A postural reflex evoked by brief axial accelerations.

Impulsive stimuli were used to evoke postural reflexes in healthy subjects (n = 10) and avestibular patients (n = 2). Electromyographic (EMG) activity was recorded with subjects standing erect, feet together with eyes closed and leaning forward to activate their leg muscles. EMG was recorded bilaterally from over the soleus muscles, rectified and averaged. The evoked response was biphasic and consisted of a short latency (SL) and a smaller medium latency (ML) response, at approximately 60 and 80 ms post-stimulus, respectively. Reflex amplitude was expressed as a percentage of the prestimulus level. Stimulus rise times of 10 and 14 ms evoked the largest responses. SL responses were largest when stimulating at the vertebra prominens (C7) or over the mid-thoracic region (C7: 56 ± 42%, mid-thoracic: 56 ± 51%). Positive (push) stimuli evoked larger responses than negative stimuli which evoked an initial inhibition (C7 positive: 56 ± 42%, C7 negative: -12 ± 5%). Similarly, increasing intensity also produced larger responses (standard intensity: 56 ± 42%; increased intensity: 80 ± 53%). There was no apparent EMG response in the sitting condition, despite activation of calf muscles, indicating the reflex was dependent on posture. Avestibular patients had well-formed responses similar to normals (patients: SL = 137.5 and 84.5%, ML = -11.6 and -16.2%) with mildly prolonged latencies. This study has described the basic properties of a reflex evoked by impulsive accelerations of the trunk that we propose is not primarily dependent upon vestibular afferents and is likely to have a role in normal postural stability. A truncal origin of the receptors mediating the response best fits our observations and is an alternative explanation for some previous reports about postural reflexes in response to limb displacements.