Abstract

In addition to its role in maintaining an upper limit on motor unit firing frequency, Renshaw cell inhibition has been hypothesized to serve as an important mechanism in the regulation of force output during co-contraction. Renshaw cells have been shown to inhibit the La inhibitory interneurons, thus providing a disconnection of reciprocal inhibition. This action prevents a cumulative decrease in voluntary activation of the agonist and antagonist motor pools during co-contraction. Because of this role, Renshaw cell inhibition may play an important role in any postural control that requires co-contraction for joint or segment stabilization. In an effort to measure Renshaw cell activity impacting on the soleus motor pool, Pierrot-Deseilligny & Bussel (1975) piloted an indirect protocol in which a test H-reflex (H1) is conditioned by a maximal motor response. This conditioning stimulus results in a second H-reflex (H!) that when compared in a ratio to H1 provides an inverse representation of current Renshaw cell activity (i.e. the higher the ratio, the lower the Renshaw cell activity). PURPOSE: In an effort to evaluate the functional effect of Renshaw cell activity modulation, this study compared the results of this conditioning protocol under two conditions: supine and standing. METHODS: Subjects were 4 females, and 4 males (age: 20.3 ± 1.2 years). Test (amplitude = 20% M-max) and conditioned soleus H-reflexes were collected at both supine and standing conditions with foot angle and hip rotation standardized. Inter-stimulus interval was 10 ms with the test H-reflex preceding the conditioning stimulus. A repeated-measures t-test was used to assess differences in H!/H1 ratio between conditions. RESULTS: All subjects significantly depressed the H!/H1 ratio 57.5% ± 12.8% from supine to standing (t = 4.75, p < .05). H!/H1 ratios were 0.86 ± 0.24 during supine tests and 0.35 ± 0.08 during standing tests. CONCLUSION: Using the supine condition as a baseline, it is apparent that as the co-contraction requirement of the system increased to control static balance, Renshaw cell activity also increased. Since the static postural control of standing does not require an increase in soleus motor unit firing frequency it is unlikely that much of the increase in Renshaw cell activity is due to activation via recurrent motor axons. Therefore, this increase in Renshaw cell activity is most likely a central strategy allowing better control of motor output during a condition of sustained co-contraction. These results further support the role of Renshaw cells in postural control.

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