The timing and the amount of agonist facilitation and antagonist inhibition of varying ankle dorsiflexion force in man.

Experiments were conducted to investigate whether or not reciprocal descending commands occur simultaneously in the brain as well as the phasic changes in the agonist facilitation and the antagonist inhibition involved in the initiation of varying forces of ankle dorsiflexion. The H-reflex and reaction time method were combined. Results showed that: (1) the agonist and the antagonist motoneurones via Ia inhibitory interneurones simultaneously receive a subliminal facilitation and inhibition from the brain before the EMG onset; and (2) the onset of the agonist facilitation and the antagonist inhibition were not modified by different contraction forces; however, the extent of facilitation and inhibition just after the EMG onset were closely related to the amount of contraction forces. These findings suggest that the descending motor command activates parallel agonist alpha-motoneurones and Ia inhibitory interneurones. However, direct action of the descending command prior to the EMG onset may not be strong enough to drive the Ia interneurone for suppressing the antagonist motoneurones, and the agonist facilitation and the antagonist inhibition just after the EMG onset increase as the voluntary effort is strengthened.