EXPERIMENTAL NEUROLOGY loo, 265-273 (1988) Coexistent Hoffmann Reflexes in Human Leg Muscles Are Commonly Due to Volume Conduction ROBERTS.HUTTON,ROLANDR. ROY, ANDV.REGGIEEDGERTON' Department of Psychology, University of Washington, Seattle, Washington 98195; and Brain Research Institute and Department of Kinesiology, University of California, Los Angeles, California 90024 Received December I I, 1986; revision received September 30, I987 The existence of “concomitant” (coexistent) electromyographic reflex responses in soleus and tibialis anterior muscles, produced by posterior tibia1 nerve stimulation, has been cited as evidence for “reciprocal excitation” between these antagonistic mus- cles normally reflexly linked by reciprocal inhibition. Using the Hoffmann reflex pro- cedure and posterior tibial nerve stimulation, the existence of true reciprocal excita- tion was tested in six subjects with no neuromuscular impairment. Coexistent EMG responses were observed in all subjects. In no instance, however, could the tibialis anterior EMG volley to posterior tibia1 nerve stimulation of the soleus muscle be antidromically blocked by common peroneal nerve stimulation applied at 10 to 20 ms offset latencies. A second stimulation pulse applied to the common peroneal nerve at similar offset latencies did antidromically block a tibialis anterior reflex response to common peroneal nerve stimulation. Therefore, volume conduction of reflex activity from the posterior tibia1 compartment to the anterior tibia1 compartment was a com- mon observance. We suggest that coexistent EMG reflex responses, presumed to re- flect reciprocal excitation, should be tested by the procedure described to reject the possibility of EMG cross-talk as a confounding variable or as the actual variable under investigation. 0 1988 Academic Press, Inc. Abbreviations: CPN-common peroneal nerve, H-Hoffmann, PTN-posterior tibia1 nerve, SOL-soleus, TA-tibialis anterior. ’ The authors extend their gratitude to Phoebe Bosche for typing the original and revised manuscripts and to the Universities of California, Los Angeles, and Washington for travel fund support for Dr. Robert S. Hutton. Appreciation is also extended to Dr. Robert J. Gregor for the electromyographic preamplifiers and recording electrodes used in this study. Please send reprint requests to Dr. Robert S. Hutton, Dept. of Psychology, NI-25, Univ. of Washington, Seattle, WA 98195. 265 0014-4886/88 $3.00 Copyright 0 1988 by Academic Press, Inc. All rights ofreproduction in any form reserved.