Shoulder muscle activity magnitude and timing in individuals with specific scapular movement patterns

Relationships between specific muscle activity alterations and specific scapular motion patterns have not been well established. The purpose of this study was to identify muscle activation magnitude and timing in two specific scapular movement groups (excessive scapular anterior tilt, scapular lateralization). Surface electromyography (EMG) was captured simultaneously with biplane video radiography and optical motion capture during two repetitions each of shoulder flexion, abduction, and unrestricted overhead reaching in 45 adults with shoulder pain. The following muscle activation magnitudes measured with EMG were assessed individually and in ratios: lower trapezius (LT), serratus anterior (SA), upper trapezius (UT), anterior deltoid (AD), and middle deltoid (MD). The timing of each muscle's initiation was calculated as a percent of humerothoracic range of motion. For EMG magnitude, significant interactions including group were found for AD, LT/SA, and LT/AD. Findings suggest a relative reduction of SA in both groups in the constrained motions compared to the self-selected motion and a reduction in LT in the lateralization group in early motion. For EMG timing, no group effects were significant, however findings suggest delays in SA and LT activation in both groups. The findings provide evidence that individuals with different scapular movement patterns may have distinct muscle activity patterns which can inform patient-specific rehabilitation interventions.