漸増的筋力発揮と持続的筋力発揮の中枢性制御機構 : 磁気刺激法(TMS)による解析

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Although numerous reports of various muscle contraction modes, for example isometric and isotonic ones, were accumulated, it is still not clear whether neurophysiological mechanisms related to these muscle contraction modes are same or not. In particular, it is not well known how the isometric grading (dynamic) and tonic (static) muscle contractions are controlled in spite of the same muscle contraction mode. Thus, using transcranial magnetic stimulation (TMS) in the present study, we investigated differences of excitability changes between these two different muscle contraction modes. In the first experiment, using low and high TMS intensities, motor evoked potentials (MEPs) between fast (50% MVC at one second) and slow (50% MVC at three seconds) dynamic muscle contractions were examined. Additionally, active MEP thresholds were also examined between fast dynamic and static muscle contraction modes. In the second experiment, we investigated excitability changes induced by seven different TMS intensities between dynamic and static muscle contractions. That is, we calculated MEP amplitudes/background EMG activities (MEP/B.EMG) ratio and compared them between these two different muscle contractions. The present results were described as follows : 1) MEP amplitudes in dynamic muscle contraction were significantly larger than those in static one at 10% MVC level, 2) however, these differences disappeared at 30% MVC level, 3) there were no differences of MEP amplitude between slow dynamic and static muscle contractions, 4) the active MEP threshold in fast dynamic muscle contraction was lower than that in static one, and 5) MEP/B.EMG ratios in dynamic muscle contraction were larger than those in static one under weak TMS intensities(1.1-1.4xth). Based on these results, it might be presumed that at low muscle contraction levels, subliminal fringe of spinal motoneurons in dynamic muscle contraction is larger than that in static one. Because discharge rate of human muscle spindle activity increases linearly with muscle contraction speed and consequently susceptibility of spinal motoneuron pool might be different in proportion to muscle spindle afferents induced by different muscle contraction modes.
日本運動生理学会の論文
2005-09-30

漸増的筋力発揮と持続的筋力発揮の中枢性制御機構 : 磁気刺激法(TMS)による解析

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