Selective facilitation of short latency postural reflexes by instability.

We sought to determine if enhancement of postural reflexes occurs specifically in response to instability induced by leaning and the possible neural level at which this may be mediated. Healthy participants (n = 10; 37 ± 15 years) were asked to adopt leaning postures to increase postural instability. Recordings of postural, eye and neck muscles and from over the cerebellum were made during stable upright stance and with the feet plantarflexed or dorsiflexed. The latter postures were used to modulate tonic activity levels in the soleus and tibialis anterior (TA) muscles to a similar degree to leaning. External perturbations were delivered to the trunk [vertebra prominens (C7) and upper sternum] and at the mastoids. C7 stimulation produced larger short latency (SL) reflex amplitudes in soleus during forwards lean than upright stance, when using matched tonic levels, although SL amplitudes in TA in response to sternal stimulation were unaffected when tonic activity was matched. Cerebellar evoked responses were larger during anterior than posterior lean for truncal stimuli. Mastoid stimulation generally evoked larger responses in the legs during lean than during upright stance with matching tonic EMG levels. Vestibular and non-vestibular responses recorded from the sternocleidomastoid muscles were often larger during posterior lean, independent of the direction of mastoid stimulation. Short-latency vestibular evoked cerebellar responses and extraocular responses were unaffected by posture. Facilitation of postural reflexes by instability occurred for axially-evoked postural reflexes in soleus but not TA. We propose that this effect is mediated by modulation of descending brainstem projections.