Abstract
BACKGROUND: Myofascial trigger points (MTrPs) in the upper trapezius are prevalent in individuals with shoulder pain and may interfere with neuromuscular control and kinematics during shoulder movements. The purpose of this study was to compare shoulder muscle activation patterns and kinematic parameters, particularly scapulohumeral rhythm (SHR), between individuals with upper trapezius MTrPs and the healthy group during shoulder abduction. METHODS: An observational study was conducted with participants assigned to either a trigger point group (n = 15) or a healthy group (n = 13). Surface electromyography (EMG) recorded activation of eight shoulder muscles during abduction. Kinematic analysis quantified SHR, scapular, and glenohumeral motion. Statistical comparisons were made using independent t-tests with SPSS version 17, considering a p-value of less than 0.05 as significant. RESULTS: No statistically significant differences were observed in peak EMG amplitudes, onset timing, or root mean squared activity between groups for any muscle ( p > 0.05). However, the trigger point group exhibited a significantly prolonged rise time (1.21 ± 0.51) compared with the healthy group (0.68 ± 0.39; d = 0.90, p < 0.001), as well as an increased SHR (4.84 ± 0.77) compared with the healthy group (4.23 ± 0.50; d = 0.90, p = 0.026), respectively. These findings suggest a disruption in glenohumeral-scapular coordination, potentially indicative of compensatory neuromuscular adaptations associated with the presence of trigger points. Other kinematic variables, including maximum scapular upward rotation and anterior tilt, did not differ significantly. CONCLUSION: Upper trapezius MTrPs are associated with increased SHR and prolonged rise time during abduction, reflecting altered shoulder coordination. Combining kinematic assessments with advanced EMG timing analyses may provide more sensitive markers of subtle neuromuscular changes associated with MTrPs.