Reliability of Masseter and Temporalis EMG recorded in Habitual Environments

Objectives: To determine the reliability of self-recordings made using portable EMG equipment in habitual environments.
Methods: Eighty-six subjects (52 women, 34 men; average age: 33±11 years) gave informed consent to participate in IRB-approved protocols. Each subject attended 2 laboratory visits. At both visits the same standardized molar biting tasks were performed on one side at a time while bite-force and EMG signals from bilateral masseter and anterior temporalis muscles were recorded via a pre-calibrated force transducer and bipolar surface electrodes, respectively. Root-mean-square muscle activities (mV) were plotted versus bite-force (N) for each muscle acting unilaterally and contralaterally to the bite-force and the resultant regression relations were used to calibrate subject- and muscle-specific ambulatory recordings. At visit 1 subjects learned to use the portable EMG recording equipment. At visit 2 subjects returned data and equipment after recording from masseter and anterior temporalis muscles on one selected side for 3 days and 3 nights in their habitual environments. Self-recordings for each subject and muscle were characterized relative to thresholds of EMG/20N bite-force (T20N) using muscle duty factors (DF=amount of time muscle activity at T20N during a given recording period, %). Reliabilities of muscle DFs for 6 recordings overall and 3 recordings for each time-period were assessed using Intraclass Correlation Coefficients (ICC), where by definition, ICC: ≤0.40=low, 0.40-0.70=moderate, >0.70=high, if lower 95% confidence interval (CI) limit >0.00.
Results: Reliabilities of DFs overall were high for masseter (ICC=0.74) and low for temporalis (ICC=0.42) muscles (Table). Similarly, DF for day and night recordings were better for masseter (ICC=0.63 and 0.87, respectively) compared to temporalis (ICC=0.43 and 0.24, respectively) muscles.
Conclusions: Self-recordings of masseter muscle activities in habitual environments were highly reliable overall and particularly at night.