Weakness is the primary contributor to finger impairment in chronic stroke

Objective: To assess the relative contributions of several neurologic and biomechanic impairment mechanisms to overall finger and hand impairment in chronic hemiparetic stroke survivors. Design: Repeated-measures design. Setting: Clinical research laboratory. Participants: Thirty stroke survivors with chronic hemiparesis. Fifteen subjects had severe hand motor impairment and 15 had moderate impairment, as measured with the Chedoke-McMaster Stroke Assessment. Interventions: Not applicable. Main Outcome Measures: The biomechanic factors stiffness and resting flexion torque, together with the neurologic factors spasticity, strength, and coactivation, were quantified by using a custom hand manipulator, a dynamometer, and electromyographic recordings. Both passive and active rotations of the metacarpophalangeal joints of the fingers were examined. Results: Although subjects in the severely impaired group exhibited statistically greater passive stiffness and resting flexion torque than their moderately impaired counterparts (P <.05), the overall effect of these biomechanic changes appeared small in relation to the deficits attributable to neurologic changes such as spasticity and, especially, weakness. In fact, weakness in grip strength and isometric extension accounted for the greatest portion of the variance between the 2 groups (eta(2) =.40 and eta(2) =.23, respectively). Conclusions: Thus, deficits in hand motor control after stroke seem to derive mainly from weakness, which may be attributable to the loss of descending corticospinal pathway activation of motoneurons.