This study investigated the reflex control of postural sway during human bipedal stance. The experiments were designed to: (i) find evidence for the operation of 'stretch reflex' pathways during quiet stance, (ii) determine the bandwidth of the reflex response, (iii) describe the reflex transmission characteristics in standing subjects, and (iv) assess the ability of subjects to make a task-dependent change in the reflex. 2. A continuous random perturbation that did not threaten stability was applied at waist level to nine standing subjects. The effects of the perturbation on ankle torque, ankle movement and soleus electromyographic activity (EMG) were identified by cross-correlation. The bandwidth of the reflex response and the transmission characteristics of reflexes that respond to ankle movement were identified by spectral analysis. Changes in these reflex responses were investigated when subjects attempted to stand as still as possible, had their eyes closed, or balanced a load equivalent to their own body in a situation in which neither visual nor vestibular reflexes would be activated. 3. When standing, a reflex response coherent with the perturbation was seen in soleus EMG at frequencies up to 5 Hz, with maximal coherence at 1.0-2.0 Hz. Reflex gain increased with frequency, and there was a frequency-dependent phase advance of soleus EMG on ankle movement reaching 135 deg at 3 Hz. When attempting to minimize sway, subjects produced a more coherent reflex response and significantly increased reflex gain. 4. The response and transmission characteristics of the lower limb proprioceptive reflex in freely standing subjects were similar to those in subjects balancing a load at the ankle, a situation in which vestibular and visual inputs could not contribute. 5. It is concluded that reflex feedback related to ankle movement contributes significantly to maintaining stance, and that much of the reflex response originates from lower limb mechanoreceptors stimulated by ankle rotation. Although reflex gain may be relatively low during quiet stance it can be increased when necessary to maintain stability.
