Strength and functional correlates of reachable workspace in facioscapulohumeral muscular dystrophy

Patients with FSHD can develop scapular winging and loss of ability to raise arm. Reachable workspace (RWS) is able to describe upper extremity (UE) reaching ability. We analyzed data from 175 clinically affected and genetically confirmed adults with FSHD enrolled in the Clinical Trial Readiness to Solve Barriers to Drug Development in FSHD (ReSolve) study. Our objective was to 1) investigate the relationship between UE reaching ability and muscle strength (using quantitative muscle testing, QMT); and (2) describe how different reaching abilities correspond to patient-reported function at home. There were strong correlations between total relative surface area (i.e., upper extremity reachability in the frontal plane) and UE strength measured by QMT. For each successive 25 % decrease in total relative surface area, there was a corresponding mean 18 % decrease in predicted UE strength and a mean 9-point decrease in the Upper Extremity Functional Index. Individuals that could not reach the right upper medial quadrant reported more difficulty performing various upper extremity activities of daily living and had decreases in the physical function as reported by the PROMIS-57 questionnaire. RWS provides a meaningful measurement of upper extremity strength and function in patients with FSHD.