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Introduction: Normalizing the amplitude of muscle activity measured by surface electromyography (sEMG) helps to reduce variability when comparing different participants or experimental conditions. Although some authors have provided recommendations for normalization of upper trapezius (TRA) muscle sEMG (Mathiassen et al. 1995; Cid et al. 2017 and 2020; Burden 2010), there is no defined consensus in the literature. The aim of this study was to determine whether the method of normalizing the activity of the TRA muscle, using maximal (MVC) vs. submaximal voluntary contractions (SUBVC), affects results related to different work postures used when performing a computer task, i.e., a low-level workload.

Methods: Thirty-one right-handed women performed 5 times a standardized pointing-clicking task with a mouse in 4 working postures (sitting on an office chair, sitting on a ball, standing and pedaling on an ergocycle). The sEMG activity of the right TRA muscle was recorded. Two methods of voluntary isometric contractions were performed: MVC and SUBVC. For each, participants stood with their upper limbs abducted at 90° and performed two 5s-contractions. For MVC, a resistance was applied by a non-elastic strap placed on the participants’ right elbows. They were required to perform a maximal isometric abduction of their shoulder. For SUBVC, participants held a 1 kg dumbbell in their hand and should maintain the position. To determine the effect of working postures on the 10^th, 50^th and 90^th percentiles of the RMS values of the sEMG signal, linear mixed models were used. Significant differences at a 5% threshold between postures were determined using a Bonferroni post-hoc test. Effect sizes were then assessed using Cohen’s d, where d ≤ 0.2 indicates a very small effect, 0.2 < d ≤ 0.5 a small effect, 0.5 < d ≤ 0.8 a medium effect and d > 0.8 a large effect.

Results: With regard to significance, the results were the same for both normalization methods. The 10^th, 50^th and 90^th percentile values were higher in sitting on a ball compared to the other 3 postures. The 10^th percentile values were higher while pedaling on an ergocycle compared to sitting on an office chair. The 50^th and 90^th percentile values were higher while pedaling on an ergocycle compared to standing and sitting on an office chair. However, with regard to effect size, the results differed depending on the normalization method used. Using the SUBVC normalization method, the differences between postures showed very small (8 differences) or small (6 differences) effect sizes. Using the MVC normalization method, effect sizes were generally greater, with 2 very small differences, 9 small differences and 3 medium differences.

Discussion: Considering only the significant differences, normalizing by MVC or SUBVC allowed to observe the same results. However, effect sizes were higher when TRA muscle normalization was performed with MVC compared to SUBVC. Thus, the MVC method more clearly highlighted the differences between the working postures in this study, conducted on women, which simulated a computer work task involving a low-level workload.

Conclusion: Researchers and practitioners should be aware of the impact of the sEMG normalization method on the results, and therefore, the interpretation of their study data.