Relative scapular muscle activity ratios are altered in subacromial pain syndrome

Coordinated muscle activity is needed for synchronized joint motion and stability. Characterizing relative scapular muscle activity deficits in participants with shoulder pain will provide foundational knowledge to develop rehabilitation programs.

Participants were recruited with subacromial pain syndrome and an asymptomatic control group matched for age, gender, and dominant arm (N = 56). Surface electromyographic muscle activity was recorded from the upper, middle, and lower trapezius (UT, MT, LT) and serratus anterior (SA) during 5 repetitions of a weighted arm elevation task. Muscle activity was normalized to a reference contraction and then expressed as UT/MT, UT/LT, UT/SA, and LT/SA ratios. Ratios were compared between groups and across 3 arm angle intervals during ascending and descending elevation.

A 2 × 3 mixed-model analysis of variance yielded a group main effect for the UT/LT ratio, with a higher ratio in the subacromial pain group during ascending (mean difference, 0.92; P = .008) and descending (mean difference, 0.70; P = .030). For the LT/SA ratio, there was a group effect: a lower ratio in the subacromial group during ascending (mean difference, -0.25; P = .026) and descending (mean difference, -0.51; P = .032). There were no differences for the UT/MT or UT/SA.

There is a disruption in coordination between the LT and SA and the UT and LT during an arm elevation task in patients with subacromial pain syndrome. The LT was part of both altered ratios, indicating the relative importance of the LT. Future research should determine if exercises aimed at restoring the dysfunctional LT/SA and UT/LT force couples are beneficial to reduce shoulder pain and disability in patients with unilateral shoulder pain.