Overlap Between Facial and Masticatory Muscle Surface EMG Signals

Surface electromyography has been used extensively in dentistry to evaluate masticatory muscle activity under a variety of situations.  Most commonly the masseter and the anterior portion of the temporalis muscles are recorded.  These muscles are in close proximity to a number of facial muscles that, when actively contracting, emit signals that could comingle with the masseter and temporalis signals.

Objectives: The objective of this study was to determine the extent and circumstances under which facial muscle activity may be mixed with masticatory activity when measured with surface electromyography (EMG).

Methods: Surface EMG electrodes were placed over the anterior temporalis and masseter muscles of 10 healthy, asymptomatic subjects who consented to participate.  Resting and clenching activity was recorded while subjects exhibited each of the following expressions; 1) passive facial expression, 2) smile broadly, 3) grimace and 4) act surprised.  These expressions activate a number of facial muscles such as; a.) frontalis and orbicularus oculi near the temporalis and b.) buccinator, risorius and zygomaticus major in close proximity to the masseter.  Average values under each condition were calculated for each subject.  Student’s paired t test was used to compare the passive condition to each of the 3 active conditions (alpha = 0.05). 

Results: Passive resting activity was significantly lower (all muscles) than for each of the 3 conditions with active facial expressions (p < 0.0000 for all 3 comparisons).  Clenching activity with a passive facial expression did not show any significant difference compared to the 3 facially active conditions (p > 0.4 for all 3 conditions). 

Conclusions: Active facial expressions can significantly increase the microvolt levels of the masseter and anterior temporalis muscles when surface electrodes are used for EMG recordings.  However, smiling, grimacing and a look of surprise did not significantly affect the clenching microvolt levels of these asymptomatic subjects.