A TOPOGRAPHICAL ANALYSIS OF SPECTRAL ELECTROMYOGRAPHIC DATA OF THE HUMAN MASSETER MUSCLE UNDER DIFFERENT FUNCTIONAL CONDITIONS IN HEALTHY-SUBJECTS

In 20 healthy subjects, 16-channel surface electromyograms (EMG) were recorded with the mandible in its postural position, during compensati on for forces applied from ipsilateral, contralateral and frontal posi tions, and during force-constant biting on st unilaterally placed forc e transducer. After artefact elimination, EMG power spectra were calcu lated on the basis of the original EMG curves via fast Fourier transfo rmation. Using spectral EMG characteristics, EMG maps were computed by means of an interpolation algorithm and an imaging procedure. Spectra l EMG maps were demonstrated for all the test conditions. Significant differences of myoelectrical activity were verified between the 16 ele ctrode positions. Moreover, the levels and/or the topographical distri butions of spectral EMG powers differed significantly between the test conditions. During biting as well as with contralaterally and frontal ly applied forces, the highest EMG activity was found in the inferior third of the masseter and the lowest in the superior third. With ipsil aterally applied forces, the topographical distribution of the total E MG power was completely changed. The power maximum was frequently demo nstrated in the superior, especially the posterosuperior, third of the muscle, the minimum in the inferior third. In the postural position o f the mandible, EMG activity decreased from the anterior to the poster ior regions of the masseter. If the bite force or the horizontally app lied forces were enhanced, EMG activity increased significantly but th e EMG map structures were only changed in some details. The topographi cal distribution of myoelectrical activity demonstrated by EMG maps ch aracterizes the intramuscular activation patterns of distinct masseter functions. The patterns found conformed with the muscle's functional morphology. Changes in normal topographical EMG distribution could the refore be considered as an indication of muscular dysfunctions.