Normalizing surface electromyographic measures of the masticatory muscles : Comparison of two different methods for clinical purpose

Purpose - To compare a new normalization technique (wax pad, WAX) with the currently utilized cotton roll (COT) method in surface electromyography (sEMG) of the masticatory muscles. Methods - sEMG of the masseter and anterior temporalis muscles of 23 subjects was recorded while performing two repetitions of 5s maximum voluntary clenches (MVC) on COT and WAX. For each task, the mean value of sEMG amplitude and its coefficient of variation were calculated, and the differences between the two repetitions computed. The standard error of measurement (SEM) was calculated. For each subject and muscle, the COT-to-WAX maximum activity increment was computed. Participant preference between tasks was also recorded. Results - WAX MVC tasks had larger maximum muscular activity than COT MVC tasks (P<0.001), with COT-to-WAX maximum activity increments of 61% (temporalis) and 94% (masseter) (P=0.006). WAX MVC had better test-retest repeatability than COT. For both MVC modalities, the mean amplitude (P>0.391) and its coefficient of variation were unchanged (P>0.180). The WAX task was the more comfortable for 18/23 subjects (P=0.007). Conclusion - WAX normalization ensures the same stability level of maximum muscular activity as COT normalization, but it is more repeatable, elicits larger maximum muscular contraction, and is felt to be more comfortable by subjects.