J. Electromyogr. Kin&d. Vol. 5, No. 4, pp. 251-257, 1995 Copyright 0 1996 Elsevier Science Ltd. All rights reserved Printed in Great Britain 1050-6411(95)00011-9 105M411/95 $10.00 + 0.00 Normalizing Upper Trapezius EMG Amplitude: Comparison of Different Procedures Shihan Baa’, Svend Erik Mathiassenl and J&-gen Winkelly* ‘Division of Applied Work Physiology, National Institute of Occupational Health, S-l 71 84 Solna; 2Department of Occupational and Environmental Medicine, University Hospital, Lund, Sweden Summary: Different procedureshave been used for normalization of upper trapezius electromyographic(EMG) amplitudes. This complicates comparisons between studies. The present study aimed at investigating the influence of some commonly usedtrapezius EMG normalization procedures on the results of ergonomic analyses, as well as the test-retest repeatability of these procedures. EMG activity from the upper trapezius was recorded during an occupational task. The EMG activity was then normalized by seven different normalization procedures. It was shown that at the group level, a unilateral shoulder elevation maximal voluntary electrical (MVE) activation procedure gave 1.2 times higher occupational load estimates than a corresponding bilateral MVE. At the group level, the median load during the occupational task was 1.6 times higher when expressedas %MVC (maximal voluntary contraction) obtained from a power regressionof relative force on EMG amplitude than when expressed as %MVE determinedfrom a singlemaximal shoulder elevation. Normalizations in terms of a submaximal reference voluntary electrical (RVE) activation had similar test-retest repeatability in terms of the coefficient of variation (CV: ll-13%) as normalizationsin terms of an MVE (CV: ll-15%), but the power regression procedures had considerably larger CVs (21-36%). The paper provides a basis for comparing previous studies using different normalization methods,aswell asa qualitative evaluation of normalization methodsfor future use. Key Words: Calibration-Electromyography-Ergonomics-Muscle load- MVC-Ramp. J. Electromyogr. Kinesiol., Vol. 5. 251-257, December INTRODUCTION Surface electromyography (EMG) has been exten- sively used in ergonomic studies to estimate upper trapezius muscle load9. EMG amplitude is nor- malized in order to allow comparisons between Accepted March3, 1995. Correspondence and reprint requests to S. Bao, Division of Applied Work Physiology, NationalInstitute of Occupational Health, S-171 84 Solna, Sweden. subjects, days, muscles, or studies”. A number of different normalization procedures have been used in the literature, which impedes comparisons between different investigations. Only a few studies have compared the results obtained by different normaliz- ation procedures quantitatively as well as qualitat- ively. A common normalization method is to express EMG activities during occupational tasks (EMG) as a percentage of the electrical activity produced 251