Neuromuscular control of curved walking in people with stroke: Case report.

INTRODUCTION Current approaches to gait assessment following stroke primarily measure impairments during straight walking as opposed to skilled tasks that require the ability to adapt the basic walking pattern. Recent studies have postulated that impairments in gait that persist after stroke, such as slow speed and spatial-temporal asymmetry [1-2], may limit the ability to adapt the gait pattern as required to mobilize safely in the community [3]. Turning and curved walking create a unique set of demands on the neural processes involved in the control of medial-lateral (ML) stability and forward progression that are greater than the requirements for straight walking [4-8]. Given the significance of turning in everyday mobility [9-10], as well as the higher risk of falls and injuries while turning [11-15], it is surprising that there has been very little research to date on the capacity to adapt straight walking in ambulatory individuals with stroke. Some clinical assessments of mobility, the Timed "Up and Go" test (TUG) and Emory Functional Ambulation Profile, include a turning task to examine adaptation to straight walking. Both measures have been correlated with balance function [16-17], walking speed [17-18], and lower-limb motor impairment [19-20] in the stroke population. Previous findings have demonstrated that individuals with chronic stroke require a greater number of steps and more time to complete a turn to both the paretic and nonparetic sides than controls [21]. Furthermore, turning capacity at 180[degrees] was correlated with single-support time asymmetry, suggesting greater difficulty for individuals with more severe gait impairments [21]. Similarly, performance on the Figure-8 Walk Test shows that individuals with chronic stroke take twice as long to complete a curved walking task as nondisabled controls [22]. Although timed tests of functional ambulation offer an easy and efficient measure of gait performance in the clinical setting, they do not expose underlying challenges in the motor control of changing directions during walking that can be determined by examining the center of pressure (COP) trajectory. Curved walking imposes greater ML instability than straight walking [4-7] because the COP needs to shift toward the lateral edge of the inner foot [7,23]. People with stroke have impairments in the regulation of frontal plane stability, as indicated by a reduction in weight-shifting capacity while standing [24-26]. However, the extent to which ML control is affected during curved walking remains unknown. Examining the COP trajectory may provide information on the capacity to turn or walk along curved paths. Individuals with stroke demonstrate less ML variability and shorter anterior-posterior (AP) displacement in the COP trajectory under the paretic foot than the nonparetic foot during straight walking [27]. These findings suggest poor control of ML stability and forward progression, which coincides with a reduced single support time on the paretic side [27]. In addition, this may lead to difficulty adapting the gait pattern to match the greater demands required of curved walking. Modulation of muscle activation patterns has been identified with walking along curved pathways [28-29]. The inner leg of the turn shows reduced amplitude of activity in muscles located in the medial compartments of the lower limb, whereas those in the lateral compartments show increased amplitude with increasing path curvature; however, the opposite effect is observed for the outer leg [28]. These effects are consistent with a shift in foot pressures toward the lateral aspect of the inner foot [29]. One study investigating neuromuscular strategies for curved walking in people with chronic stroke found reduced adaptation in the gastrocnemii and hamstring muscles and a shift in foot pressures to increased curved walking paths compared with age-matched controls [29]. In addition, physiological evidence shows that the gastrocnemii muscles contribute to active control over ML ankle motion to stabilize the body in the frontal plane and may be related to the degree of challenge to stability [30]. …