System identification of human stretch reflex dynamics: tibialis anterior.

System identification methods have shown that the stretch reflex in the human ankle extensor (triceps surae, TS) may be modeled as velocity feedback via a single, short latency pathway containing a uni-directional rate-sensitive nonlinearity. Evidence of differences in the reflex organization in flexors and extensors led us to use the same methods to examine stretch reflexes in the ankle flexor (tibialis anterior, TA). Five normal subjects maintained a tonic contraction of TA while subjected to repeated, stochastic perturbations of ankle position. Position, torque, and smoothed, rectified EMGs from TA and TS were recorded and ensemble averaged over 25 stimulus presentations. Linear impulse response functions relating TA EMG to ankle velocity were then determined. The results confirmed the existence of significant differences between stretch reflexes in TA and TS. In particular: 1. TA impulse response functions were characterized by two distinct peaks of excitation separated by a period of inhibition. The impulse response function amplitude increased with increasing mean torque and decreased with displacement amplitude although the sensitivity of the two peaks to these changes was different. This was interpreted as suggesting that TA stretch reflexes involve two separate pathways both involving muscle spindle information. 2. TA stretch reflexes modulate tonic activity much less than do those in TS. 3. The strong uni-directional rate-sensitive nonlinearity seen in TS was not present in TA. The functional significance of these differences remains to be determined.