The Face of Pain ‐ A Pilot Study to Validate the Measurement of Facial Pain Expression with an Improved EMG Method

OBJECTIVE: The purpose of this pilot study was to establish the validity of an improved facial electromyogram (EMG) method for the measurement of facial pain expression. BACKGROUND: Darwin defined pain in connection with fear as a simultaneous occurrence of eye staring, brow contraction and teeth chattering. Prkachin was the first to use the video‐based Facial Action Coding System to measure facial expressions while using four different types of pain triggers, identifying a group of facial muscles around the eyes. METHOD: The activity of nine facial muscles in 10 healthy male subjects was analyzed. Pain was induced through a laser system with a randomized sequence of different intensities. Muscle activity was measured with a new, highly sensitive and selective facial EMG. RESULTS: The results indicate two groups of muscles as key for pain expression. These results are in concordance with Darwin′s definition. As in Prkachin′s findings, one muscle group is assembled around the orbicularis oculi muscle, initiating eye staring. The second group consists of the mentalis and depressor anguli oris muscles, which trigger mouth movements. CONCLUSIONS: The results demonstrate the validity of the facial EMG method for measuring facial pain expression. Further studies with psychometric measurements, a larger sample size and a female test group should be conducted.