Environmental changes in soleus H-reflex excitability in young and elderly subjects.

The purpose of this study was to examine the role of task complexity on soleus H-reflex modulation in young and elderly subjects. Twelve young (mean age = 29.2yrs) and 10 elderly (mean age = 75.1 yrs) apparently healthy individuals were examined under four experimental conditions: (1) eyes open on normal floor surface; (2) eyes open on foam floor surface; (3) eyes closed on normal floor surface; and (4) eyes closed on foam surface. Under each condition, soleus H-reflexes, and background electromyography (EMG) (40 ms window prior to stimulation in both the soleus and the tibialis anterior) were recorded. Postural sway in each condition was also assessed using a Kistler force platform. Results indicated differential reflex modulation between young and elderly subjects. Specifically, young subjects depressed the amplitude of the reflex as task complexity was increased. Elderly subjects, on the other hand, increased the amplitude of the H-reflex when utilizing visual cues, but decreased H-reflex amplitude when vision was occluded. Postural sway in both groups increased as task complexity was increased. There existed a significant difference in the relationship between tibialis anterior activation and soleus H-reflex between the two groups. These results demonstrate differential reflex adjustments between young and elderly subjects as task complexity is increased, and may provide useful information pertaining to postural control in the elderly.