Background: Scapular dyskinesis is an observable alteration of shoulder motion, specifically related to the scapula, associated with shoulder injury. Clinicians attribute dyskinesis to scapular muscle impairments without evidence to support this relationship. Purpose: To determine whether modifiable factors including isometric scapular strength and muscle morphology are associated with dyskinesis. Subjects: Five individuals with dyskinesis and one with normal motion participated. Methods: Scapular motion was assessed using a reliable and validated method. æSerratus anterior (SA) and lower trapezius (LT) muscle morphology was evaluated with measures of thickness at rest and with contraction against gravity using ultrasonography. Percent change of muscle thickness of the SA and LT was calculated from two trials of each contraction condition. æTwo trials of maximum muscle (SA; LT) strength 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 only 15.6% and 36% in the individual with normal scapular motion. There was no correlation between normalized LT and SA strength and change in muscle thickness with contraction (p>0.05). Conclusions: A larger increase in SA and LT muscle thickness suggests intrinsic muscle deficits exist in individuals with dyskinesis, yet further study that includes change in muscle thickness with a maximum isometric strength test is warranted. Change in scapular muscle thickness is a modifiable factor that supports clinician treatment choices for individuals with dyskinesis in shoulder rehabilitation and injury prevention programs.