Modulation of stretch reflex excitability in quiet human standing

Stretch reflex excitability was measured during quiet standing by using a bilateral electro-hydraulic actuator to apply perturbations of angular position to the ankle. Subjects were instructed to stand quietly while pulse displacements were applied at random times. Position, torque, gastrocnemius-soleus EMG, tibialis anterior EMG, heel position, tibia angle, femur angle, and sacrum angle were measured. Background torque and reflex excitability varied substantially from trial to trial---reflex torque decreased as the background torque level increased; while reflex EMG increased when background torque increased. This behavior is consistent with previous findings in prone subjects. Reflex torque for a given activation level was found to vary with the initial torque derivative---reflex excitation was greater for negative torque derivatives than their positive counterparts. These findings suggest that reflex excitability in quiet human standing is modulated to optimize balance