Ultrasound evaluation of the subacromial space in healthy subjects performing three different positions of shoulder abduction in both loaded and unloaded conditions

Background: Subacromial space (SS) narrowing may result in unsafe contact of rotator cuff tendons and bursa with the acromion, possibly promoting rotator cuff pathologies and pain. We measured SS in asymptomatic subjects after three movements employed for shoulder muscles' training.

Participants and main outcome measures: Twenty-five males (age 24±5years; stature 175±8 cm; body mass 74±12 kg) underwent SS ultrasound evaluation while executing with (WL) and without (NL) 4-kg load: 90° shoulder abduction (SA) without humerus rotation (90°_neutral); 90° SA maximal humeral external rotation (90°_er); maximum SA (above 90°) during upright row (UR). For inter-operator reliability analysis, a subset of 12 subjects was assessed by two operators.

Results: Intra/inter-operator reliability coefficients were high (ICC>0.87; standard error of measurement range: 0.23-0.51 mm; minimum detectable change range: 0.61-1.79 mm). ANOVA revealed significant differences between loads (p = 0.003, η² = 0.37) and angles (p < 0.001, η² = 0.72), and interaction (p = 0.004, η² = 0.45). Normalized SS values WL were higher at 90°_er vs. 90°_neutral and UR (p < 0.001 for both). Differences in NL were found between 90°_neutral and both 90°_er (p = 0.002) and UR (p = 0.009).

Conclusions: When holding a 4-kg load, SS is larger at 90°_er than at 90°_neutral and UR shoulder abduction positions. In unloaded condition, SS at 90°_neutral is reduced compared to 90°_er and UR.