¹McGill University, Department of Kinesiology and Physical Education, Montréal, Canada
²Karolinska Institutet, Stockholm, Sweden
³National Institute of Occupational Health (STAMI), Oslo, Norway
⁴Aarhus University Hospital, Department of Occupational and Enviromental Medicine, Aarhus, Denmark
⁵Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden, The Centre for Occupational and Environmental Medicine, Region Stockholm, Stockholm, Sweden
⁶Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden

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Introduction: Many work tasks involve the use of both arms to execute in-phase bilateral movements above shoulders, with prolonged work causing fatigue and risk of injuries. However, bilateral limb asymmetry has been proposed to be a possible cause for injuries. Additionally, the dominant arm is more predisposed to musculoskeletal injuries. Should we assume that both arms’ exposure to injury risk factors is the same during a bilateral task? The purpose of this study was to measure the effects of sex and fatigue on activity of bilateral shoulder muscle activity during a bilateral fatiguing overhead shoulder flexion task, in right-handed or ambidextrous individuals. In addition, we studied whether the previously demonstrated increase in movement-to-movement variation occurs also during the bilateral task to the same extent in both arms, and whether this is affected by sex and/or handedness.

Methods: Twenty-five healthy adults, (13 males), were recruited to complete a bilateral overhead fatiguing task with repetitive shoulder flexions from 90° to 135°. Exposure was measured bilaterally using electromyographic data from six shoulder muscles, and their activation amplitude (RMS) and its movement-to-movement variability (SD) were quantified. Data was analyzed separately for right-handed (n = 18) and ambidextrous (n = 6) males and females, with the strength of their laterality measured using the Edinburgh handedness scale.

Results: With fatigue, right-handed females had greater anterior deltoid activation in the dominant arm only (p = 0.04), whereas ambidextrous females showed increased activation of their right infraspinatus (p = 0.005). All females had greater middle deltoid activation variability in the left arm compared to the right (p < 0.01). In contrast, right-handed males had more middle deltoid activation amplitude and variability in their left arm (p = 0.03; 0.009), but had otherwise fewer arm differences.

Discussion: Our results indicate more impact of the bilateral fatiguing task on arm electromyographic asymmetries in females. In our experiment, the average time to fatigue-failure was under 5 minutes, which was enough to elicit significant asymmetry of muscle activation. Therefore, we recommend frequent breaks after a few minutes of work when feasible, in order to avoid asymmetrical loadings during bilateral manual work, especially in females, who have both higher injury risk and more bilateral asymmetry. In the future, studies investigating variations at work, such as with job rotation or alternating arm during unilateral work, should be conducted in order to determine if they can effectively induce more variation, and this reduce injury exposure, during bilateral manual work. This should be measured using wearable technology that measures muscle activity.

Conclusion: Our results suggest that job rotation such as alternating arms may pose a greater injury risk in females, since their shoulders show a clearer side dominance. To enhance worker health and safety, handedness and sex should be considered when implementing workplace changes. For instance, our results could have implications on job rotation strategies that alternate use of hands being easier to implement for males.