Crossed-spinal conditioning of the soleus H-reflex: The effects of age and physical activity

The aging process results in several biological changes to the spinal motor system resulting in decreased neuromuscular function and deficits in motor performance such as reduced postural stability and increased fall-risk. One understudied mechanism in neural aging research is the role of physical activity in maintaining or restoring spinal cord reflex function, in particular the impact of physical activity on crossed-spinal reflexes and bilateral lower limb coordination during aging. Therefore, the purpose of this study was to examine the time-course modulation of the soleus H-reflex following a conditioning stimulation from the contralateral common peroneal nerve in both sedentary and highly physically active young and elderly adults. Each subject's physical activity level was determined by their responses to the International Physical Activity Questionnaire. To assess crossed-spinal reflex modulation, conditioning stimuli were delivered to the common peroneal nerve at the fibular head of the left leg (160 Hz train, 20ms duration at 1.2x motor threshold). This was followed by test stimulation to the posterior tibial nerve at the popliteal fossa in the right leg to evoke a soleus H-reflex (the control amplitude of H-reflex was adjusted at 50% of the maximal H response). The following conditioning intervals were examined: control (H-reflex only), 25 ms, 50 ms, 75 ms, 150 ms, and 300 ms. All testing was performed with the subjects in the prone position (non-weight bearing). A significant interaction was found between conditioning interval and age in which young subjects demonstrated H-reflex facilitation at 50 ms (19.3%) and elderly subjects demonstrated H-reflex facilitation at 150 ms (30.0%). Interestingly, in both elderly and young physically active subjects this reflex facilitation was not observed. It is concluded that the aging process results in impairment to crossed-spinal muscle afferent communication and that participation in physical activity contributes to greater crossed-spinal reflex stability in both the young and elderly, and could ultimately lead to greater postural control.