Effect of Air-Splint Pressure on the Soleus Stretch Reflex during a Voluntary Ramp Plantar Flexion

Circumferential pressure (CP) applied to the limb has been shown to decrease muscle activity in subjects without neuromuscular disorders and in individuals with spinal cord injury and cerebrovascular accidents. Thus far, studies estimating the CP efficacy with respect to reflex excitability of motoneurons mainly used the H reflex technique on a resting muscle. The purpose of our study, therefore, was to investigate the effect that CP exerts on the soleus stretch reflex (SSR) when superimposed onto a voluntary ramp plantar flexion movement in subjects without neuromuscular disorders. Forty-eight subjects volunteered for this study. SSRs were investigated before, during, and after the application of pressure to the calf. An inflated air-splint connected to a pressure transducer was used to administer and measure the pressure set to 45-50 mm Hg. The SSRs were elicited by dorsiflexing the subject's ankle by 10 deg at 180 deg/sec, while the subject plantarflexed against a moving footplate at 20% of the maximum voluntary contraction through a 30 deg arc at 90 deg/sec. Twenty-five SSRs were recorded and averaged for each experimental phase; peak-to-peak amplitudes were measured and normalized, and reflex latencies were also measured. Friedman Repeated Measures Analysis of Variance on Ranks was used to analyze the differences in the SSR latency and amplitude from the baseline values. No significant general difference in the SSR amplitude was found during pressure application, although individual responses varied widely. The post-pressure values returned to the baseline, and the differences were insignificant. The reflex latencies were also unchanged with respect to the baseline levels. Thus, the CP inhibitory effect on reflex excitability of motoneurons is mild, on average, and variable when a voluntary movement is a condition. The CP technique may not be as efficacious in reducing muscle hyperactivity as was previously thought.