This study aimed to identify critical physiological outcome variables underlying reduced upper extremity task performance in Duchenne muscular dystrophy (DMD). These critical variables were used to propose an explanatory biophysical model of the upper extremity working mechanisms in DMD. Twenty-three DMD patients (8–21 years) participated in this study. Correlations with Brooke scale and Performance of Upper Limb (PUL) score were very high for maximal active joint angle, high for maximal muscle torque and maximal surface electromyography amplitude, and moderate for mean echogenicity and maximal passive joint angle. Multivariable regression analysis showed that maximal active joint angle and maximal muscle torque were significantly associated with Brooke score (R ² = 0.91). Maximal active joint angle, maximal passive joint angle, and maximal muscle torque were significantly associated with PUL score (R ² = 0.94). Based on the most critical physiological outcome variables, we constructed an exploratory biophysical model of the working mechanisms leading to limitations in upper extremity task performance. Better insights in these working mechanisms could support clinical management of upper extremity limitations and facilitate the development of interventions. In addition, the model could form the basis for new multi-layered outcome measures for clinical trials.

Background: Therapeutic management of upper extremity (UE) function of boys and men with Duchenne Muscular Dystrophy (DMD) requires sensitive and objective assessment. Therefore, we aimed to measure physiologic UE function of healthy subjects and DMD patients in different disease stages, and to evaluate the relation between these physiologic measures and functional UE scales. Methods: Twenty-three DMD patients and twenty healthy controls (7-23 years) participated in this explorative case-control study. Maximal muscle torque, maximal and normalized surface electromyography (sEMG) amplitudes, muscle thickness, echogenicity and maximal passive and active joint angles were measured. At activity level, Brooke upper extremity rating scale and the Performance of Upper Limb (PUL) scale were used. Results: Outcome measures related to proximal UE function could discriminate between disease stages. Increased normalized sEMG amplitudes were found in patients, even in early disease stages. Maximal active joint angles showed the strongest relation to Brooke scale (R ² = 0.88) and PUL scale (R ² = 0.85). Conclusions: The decline of muscle functions precedes the decline in performance of UE activities, and therefore may play a role in early detection of UE limitations. Increased sEMG levels demonstrate that DMD patients use more of their muscle capacity compared to healthy subjects, to perform daily activities. This might result in increased fatigability. Active maximal joint angles are highly related to functional scales, so preserving the ability to use the full range of motion is important for the performance of daily activities. Close monitoring of active joint angles could therefore help in starting interventions that minimize functional UE decline in DMD patients timely.