Altered digit force direction during pinch grip following stroke.

This study examined grip force development in individuals with hemiparesis following unilateral stroke. Eleven patients with chronic stroke with severe hand impairment and five age-matched neurologically intact subjects grasped an instrumented object between the index finger and thumb while fingertip forces, digit posture, and muscle electromyographic activity were recorded. We tested a range of different grip conditions with varying grip sizes, object stability, and grip force level. We found that fingertip force direction in the paretic digits deviated from the direction normal to the grip surface by more than twice as much as for asymptomatic digits. Additionally, the paretic thumb had, on average, 18% greater deviation of grip force direction than the paretic index finger. This large deviation of finger force direction for the paretic digits was consistently observed regardless of grip size, grip force level, and object stability. Due to the large deviation of the force direction from the normal direction, the paretic digits slipped and moved more than 1 cm during 55% of all grasping trials. A regression analysis suggests that this altered grip force direction was associated with altered hand muscle activation patterns, but not with the posture at which the digit made contact with the object. Therapies to redirect the force direction at the digits may improve stroke survivors' ability to stably grip an object.