Investigation of the association between shoulder flexion range of motion and deltoid and trapezius muscle activity after reverse total shoulder arthroplasty.

[Purpose] We aimed to evaluate deltoid and trapezius muscle activity during shoulder flexion postoperatively in participants who had undergone reverse total shoulder arthroplasty (rTSA) and identify factors contributing to active flexion range of motion (ROM). [Participants and Methods] We included 18 participants who had undergone rTSA and were followed for at least one year postoperatively. Surface electromyography recorded muscle activity of the deltoid and trapezius at 30° of shoulder flexion, approximately 8 months after surgery. Muscle activity was normalized as % reference voluntary contraction (%RVC). Based on active flexion ROM, participants were categorized into a good group (≥120°) and a poor group (<120°). [Results] The good group included 14 participants (3 males, 11 females, mean age 75.4 ± 5.7 years), and the poor group included 4 participants (1 male, 3 females, mean age 74.5 ± 11.4 years). The good group showed greater anterior deltoid activity, while the poor group exhibited higher activity in the posterior deltoid and lower trapezius. [Conclusion] We identified anterior deltoid activity as a positive factor for achieving good shoulder flexion ROM after rTSA. In contrast, increased activity in the posterior deltoid and lower trapezius may reflect compensatory strategies that inhibit optimal flexion.