Inhibition of Group Ia Afferents Between Brachioradialis and Flexor Carpi Radialis in Humans: A Study Using an Electromyogram-Averaging Method

PURPOSE: Our previous studies using a poststimulus time histogram method demonstrated inhibitory spinal reflex arcs (inhibition) between the brachioradialis (BR) and flexor carpi radialis (FCR) in humans. Group I afferents mediated the inhibition through an oligosynaptic path. In this study, effects of the inhibition on excitability of the motoneuron pools were examined, and we tried to clarify which afferents of group Ia or Ib are responsible for the inhibition. METHODS: We evaluated the effects of low-threshold afferents between BR and FCR on FCR and BR motoneuron pools, respectively, using an electromyogram-averaging method in 14 healthy human subjects. Changes of rectified and averaged electromyogram of BR by electrical conditioning stimulation with the intensity below the motor threshold to the median nerve branch innervating FCR (FCR nerve) and those of FCR by the stimulation to the radial nerve branch innervating BR (BR nerve) were evaluated. RESULTS: The stimulation to the FCR and BR nerves produced an early and significant trough of rectified and averaged electromyogram of BR and FCR, respectively, in all the subjects. The amount of inhibition of BR and FCR was 13.2 ± 3.4% (mean ± SD) and 14.2 ± 1.4%, respectively. The trough of BR and FCR diminished by tonic vibration stimuli to a respective FCR and BR. Such a trough was never provoked by cutaneous stimulation. CONCLUSIONS: The inhibition between BR and FCR depresses excitability of the FCR and BR motoneuron pools, respectively. Group Ia afferents should mediate the inhibition.