THE INFLUENCE OF THE TYPE OF CONTRACTION ON THE MASSETER MUSCLE EMG POWER SPECTRUM

Different behaviours of the EMG power spectrum across increasing force levels have been reported for the masseter muscle. A factor that coul d explain these different behaviours may be the type of contraction us ed, as was recently shown for certain upper limb muscles5. The purpose of this study was to compare, between two types of isometric contract ions, the behaviour of EMG power spectrum statistics (median frequency (MF) and mean power frequency (MPF)) obtained across increasing force levels. Ten women exerted, while biting in the intercuspal position, three 5 s ramp contractions that increased linearly from 0 to 100% of the maximal voluntary contraction (MVC). They also completed three ste p contractions (constant EMG amplitude) at each of the following level s: 20, 40, 60 and 80% MVC. EMG signals from the masseter muscle were r ecorded with miniature surface electrodes. The RMS, as well as the MPF and MF of the power spectrum were calculated at 20, 40, 60 and 80% MV C for each type of contraction. As expected, the RMS values showed sim ilar increases with increasing levels of effort for both types of cont ractions. Different behaviours for both MPF (contractionforce interac tion, ANOVA, P<0.05) and MF (contractionforce interaction, ANOVA, P>0 .05) across increasing levels of effort were found between the two typ es of contraction. The use of step contractions gave rise to a decreas e of both MPF and MF with increasing force, while the use of ramp cont ractions gave rise to an increase in both statistics up to at least 40 % MVC followed by a decrease at higher force levels. These findings su ggest that the type of contraction used does influence the behaviour o f the spectral statistics across increasing force levels and that this could explain the differences obtained in previous studies for the ma sseter muscle.