Association between speckle-tracking ultrasonography and isometric load of shoulder muscles in patients with subacromial pain syndrome – A cross-sectional study

Background: Subacromial pain syndrome is the leading cause of shoulder pain, commonly affecting the supraspinatus and infraspinatus rotator cuff muscles. Speckle-tracking ultrasonography offers a direct, non-invasive method to assess muscle function by measuring percentage deformation (strain) during contraction, but its application in patients with subacromial pain syndrome is unknown. The aim of the study was to investigate whether differences in muscle strain in the supraspinatus and infraspinatus muscles were associated with increasing external isometric muscle force in patients with subacromial pain syndrome. Methods: This cross-sectional study assessed 24 patients with subacromial pain syndrome using speckle-tracking ultrasonography during submaximal isometric contractions (20 % to 80 % of maximal voluntary isometric contraction) of the supraspinatus and infraspinatus muscles. Muscle strain was analyzed with the commercial EchoPAC™ software. Associations between strain and external isometric force were evaluated using repeated measures ANOVA and multiple linear regression analyses (adjustment for pain, sex, and age). Findings: Muscle strain generally increased with external loads (p < 0.001), except between 60 % and 80 % of maximal voluntary isometric contraction for supraspinatus (p = 0.12). Strain explained a large proportion of the variance in muscle force for both supraspinatus (r ² = 0.55, p < 0.001) and infraspinatus (r ² = 0.69, p < 0.001). Interpretation: Strain assessed via speckle-tracking ultrasonography explained a large proportion of the variation in supraspinatus and infraspinatus muscle force among patients with subacromial pain syndrome. However, from a clinical perspective, its diagnostic accuracy in distinguishing between healthy and pathological skeletal muscle conditions remains to be established.