Effects of age vestibular and visual systems on the soleus H-reflex

The vestibular system, visual and proprioceptive pathways provide information about control of posture, movement and balance. Loss of postural control directly leads to a greater incidence of falling in the elderly population causing serious health problems. One important neuromuscular mechanism instrumental in the control of posture and balance is the reflex system. However, the age-related changes of vestibular and visual systems and their relationship with the reflex system are not clear. The purpose of this study was to investigate the effects of age, the vestibular and the visual systems on the modulation pattern of the soleus H reflex. Seventeen neurologically healthy volunteers were categorized by age in two groups: young (n = 8, mean age = 22.1 ± 5.0 yr.) and elderly (n = 9, mean age = 59.3 ± 12.8 yr.). Maximal soleus H-reflex (H-max) and motor response (M-max) amplitudes were determined prior to testing at each condition while subjects were lying supine on a tilt table for standardization. Stimulation intensity was set to evoke a 5-10% M-wave on each trial. Participants received 5 test H-reflex stimuli in two conditions, static 60o and dynamic 60o on a tilt table. Both tilt conditions were performed with vision and no vision. A 3-way repeated-measures analysis of variance (ANOVA) 2 (groups: young/old) x 2 (condition: static/dynamic) x 2(vision: vision/no vision) was used to assess changes in H-reflexes. All data were expressed relative to the H-reflex amplitude at 0o static on the tilt table. The results showed a significant 3-way interaction ( p = .038). The old group showed greater H-reflex amplitude in the no vision condition at static 60o (vision:0.97; no vision:1.23) whereas in the young group less modulation was demonstrated in the same condition (vision:1.15; no vision:1.12). These results suggest in young subjects the vestibular system produced a suppression of the H-reflex with or without visual input; however, in the old group vision was necessary for this suppression. The interaction between the visual and vestibular systems as we age needs to be further explored.