Reciprocal inhibition of proprioceptive reflexes in man revealed by a threshold shift technique

Variations in soleus H-reflex threshold following single conditioning stimuli to tibialis anterior afferents were studied in man by probit analysis of quantal electromyographic responses. The H-reflex threshold was elevated in 6 out of 7 subjects (conditioning-test interval, 2 ms) when conditioning strengths were greater than about 0.70 X tibialis anterior motor threshold: the degree of elevation was exponentially related to conditioning strength. Analysis of response variability indicated that in some cases weaker conditioning was also effective. These findings are consistent with the view that reciprocal inhibition effectively modulates the threshold, rather than the amplitude, of proprioceptive reflex discharge.