The importance of somatosensory information in triggering and scaling automatic postural responses in humans.

To clarify the role of somatosensory information from the lower limbs of humans in triggering and scaling the magnitude of automatic postural responses, patients with diabetic peripheral neuropathy and age-matched normal controls were exposed to posterior horizontal translations of their support surface. Translation velocity and amplitude were varied to test the patients' ability to scale their postural responses to the magnitude of the translation. Postural response timing was quantified by measuring the onset latencies of three shank, thigh, and trunk muscles and response magnitude was quantified by measuring torque at the support surface. Neuropathy patients showed the same distal-to-proximal muscle activation pattern as normal subjects, but the electromyogram (EMG) onsets in patients were delayed by 20-30 ms at all segments, suggesting an important role for somatosensory information from the lower limb in triggering centrally organized postural synergies. Patients showed an impaired ability to scale torque magnitude to both the velocity and amplitude of surface translations, suggesting that somatosensory information from the legs may be utilized for both direct sensory feedback and use of prior experience in scaling the magnitude of automatic postural responses.