The role of the suprasprinal center during soleus stretching reflexes with simultaneous vibration

[Purpose] We examined whether monoaminergic brain stem centers contribute to reflexive soleus (Sol) activity when vibration is applied to ankle joints on a moving platform. [Methods] Ten male subjects (23-35 years) stood with their eyes closed on a movable platform. Vibrators (92 Hz) were applied to the malleolus and Achilles' tendon. Sol electromyographic (EMG) responses of short- (SLR) and medium-latency reflexes (MLR) during platform movement were collected under the control, Sol vibration (SV), and malleolus vibration (MV) conditions. The SLR, MLR areas and their latencies were measured. [Results] The Sol SLR and MLR onsets were significantly delayed under the SV and MV conditions compared to the control condition. The intercept of the regression line under the MV conditions was significantly greater than under the SV condition. [Conclusion] Delays of SLR and MLR onset under the SV and MV conditions might correspond to the length of time required for temporal summation of α-motoneurons due to inhibition of afferent fibers. A rise in the intercept of the regression line under the MV condition means an increase of MLR area. That is, the monoaminergic brain stem centers compensated for stimulation of the group II interneuron via ankle joint afferents acting against the inhibition of the stimulation of Sol α-motoneurons.