Abstract
Background: Although several studies have compared muscle activity in 'healthy' and 'unhealthy' shoulders, studying 'healthy' shoulders alone could improve the understanding of shoulder biomechanics. Objective: This study aims to describe the electromyographic activity of several shoulder muscles during a full range of free active flexion, as well as during abduction and scaption movements, and to compare gender differences in subjects with no history of shoulder pain or pathology. Methods: A cross-sectional descriptive study was conducted with 34 subjects aged between 18 and 60 years of both genders. The activity of the anterior, middle, and posterior deltoid, serratus anterior, infraspinatus, latissimus dorsi, and teres major muscles was measured using surface electromyography. Root Mean Square (RMS) values were calculated as a percentage of Maximal Voluntary Isometric Contraction (MVIC). Results: Regardless of whether they are considered agonists or antagonists, these muscles were active, with no statistically significant differences (Mann-Whitney U test), during both the lifting and lowering phases of the studied movements. Statistically significant differences between movements were observed only in the deltoid (Kruskal-Wallis H test, p < 0.004), which was more active during abduction. Women showed statistically significant muscle activity increase compared with men in some movements, except in the infraspinatus muscle-for example, in the three parts of the deltoid during the lifting phase of scaption (ANCOVA, p = 0.002-0.024). Conclusions: In this sample, the shoulder muscles studied showed comparable activity, acting as agonists or antagonists during shoulder elevation. These findings are exploratory and may help inform future studies on muscle activation in healthy shoulders during more varied functional tasks.