Coordination of the Sub-Regions of the Supraspinatus and Deltoid Muscles During Shoulder Scaption: a Shear Wave Elastography Study

Abstract

Background. Understanding the functional role of the supraspinatus (SSP) and deltoid muscles during shoulder motion is a basis for understanding rotator cuff pathology and for the development of appropriate rehabilitation protocols. The purpose of this study was to elucidate the coordination, focusing on activation timing, of the sub-re­gions of the SSP muscle and of the middle region of deltoid muscle during scapular plane abduction (scaption) using shear wave elastography (SWE). Methods. Twelve healthy male volunteers without any restrictions in their shoul­der joints, altered posture or scapulothoracic dyskinesis were recruited to this study. Participants were instructed to sit on a chair with their back against the back-rest. Measurements of the non-dominant arm were obtained at rest and during isometric contraction at a neutral position and every 15° interval from 30° to 150° during scap­tion. Muscle activity was defined as the difference in SWE-measured stiffness between resting outcomes and those measured during muscle contraction (deltaE = stiffness during contraction – stiffness at rest). Results. The anterior-middle sub-region of the SSP muscle and the middle region of the deltoid muscle presented a mountain-shape type curve with peaks at 60° (146.2 ± 26.6 kPa) and at 90° (142.0 ± 25.9 kPa), respectively. The anterior-superficial sub-re­gion of SSP muscle peaked at 30° (102.1 ± 27.4 kPa), linearly decreasing thereafter. Conclusions. The anterior-superficial sub-region of the SSP muscle showed activity during initial range of motion, while the anterior-middle sub-region showed activity at early mid-range. On the contrary, the middle region of the deltoid muscle showed increased SWE-measured activity at late mid-range. A more refined approach focusing on the muscles sub-regions may lead to improved rehabilitation protocols.

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