We investigated the surface EMG of the trapezius muscle during static isometric contraction (abduction 60°, see [1]) with a rectangular matrix of electrodes (7x6). The area of maximal electrical activity was found in the upper part of the trapezius muscle [1]. The cross covariance functions between bipolar channels show the propagation of action potentials in the electrode rows parallel to the line between C7 and Acromion and therefore parallel to the muscle fibre direction (each row consists of 7 electrodes or 6 bipolar channels respectively) via time shifts. High pass filtering of these signals with about 100 Hz suppresses the propagating peaks between mid and the border of each row (i.e. in channels 2 and 5) whereas the time shifts between the mid of the rows (channels 3 and 4) and their border (channels 1 and 6) increase. This behaviour is a hint to deap sources. I.e. the main part of the force is generated by deep muscle fibres.

[1] N. P. Schumann et al. Topographische Verteilung myoelektrischer Aktivität - armhaltungsbezogene Aktivierungsmuster und der Einfluß von Schmerz. In R. Grieshaber, M. Stadeler, H.-Ch. Scholle (eds.) 12. Erfurter Tage, Bussert & Stadeler, Jena 2006 (ISBN 3-932906-73-X), S. 429–

443