Affected and unaffected quantitative aspects of grip force control in hemiparetic patients after stroke

Adequate grip force modulation is critical to manual dexterity and often impaired in hemiparetic stroke patients. Previous studies in hemiparetic patients suggest that aspects of grip force control may be differently affected by the lesion. We developed a visuomotor power grip force-tracking task allowing quantification of tracking error, force variability and release duration. We investigated force control in 24 chronic stroke patients with varying severity of hemiparesis and in healthy control subjects. Force tracking was performed at 10, 20, and 30% maximal voluntary contraction (MVC). Control subjects were also tested at absolute force levels similar to those of the patients. Patients tracking with their paretic hand at similar relative (%MVC) grip force levels showed increased error, force variability and release duration, but surprisingly, there was no difference in tracking error or variability between patients and control subjects performing at similar absolute force levels. Furthermore, patients improved their tracking performance across repeated blocks similar to control subjects. Release duration, however, was increased (also in the non-paretic hand), was force-independent and did not correlate with MVC strength. Of the three performance measures, only release duration explained some of the variance in arm and hand function (Frenchay Arm Test score), independent of MVC strength. The findings show (i) that hemiparetic stroke patients preserve the ability to modulate (generate and maintain) power grip force within their limited force range and (ii) that MVC grip strength and duration of grip release are differently affected and are two complementary predictors of arm function after stroke.