Click evoked EMG responses in sternocleidomastoid muscles: characteristics in normal subjects

Recordings were obtained from a total of 25 normal subjects of the electromyographic (EMG) responses in the sternocleidomastoid muscle (SCM) to intense sound stimuli. While previous authors have demonstrated these responses exist, it has remained unclear whether the EMG response is unilateral or bilateral in nature. Accordingly, we chose a remote site, linked-wrists, for our reference electrodes so that we could be certain that no significant volume conduction of potentials could occur from the source in the SCM to the reference site. When this was done we found that if the sternum was used as a reference site, as was the case in previous studies, some subjects exhibited bilateral responses while in others, the response was ipsilateral. However, with linked-wrists as the reference site, responses were always purely ipsilateral. Furthermore, recordings that used the sternum or the ipsilateral mastoid process as active sites and linked-wrists as a reference, exhibited responses which were inverted. Thus, both the sternum and the ipsilateral mastoid process are electrically active due to volume conduction from the nearby source in the SCM. The ambiguity in previous recordings can be attributed to the use of these active sites as a reference. When SCM responses are recorded versus a remote, electrically inactive site, the responses are purely ipsilateral.