Patients With Spinal Muscular Atrophy Use High Percentages of Trunk Muscle Capacity to Perform Seated Tasks

OBJECTIVE: The aim of the study was to investigate trunk function during seated upper limb tasks in patients with spinal muscular atrophy types 2 and 3. DESIGN: Seventeen persons with spinal muscular atrophy and 15 healthy controls performed several tasks when sitting unsupported, such as reaching (and placing) forward and sideward. Joint torque and muscle activity were measured during maximum voluntary isometric contractions. Three-dimensional kinematics and normalized muscle activity were analyzed when performing tasks. RESULTS: Trunk joint torques were significantly decreased, approximately 45%, in patients with spinal muscular atrophy compared with healthy controls. Active range of trunk motion was also significantly decreased in all directions. When performing tasks, the average back muscle activity was 27% and 56% of maximum voluntary isometric contractions for healthy controls and spinal muscular atrophy and for abdominal muscles 10% and 44% of maximum voluntary isometric contractions, respectively. Trunk range of motion did not differ when performing daily tasks. CONCLUSIONS: The trunk of patients with spinal muscular atrophy is weaker compared with healthy controls, reflected by reduced trunk torques and decreased active range of motion. In addition, patients with spinal muscular atrophy use high percentages of their trunk muscle capacity to perform tasks. Clinicians should take this into account for intervention development, because using high percentages of the maximum muscle capacity results in fatigue and muscle overloading.