Foot Force Control Deficit After Stroke

Observed Misdirection of Foot Force

Using our previously described non-postural tasks (seated pushing on stationary and moving pedals), we revealed that stroke causes a misdirection of foot force. While in non-disabled individuals the muscle component of the foot force (linear force path) was directed near the center-of-mass (CM), in both chronic and acute (< 24 hours) stroke survivors the direction was biased to be anterior and lateral of the CM. This effect was present for both static and moving pedals suggesting that it is not a timing error, but rather, a coordination error wherein the relative torques at the hip and knee are slightly biased relative to that present in non-disabled and non-paretic limbs.

Significance

Highly preserved force path linearity in the paretic limb suggests that the coordination responsible for keeping the joint torques proportional through the push effort are preserved post-stroke, and thus, may be a spinal mechanism (consistent with spinal ‘motor primitives’). The misdirection of the force results from the choice of a specific muscle activation ratio which implicates higher brain centers damaged by the stroke.

These post-stroke observations and the walking patterns of newly-walking toddlers suggest that stroke may be causing a regression to a primitive control strategy. One compensation for the anterior misdirected foot force is to flex at the hip joint to better align the force vector with the CM. Toddlers adopt that posture in early months of walking indicating that they too have a foot force control that is biased anteriorly. This may explain why all stroke survivors’ force misdirection was anteriorly biased; they all were using the innate control strategy. Via some mechanism, the stroke prevented access to the behavior learned as a toddler. These ideas have profound implications for the recovery of walking after stroke and guiding the development of walking for those with cerebral palsy.

Relevant Publications:

Rogers LM, Brown DA, Gruben KG: Foot force direction control during leg pushes against fixed and moving pedals in persons post-stroke. Gait and Posture, 19(1):58-68, 2004.

 

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