Restoration of frequency-dependent depression of the H-reflex by passive exercise in spinal rats

Study design: Hyper-reflexia, measured as a decrease of low frequency-dependent depression of the H-reflex, is known to occur in both humans and animals after spinal cord injury (SCI). Previous studies have shown that passive exercise for 3 months could be used to restore low frequency-dependent depression of the H-reflex after SCI. Objective: To determine the effects of various periods of time on the ability of passive exercise to restore low frequency-dependent depression of the H- reflex. Setting: Spinal Cord Injury Mobilization Program of the Center for Translational Neuroscience, the research arm of the Jackson T Stephens Spine & Neuroscience Institute, Little Rock, AR, USA. Methods: Adult rats underwent complete spinal cord transection at the T10 level. The hindlimbs were passively exercised in different groups of rats for 1 h/day, 5 days/ week for 15, 30, 45, 60, or 90 days, and low frequency-dependent depression of the H- reflex was tested. Results: Statistically significant low frequency-dependent depression of the H-reflex was evident by 30 days of exercise, although numerical reductions were seen even at 15 days. There was a linear decrease in low frequency-dependent depression of the H-reflex with duration of passive exercise. Conclusions: Passive exercise can restore frequency-dependent depression of spinal reflexes in a time-dependent manner if used following complete spinal transection.