Figure 2. Human muscle spindle organs are not basic kinematic sensors.

(A) Averaged responses of a representative spindle afferent from the common digit extensor muscle, during active sinusoidal movements of a single finger at 1Hz (adapted from Figure 3 of Dimitriou, 2014). Movement was constrained to the metacarpophalangeal joint (MCP) and occurred under a flexion resistive or assistive torque load, or no external load. Standard classification tests identified the afferent as a typical spindle primary (i.e. ‘type Ia’; see Fig 2 in Dimitriou, 2014). Despite virtually identical finger flexion, spindle responses to stretch varied according to joint dynamics. (B) Averaged spindle afferent population responses and equivalent muscle length changes during the classic visuomotor rotation task (both ‘B’ and ‘C’ adapted from Figure 4A and 4E of Dimitriou, 2016). Grey background bars highlight phases in early adaptation (orange) that differ substantially from baseline (black). (C) Correlating the signals shown in ‘B’ (down-sampled at 50ms) confirmed a significant relationship in the washout stage. (D) Muscle velocity (null) and changes in spindle Ia responses before movement initiation in the classic instructed-delay reaching task with the hand. Ia firing rates from extensor muscles were lower when preparing movement to visual targets associated with stretch of the spindle-bearing muscle (purple). ‘D’ and ‘E’ are adapted from Figure 2B and 6A of Papaioannou and Dimitriou, 2021. (E) Averaged signals across participants; experiments using a robotic manipulandum showed a congruent goal-directed tuning of stretch reflexes, including at the short-latency epoch (‘SLR’) in cases where the homonymous muscle was not heavily loaded before perturbation. Color coding as in ‘D’.