Effects of body orientation on maximum voluntary ARM torques

Introduction: Increased reliance on bulbospinal motor systems has been implicated in individuals with chronic stroke during maximum voluntary arm joint torque generation. Methods: Maximum isometric single-joint and multi-joint arm strength was observed in two body orientations (sitting and supine) while maintaining identical head/neck/trunk/extremity joint configurations in order to identify bulbospinal contributions to maximum joint torque generation in 11 individuals with stroke and 10 individuals without stroke. Results: During sitting, shoulder flexion was greater for both groups, whereas shoulder extension and elbow flexion, part of the 'flexion synergy,' were greater only in individuals with stroke. Conclusions: Body orientation influenced isometric arm strength, notably the constituents of flexion synergy in individuals with stroke, suggesting bulbospinal motor pathway involvement. From a practical perspective, clinical evaluation of single joint strength in the supine position may underestimate strength available during activities of daily living that are performed in an upright orientation.