Dependence of Rotator Cuff Muscle Thickness and Stiffness on Low-Level Contractions: Metrology of the Contraction Level Using Shear-Wave Imaging.

Objective: The study aimed to investigate the effects of the level of contraction during isometric shoulder abduction at different abduction angles on muscle thickness and stiffness of the shoulder girdle in asymptomatic individuals. Design: Measurement properties study. Setting: Biomechanics and motion analysis lab. Participants: Twenty individuals volunteered to participate in this study. Main Outcome Measure: The subjects were tested for morphological and mechanical properties, expressed by thickness and stiffness of the supraspinatus tendon and muscle, and upper trapezius muscle. Moreover, acromiohumeral distance was also evaluated using B-mode ultrasound and shear-wave elastography. Intervention: The thickness and stiffness of the supraspinatus and upper trapezius muscle were assessed at 3 angles of abduction (0°, 60°, and 90°) and 3 levels of contraction (0%, 10%, and 20% of the maximal voluntary isometric contraction) using ultrasonography with shear-wave imaging. Moreover, the acromiohumeral distance was measured to establish the occupation ratio during passive movement. Results: The supraspinatus and upper trapezius muscle thickness and stiffness were significantly greater at 60° shoulder abduction compared with 0°, and 90° compared with 60°, as well as significantly greater at 20% maximal voluntary isometric contraction compared with 0% and 10% maximal voluntary isometric contraction. Thickness and stiffness were significantly greater in the supraspinatus compared with the upper trapezius muscle at all 3 angles of shoulder abduction for all 3 level of contractions. The acromiohumeral distance decreased significantly from 0° to 60° and from 60° to 90°. Conclusion: Morphological and mechanical properties of the supraspinatus and upper trapezius muscles depended on the relative level of muscle contraction and the angle of shoulder abduction. [ABSTRACT FROM AUTHOR]