Evidence for task dependent weakness due to abnormal torque synergies in the impaired upper limb of hemiparetic stroke subjects: preliminary results

The purpose of this study was to quantify the degree of weakness and the possible existence of abnormal torque synergies in the upper limb of stroke subjects. Subjects produced static elbow and shoulder torques which were measured using a 6 degree of freedom (DOF) load cell for two separate protocols: uncontrolled maximum torque generation (UMTG), generation of maximum torques in a single specified joint degree of freedom (either elbow or shoulder); and controlled maximum torque generation (CMTG) which required the subject to produce maximum-submaximum elbow/shoulder torque combinations. Impaired arms exhibit weakness in all physiological degrees of freedom tested across all subjects. Furthermore, abnormal torque synergies (e.g. shoulder abduction/elbow extension) in the impaired upper limb were also identified. Finally, these abnormal torque synergies appear to compromise functional usage of the limb and produce task-dependent weakness when joint torque combinations are different from the abnormal torque synergies.