We aimed to define a standardised protocol for the electromyographic evaluation of trapezius muscle in dentistry and to assess its within- and between-session repeatability. Surface electromyography of trapezius, masseter and temporal muscles was performed in 40 healthy subjects aged 20-35years during shoulder elevation, and maximum teeth clenching with and without cotton rolls. Two repetitions were made both within (same electrodes) and between sessions (different electrodes). Maximum voluntary clench on cotton rolls was used to standardise the potentials of the six analysed muscles with tooth contact; shoulder elevation was used to standardise the upper trapezius potentials. From the standardised electromyographic potentials, several indices (muscle symmetry; masticatory muscle torque and relative activity; total masticatory muscle activity; trapezius cervical load, percentage co-contraction of trapezius during teeth clenching) were computed; random (technical error of measurement) and systematic (Student's t-test, Analysis of Variance) errors were assessed. For all indices, no systematic errors were found between the two separate data collection sessions. Within session, limited (lower than 8%) technical errors of measurement were found for temporalis and masseter symmetry, torque and activity indices, and the trapezius cervical load. Larger random errors were obtained for trapezius symmetry and total masticatory muscle activity (up to 20%). Between sessions, no significant differences were found for trapezius co-contraction. In conclusion, a protocol for the standardisation of trapezius muscle that may be used within dental clinical applications was defined, and the repeatability of masseter, temporalis and trapezius electromyographic recordings for serial assessments was assessed in healthy subjects.
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