Identification of time-varying dynamics of the human triceps surae stretch reflex. II. Rapid imposed movement.

We examined the time-varying dynamics of the human triceps surae stretch reflex before, during, and after a large stretch was imposed upon the ankle joint, during a constant voluntary contraction of 15% of maximum voluntary contraction. Stretch reflex dynamics were estimated by superimposing a small stochastic displacement on many such stretches and using an "ensemble-based" time-varying identification procedure to compute impulse response functions relating the perturbation to the evoked electromyogram (EMG) at each point throughout the task. We found that stretch reflex magnitude (relating joint velocity to EMG) varied directly with baseline EMG activity during steady-state conditions before and after the large imposed stretch. Following the large stretch and the reflex activity it evoked, both background EMG and stretch reflex magnitude declined for up to 100 ms; changes in the stretch reflex were substantially greater in magnitude and followed a different time course from the corresponding changes in background EMG, however, indicating that stretch reflex properties were modulated independently of motoneuron pool activation level. Based on timing and the invariance of stretch reflex dynamics across time, it is argued that this behavior is largely mediated via peripheral neural mechanisms. This peripheral modulation of the stretch reflex presumably supplements various descending influences to adjust reflex properties.