Comparison of Heteronymous Monosynaptic Ia Facilitation in Young and Elderly Subjects in Supine and Standing Positions

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Abstract
Background The control of posture and balance is a primary concern among the elderly. Postural instability has been identified as a contributor to the greater incidence of falling among this segment of the population. One important neuromuscular mechanism identified as important in the control of posture and balance is the segmental reflex system. The purpose of this study was to examine the role of presynaptic inhibition in modulating the reflex system in young and elderly subjects. Methods To estimate the influence of body position on presynaptic inhibition to the soleus motor pool between young and elderly subjects, 11 young (mean age = 23.9 yrs.) and 9 elderly (mean age = 72.1 yrs.) subjects were examined in two different body positions: supine and standing. This study utilized the heteronymous facilitation protocol, as described by Hultborn et at. (1987). to estimate presynaptic inhibition of the la afferent pathway onto the soleus o-motoneuron pool. Maximal soleus H-reflex (H-max) and motor response (M-max) amplitudes were determined prior to testing at each condition, and the H-max/M-max ratio at each body position was determined. To estimate presynaptic inhibition at each body position, subjects received 24 test soleus H-reflex stimuli (~15% M-max), and 24 soleus H-reflexes conditioned by stimulation of the ipsilateral femoral nerve. Results Results demonstrated a significant decrease in H-max/M-max ratio from supine (66.1%) to standing (56.8%) for the young subjects, whereas the elderly subjects demonstrated no changes in the H-max/M-max ratio between body positions (39.8% supine; 39.8% standing). The conditioning stimulus produced a significant change in the test reflex for the young subjects during supine testing (51.1% increase) but not standing (3.4% increase). The elderly subjects demonstrated no significant changes in the test reflex produced by the heteronymous conditioning at either condition (17.6% increase supine; 4.9% increase standing). Conclusions These results demonstrate differential effects of both H-reflex modulation and heteronymous conditioning for elderly subjects when compared with young adults. These differences may be an adaptive phenomenon of the aging neuromuscular system, exemplified by a decreased ability to modulate the reflex system in the elderly group.