FACIAL AND SHOULDER ELECTROMYPGRAPHIC RESPONSE TO HEAT STIMULATION AT DISTINCT INTENSITY LEVELS

Reliable objective pain assessment is necessary to provide adequate pain relief treatments, especially in case of non-communicative patients. In this sense we address the use of electromyography (EMG) in the context of experimental pain induction. Thus, we aimed to analyze whether thermal painful stimulation can elicit stable electromyographic responses among several random stimulations for the Corrugator, Zygomaticus and Upper Trapezius muscles. Bipolar differential surface EMG were acquired with a sampling rate of 512 Hz, gain of 24, and ADC resolution of 24 bits. Raw data were processed in Matlab with 4th order Butterworth bandpass (5-256 Hz) filter and 20th order polynomial filters. Then, the Hilbert transform was used to estimate the instantaneous amplitude, which was processed by a thresholding algorithm, so that only responses with prominences larger than 5% of the entire signal’s range were counted. Among Corrugator, Zygomaticus and Trapezius muscles, Zygomaticus was the muscle in which the responses were more often identified (over 47%), regardless of the stimulation intensity. Thermal painful stimulation at the painful threshold level elicited e l e c t r omyogr a ph ic r e sponse s i n on ly 20.7% (Corrugator) , 3 1 . 3 % (Zygomaticus) a n d 3 2 . 7 % (Trapezius) of the total number of stimuli, while thermal painful stimulation at the tolerance level elicited electromyographic responses in 56.2%, 69.8%, and 54.2% of the total number of stimuli, respectively. We observed that the higher the stimulation intensity, the larger the recognition rates of electromyographic responses for all the three muscles. We concluded that, although using randomized time and intensity stimulation design, we didn’t get stable EMG responses with respect to thermal pain stimulation. K e y w o r d s : Biomedical Signal Process ing , Electromyography, Pain Measurement.