Modulation of stretch reflex excitability during 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 time intervals. Position, torque, gastrocnemius soleus EMG, tibialis anterior EMG, heel position, tibia angle, femur angle, and sacrum angle were measured. Activation level 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. Negative torque derivatives produced greater reflex excitation then their positive counterparts. These findings suggest that reflex excitability in quiet human standing is modulated to optimize balance.