The effects of circumferential pressure on the soleus muscle F-wave in healthy subjects
Circumferential pressure (CP) was shown to decrease muscle activity in subjects without neuromuscular disorders and in individuals with spinal cord injury and cerebrovascular accidents. The mechanism for this decrease is unknown although it is hypothesized to be spinal in origin. The purpose of this study was to investigate the effect CP has on the soleus F-wave. Results will help determine the mechanism CP uses to effect motoneuron reflex excitability. Thirty-seven healthy volunteers participated. A 16cm air-splint was placed around the calf and during the pressure phase of the experiment it was inflated to 40-45mm Hg. F-waves were evoked by supra maximally stimulating (20%>Mmax) the tibial nerve with a 0.1ms pulse at 0.2Hz using a bipolar surface electrode on the skin of the popliteal fossa. Fifty F-waves were recorded before (baseline), during, and 3) after CP was applied. F-waves were then identified and mean latency, persistence, and mean F/Mmax amplitude ratios were measured and calculated. Friedman Repeated Measures on Ranks tests were conducted on each of the three parameters (p⩽0.05). No statistically significant difference was found for any of the F-wave parameters evaluated. These results were contrary to previous CP studies that observed a significant decrease in muscle activity. Possible reasons for this discrepancy are discussed.
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Circumferential pressure (CP) applied to the lower leg reduces soleus motor neuron reflex excitability (MNRE); however, the mechanism of control is unknown.
To investigate the effect that CP has on disynaptic reciprocal inhibition (DSRI) and on Ia presynaptic inhibition (IaPI) of the soleus H-reflex in healthy subjects.
DSRI of soleus motoneurons and presynaptic control of soleus group Ia afferents were measured before, during and after CP was applied to the calf. Pressure was set to 40–45 mmHg. DSRI was evaluated by observing changes in the H-reflex amplitude after a conditioning stimulus was applied to the common peroneal nerve. IaPI was assessed using two separate protocols involving conditioning of the soleus H-reflex: femoral nerve facilitation (FNS) (heteronymous) and D1 and D2 inhibition (homonymous). A change in DSRI and IaPI was determined by comparing the Hpressure, Hpost-pressure phases to the Hpre-pressure phase of the conditioned H-reflexes.
A mean 12% decrease in FNS was observed during CP (p < 0.05). D1 and D2 inhibition decreased slightly. CP did not affect DSRI.
The results show that CP applied to the calf significantly increased heteronymous soleus IaPI, but affected homonymous IaPI less. It was concluded the CP does increase IaPI of soleus motoneurons but only modestly. The change was not large enough to explain the dramatic inhibition that occurs in the (unconditioned) H-reflex amplitude when CP is applied. Therefore, IaPI is not the primary inhibitory mechanism that CP uses to lower MNRE.
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