Foot force direction control during leg pushes against fixed and moving pedals in persons post-stroke

The component of foot force generated by muscle action (F_m) during pedaling in healthy humans has a nearly constant direction with increasing force magnitude (J Mot Behav 32 (2000) 151-62; Exp Brain Res 148 (2003) 50-61). The present study investigated the effect of stroke on the control of foot force. Ten individuals with hemiparesis secondary to a cerebral vascular accident performed pushing efforts against translationally fixed and moving pedals on a custom stationary cycle ergometer. We found that while F _m direction remained constant with increasing effort in both the fixed- and moving-crank conditions for both limbs, the orientation of that force component differed between limbs. The non-paretic limb produced the same F_m orientation as seen previously in healthy humans. However, relative to the non-paretic limb, the paretic limb force line-of-action was shifted away from the hip and closer to the knee in the sagittal-plane for both pedal motion conditions. In the frontal plane, the paretic limb force line-of-action was shifted laterally, closer to parallel to the midline, for both pedal motion conditions. These shifts were consistent with previously reported lower limb muscle weakness and alterations in muscle activation observed during pedaling tasks following stroke. The finding of similar orientations for static and dynamic pushing efforts suggests that limb posture could be a trigger for relative muscle activation levels. The preservation of a constant direction in F_m with increasing force magnitude post-stroke, despite an orientation shift, suggests that control of lower limb force may be organized by magnitude and direction and that these two aspects are differentially affected by stroke.