Surface EMG mapping of the human trapezius muscle: the topography of monopolar and bipolar surface EMG amplitude and spectrum parameters at varied forces and in fatigue

Objectives: To investigate the factors affecting the topography of trapeziu s muscle EMG, multichannel recordings were made at different forces of isom etric shoulder elevation and during fatiguing exercise. Methods: Twenty-eight channels of monopolar EMG were recorded from an array of 4 x 7 electrodes placed on the upper trapezius muscle. From the monopol ar EMG and the bipolar derivations the root mean square (RMSmonopolar, RMSb ipolar) and power spectrum median frequency (MFmonopolar, MFbipolar) were c alculated. Results: The maximum RMSmonopolar was located above the middle part of the trapezius muscle, where a minimum was found for RMSbipolar. The cranial-cau dal RMS distribution shifted upwards when the Force was increased from 50 t o 100% MVC and during fatigue. MFbipolar showed a peak above the endplate r egion, where the MFmonopolar was low. During fatigue the normalized MF slop e was independent of the cranial-caudal electrode position, but MFmonopolar decreased most strongly at positions above the endplate region, where MFbi polar decreased less. Conclusions: While the changes in ME reflected metabolic properties and vol ume conduction phenomena in the muscle, changes in RMS reflected a compensa tion for the fatigue processes within the muscle. The RMS changes in fatigu e can be explained by the direction of the fibres involved in shoulder elev ation. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.