1996 年 13 巻 p. 195-204
The feasibility of a self-organized change of stretch reflex gain caused by the passive change of the ankle joint angle was discussed in the human soleus muscles. The amount of the maximal amplitude of Hoffman (H) reflex, normalized by the maximal direct motor (M) response, was used to evaluate the excitability of the soleus motoneuron. The H reflex amplitudes in 1) normal standing, 2) ten degree dorsiflexion, 3) ten degree plantarflexion, were measured in standing posture, and also in sitting posture with the same ankle joint angles, in ten neurologically intact subjects. The normalized H-reflex amplitude was decreased from 38 percent to 32 percent by dorsiflexion and increased to 44 percent in sitting posture. The amplitude in standing posture was also decreased 41 percent to 39 percent by dorsiflexion and increased 42 percent by plantarflexion. It was suggested that the motoneuron pool is inhibited by passive dorsiflexion and facilitated by passive plantarflexion. In 24 of the 30 inhibition by dorsiflexion was indicated in sitting posture; however, only 17 indicated the inhibition in standing posture. In 25 of the 30 facilitation by plantarflexion in sitting posture was indicated; however, only 16 indicated facilitation in standing posture. The tendency of the inhibition and facilitation was obvious in the sitting posture but not clear in the standing posture. Therefore, the change of the reflex loop gain appears to have no relation to the higher center control system such as that for postural control, and seems to be a local self-organized adaptation. Two hypotheses were proposed: 1) in feedback gain compensation, the change is for compensation of the muscle spindle sensitivity change, 2) in stiffness compensation, the change is for compensation of the nonlinearity of the joint stiffness. Measurement of the response to the mechanical perturbation is required to obtain answers for these hypotheses.