Could the muscle corrugator supercilii serve as a signal of pain intensity?

Background: Facial expressions are considered a prominent pain behavior and are used in pain scales developed for non-communicative patients. Objective detection of facial expressions is challenging. However, electromyography (EMG) sensors are able to detect muscle activities accurately. Muscle corrugator supercilii, drawing the eyebrows downwards and medially, is often referred as  the muscle of suffering. The aim of the study was to explore the connection between self-reported pain intensity levels and corrugator supercilii activity.  

Materials: 30 healthy working-age volunteers were included in the study. Participants with any illnesses or medication during the study were excluded. 

Methods: The pain was induced using gradually increasing continued heat or electrical stimulus, two times each in a cross-over design. Study periods consisted of mild pain and moderate/severe pain periods, defined by stimulus start, self-reported pain threshold and pain tolerance. VAS score was collected when pain tolerance was reached. EMG signal of corrugator supercilii was recorded, along with other biosignals. Data was preprocessed and standardized. Study period EMG point estimates were explored on categorized VAS levels. 

Results: During mild pain period, the EMG point estimates, categorized by reported VAS score, showed a slight increase, all similarly. During moderate/severe pain period, the EMG point estimates clearly varied between pain experiences. Eventually, the highest and lowest EMG point estimates corresponded to the highest and lowest pain scores, respectively. 

Conclusions: When pain intensity is high, the corrugator supercilii activity is strong. During the higher pain experience, the corrugator supercilii activity could serve as a potential signal of pain intensity.