Purpose. The purpose of the study was to attempt to elucidate the spinal mechanisms which influenced the final motor output in the monosynaptic reflex during different body weight-bearing orientations between elderly and young subjects. It was also to examine the modulation of the soleus H-reflex through a conditioned stimulus to the Ia afferents of the femoral nerve, and to examine the modulation of the soleus H-reflex through double stimuli to the same tibial nerve.

Methodology. Fifteen elderly subjects (mean: 67.7, SD: 4.1) and 15 young subjects (mean: 22.8, SD: 2.5) were tested. Subjects were examined under two experimental conditioning – test paradigms: (1) femoral nerve conditioning to measure heteronymous Ia facilitation (HF); and (2) paired tibial nerve conditioning to measure homosynaptic depression (HD). Each conditioning – test protocol was evoked and measured on the soleus H-reflexes during six different postural orientations: (1) supine; (2) supine inclined by 30 degrees of back support; (3) standing with 50% of body weight-bearing orientation; (4) standing with 75% of body weight-bearing orientation; (5) standing with 100% of body weight-bearing orientation with and without vision; and (6) standing with 125% of body weight-bearing orientation with and without vision.

Conclusions. The elderly group progressively increased the soleus EMG activity while they increased their body weight-bearing orientations both with and without vision, whereas the young group did not. The elderly group did not modulate the H-reflex conditioned by HF across the different body weight-bearing orientations, whereas the young did. However, the elderly group significantly decreased the H-reflex conditioned by HD, whereas the young group did not. Based on the present results, with age the elderly group may relatively activate the corticospinal pathway during the standing position, and modify their spinal mechanism including inhibitory interneurons as compared with young group.