Towards restoring scapular position and motion through mapping muscle excitation of the trapezius
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Context: Scapular dyskinesis (SD) is associated with increased risk of shoulder pain and pathology, however its etiology remains unclear. Alterations in muscle excitation of the three regions of the trapezius are believed to contribute to altered scapular kinematics, yet research regarding trapezius excitation in individuals with SD is limited. Therefore, the influence of the trapezius on SD is not fully understood. Objective: To determine if differences in the mean location of spatial distribution of excitation (BARYY) exist within each region of the trapezius during shoulder elevation (FE) and lowering (FL) in those with (DYS) and without (CON) scapular dyskinesis. Methods: Fifty-six healthy right-handed participants were assessed using the Scapular Dyskinesis Test (SDT): 28 had normal scapular motion (CON: 15 Female, 27±9 years) and 28 had dyskinesis (DYS: 22 Female, 24±7 years). Participants completed five repetitions of weighted shoulder flexion while high-density surface electromyography (HDsEMG) of the upper (UT), middle (MT), and lower (LT) trapezius was collected. Kinematics were collected to extract BARYY within 30-degree ranges (30°-60°, 60°-90°, 90°-120°) of glenohumeral flexion. A two-way (group*angle) between-within (BW) analysis of variance (ANOVA) was conducted for each grid at each angle to determine the effect of group on BARYY. Results: A significant interaction was found for LT_FE (p=.025, ηp2=.079). BARYY was more cranial in DYS, with differences of 4.6% (CON=35.2±6.6%, DYS=30.6±7.9%, p=.022) for 30°-60°, 5.0% (CON=35.1±6.7%, DYS=30.1±8.2%, p=.015) for 60°-90°, and 5.8% (CON=35.7±7.3%, DYS=29.9±8.4%, p=.008) for 90°-120°. A significant interaction was found for LT_FL, (p=.041, ηp2=.073). BARYY was more cranial in DYS at 120°-90° and 90°-60°, with differences of 5.9% (CON=36.3±6.9%, DYS=30.4±8.5%, p=.006) and 4.5% (CON=36.1±6.2%, DYS=31.6±7.9%, p=.021), respectively. No significant interactions were found for UT or MT during FE or FL. Conclusions: Differences in spatial distribution of excitation exist within the LT during both phases of shoulder flexion between individuals with SD and healthy controls. As the magnitude of the differences was small, it is unclear if this differing neuromuscular strategy contributes to alterations in scapular kinematics. No differences in excitation were observed during either phase in the UT or MT, indicating the neuromuscular strategies utilized for each group did not differ.