SOLEUS H-REFLEX GAIN IN ELDERLY AND YOUNG-ADULTS - MODULATION DUE TO BODY POSITION

Background. The control of posture and balance in the elderly is a pri mary health concern. Postural instability directly leads to a greater incidence of falling in the elderly population. One important neuromus cular mechanism instrumental in the control of posture and balance is the reflex system. The purpose of this study was to examine the gain o f the soleus H-reflex in young and elderly adults in two different bod y positions: standing and prone. Methods. Eighteen neurologically heal thy volunteers were categorized by age in two groups: young (n = 9, me an age = 23.3 yr) and elderly (n = 9, mean age = 71.7 yr). In each pos ition, the resting H-max/M-max ratio was determined. The gain of the r eflex was also assessed by instructing the subject to perform voluntar y contractions of 10, 20 and 30% of their maximum voluntary contractio n, using real-time EMG biofeedback. Data were sampled on-line using cu stom designed software (sample rate = 2 kHz). Dependent variables incl uded the average background EMG of the soleus muscle (40 ms window pri or to stimulation) and the peak-to-peak amplitude of the elicited sole us H-reflex, To examine the gain of the reflex, the peak-to-peak ampli tude of the H-reflex was plotted against the background EMG activity f or each contraction intensity. Results. Results indicated the followin g: young subjects significantly depressed the H-max/M-max ratio when s tanding (69.3% prone, 55.1% standing), whereas elderly subjects increa sed the ratio (36.1% prone, 54.54% standing). Also, the young subjects modulated the gain of the reflex from prone to standing (3.30 prone, 3.68 standing), and the elderly subjects demonstrated no gain modulati on in the different body positions (2.23 prune, 1.91 standing). In bot h body positions the young subjects demonstrated significantly higher gain than the elderly subjects. Conclusions. The results demonstrate d ifferent control strategies for young and elderly subjects between pro ne and standing body positions.