parameters rather than length, suggesting they may primarily detect muscle force. If muscle spindles respond to force rather than length, how do we still control muscle length? To address this question, we present a neuromuscular model that integrates the nonlinear dynamics of both extrafusal and intrafusal muscles and their interactions. Simulations show that if muscle spindles respond to force and activate extrafusal muscles via the alpha motor neuron, extrafusal muscle length can be regulated by intrafusal input through the gamma neuron, compensating for load changes. These findings support the idea that muscle spindles function as force receptors rather than length receptors, offering a new perspective on their neurophysiological role.