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STANDARDIZED ELECTROMYOGRAPHIC INDEXES ALLOW A RELIABLE MEASUREMENT OF MASTICATORY MUSCLES FUNCTION

Table 1. Comparison of standardized sEMG indexes obtained in two different data collection sessions( all values are percentages). No statistically significant differences were found

Masticatory sEMG standardized indexes repeatability

Session 1

Session 2

Difference

T Test 0.166 0.408 0.691 0.166 0.821

chloride bipolar surface electrodes( rectangular shape, 21x41 mm, 20 mm inter-electrode distance)( F3010, Fiab, Firenze, Italy) were positioned. The electrodes were placed on the muscular bellies parallel to muscular fibres as follows( Fig. 1):

• MM: the operator, standing in front of the seated subject, palpated the muscular belly while the subject clenched his / her teeth. The electrodes were fixed parallel to the exocanthion-gonion line and with the upper pole of the electrode under the tragus-labial commissura line.

• TA: the muscular belly was palpated during tooth clenching and the electrodes were fixed vertically along the anterior margin of the muscle( corresponding to the fronto-parietal suture) 24.

A disposable reference electrode was applied to the forehead. To reduce skin impedance, the skin was carefully cleaned prior to the electrode placement, and recordings were performed 5 min later, allowing the conductive paste to adequately moisten the skin. 2.4. sEMG recordings and measurements Instrumentation The surface EMG activity was recorded using a computerized instrument( Easymyo, 3 Technology S. r. l., Udine, Italy). The analogic sEMG signal was amplified( gain 100, bandwidth 0 – 1000 Hz, peakto-peak input range from 0 to 3600 µ V) using a differential amplifier with a high common mode rejection ratio( CMRR = 115 dB in the range 0 – 60 Hz, input impedance 100 GΩ), digitized( 24-bit resolution, 4000 Hz A / D sampling frequency), and digitally filtered( Butterworth type, high-pass filter set at 30 Hz, low-pass filter set at 400 Hz, band-stop for common 50 – 60 Hz noise). The signals were averaged over 25 ms, with muscle activity assessed as the root mean square( RMS) of the amplitude(µ V). sEMG signals were recorded for further analysis. Before the acquisition session the subjects were properly trained to elicit true teeth maximal voluntary contraction using an on-time sEMG signal visualization.

3. Results 3.1. Standardization procedure To normalize the sEMG signals two 10-mm thick cotton rolls were positioned on the mandibular second premolars / first molars of each subject and a 5-s maximum voluntary contraction( MVC) was recorded. The mean sEMG potential of each muscle obtained in that first acquisition was set at 100 %, and all further sEMG potentials were expressed as a percentage of this value(µ V / µ Vx100). 3.2. Analyzed task The sEMG activity was recorded during a 5-s MVC test in intercuspal position( IP): the subject was invited to clench as hard as possible and to maintain the same level of contraction for all the test. For each patient, the central 3 s of the MVC test were analysed, and the sEMG potential was standardized as detailed before. 3.3. sEMG data analysis Separately for each acquisition session( T1 and T2), the sEMG waves were compared by computing a series of standardized indexes using the instrument software tools: 1. the percentage overlapping coefficient( POC, unit %), an index of symmetric muscular contraction. The index ranges between 0 % and 100 %: when two paired muscles contract with perfect symmetry, a POC of 100 % is obtained. Masseter and Temporalis Anterior POCs were obtained for each patient 24, 25.

2. the torque coefficient( TORQUE, unit %) was assessed to evaluate if an unbalanced contractile activity of the contralateral Masseter and Temporalis Anterior muscles, such as that of the right Temporalis Anterior and the left Masseter, might give rise to a potential lateral displacing component. TORQUE ranges between 100 %( complete prevalence of the right Temporalis Anterior and left Masseter) and-100 %( complete prevalence of the left Temporalis Anterior and the right Masseter) 24, 25.