Shear wave elastography reveals depth- and angle-dependent engagement of rotator cuff muscle subregions during loaded...

The rotator cuff (RC) plays a critical role in stabilizing the glenohumeral joint and generating shoulder forces during elevation, yet the engagement of individual RC muscle subregions during loaded shoulder flexion remains incompletely characterized in vivo. This study used ultrasound shear-wave elastography (SWE) to quantify angle-dependent and depth-dependent engagement of supraspinatus (SSP), infraspinatus (ISP), and anterior deltoid muscle subregions during low-load shoulder flexion. Ten healthy young males performed isometric holds at shoulder flexion angles from 0°-120° while supporting an external load corresponding to 10% of maximal voluntary contraction. SWE-derived muscle modulus, as an inference of activity, was measured in anterior-superficial and anterior-middle SSP subregions, superior and middle ISP subregions, and the anterior deltoid. The anterior deltoid demonstrated a progressive rise from 0° [78.8 (42.1) kPa] to 90° [474.0 (127.2) kPa] (p < 0.001). RC subregions presented earlier engagement, with increased activity evident by 30°. Within the SSP, the anterior-middle and anterior-superficial subregions showed the highest activity at 60° [609.6 (112.4) kPa (p < 0.001) and 330.7 (120.6) kPa (p < 0.001), respectively]. In the ISP, the superior and middle subregions peaked at 60° and 90° [401.2 (149.5) kPa (p < 0.01 vs. 0°) and 312.6 (77.8) kPa (p < 0.001 vs. 0°), respectively]. These findings reveal pronounced depth- and angle-dependent differences in muscle behavior and demonstrate that deeper, capsule-adjacent subregions maintained elevated engagement across wider functional ranges. This study provides mechanistic insight into layered RC function, offering a functional framework relevant to rehabilitation strategies and surgical approaches that consider subregion-specific muscle behavior.