In-Vivo Scapular Muscle Morphology in Individuals with Scapular Dyskinesis: A Pilot Study

Abstract

Background: Observable alterations in scapular motion during arm elevation, termed dyskinesis, are present in individuals with and without shoulder pain. Dyskinesis is theorized to contribute to shoulder injury. However, there is a paucity of evidence regarding the underlying cause of dyskinesis. Purpose: To determine whether modifiable factors such as intrinsic scapular muscle morphology and maximum isometric force production are associated with dyskinesis in healthy individuals. Subjects: To date, N=5 individuals with and N=1 without obvious scapular dyskinesis participated. All subjects were free of shoulder pain. Methods: Scapular motion was evaluated with a validated and reliable clinical exam method. Scapular muscle (serratus anterior; lower trapezius) morphology was assessed with previously validated ultrasonographic measures of thickness at rest and with active contraction. Percent change in thickness during a contraction of the lower trapezius (LT) and serratus anterior (SA) were calculated from two trials each. Two trials of maximum isometric muscle force was assessed with a dynamometer, averaged, and normalized to body weight. Results: Subjects with dyskinesis demonstrated a 27.6% (SD=34.9%) and 79.9% (SD=46.2) increase in SA and LT thickness with contraction, respectively, compared to a 15.6% and 36% the individual with normal scapular motion. However, there was no significant correlation between normalized LT and SA strength and change in muscle thickness (p>0.05). Conclusions: A larger increase in SA and LT muscle thickness with the same active contraction suggests intrinsic muscle deficits exist in individuals with dyskinesis. Study with a larger sample is warranted to inform rehabilitation and shoulder injury prevention programs.