Local muscle pressure stimulates the principal receptors for proprioception

Proprioception plays a vital role in motor coordination and self-perception. Muscle spindles are considered the principal receptors for proprioception and are believed to encode stretch and signal limb position and velocity. Here, we applied percutaneous pressure on a small area of extensor muscles at the forearm while simultaneously recording spindle afferent responses, skeletal muscle activity and hand kinematics. Three target levels of press force were applied periodically on the spindle-bearing muscle when the hand was immobile ('isometric' condition) and when the participant's hand moved rhythmically about the wrist in the flexion-extension plane ('sinusoidal' condition). We show that local muscle pressure is an 'adequate' stimulus for human spindles in isometric conditions, and that pressure enhances spindle afferent responses during stretch, as hypothesized to occur due to compression of the spindle capsule. Our findings redefine basic muscle feedback and urge a reconsideration of its role in sensorimotor function and various neuromuscular pathologies. ### Competing Interest Statement The authors have declared no competing interest.