Impaired foot-force direction regulation during postural loaded locomotion in individuals poststroke

Following stroke, hemiparesis results in impaired motor control. Specifically, inappropriate direction of foot-forces during locomotion has been reported. In our previous study (Liang and Brown 2011) that examined poststroke foot-force direction during a seated, supported locomotor task, we observed that foot-force control capabilities were preserved poststroke. In this current study, we sought to better understand the mechanisms underlying the interaction of locomotor and postural control as an interactive mechanism that might interfere, poststroke, with appropriate foot-force generation. We designed an experiment in which participants performed biomechanically controlled locomotor tasks, under posturally challenged pedaling conditions while they generated mechanical output that was comparable to pedaling conditions without postural challenge, thus allowing us to monitor the strategies that the nervous system adopts when postural conditions were manipulated. We hypothesized that, with postural influence, individuals poststroke would generate inappropriate shear forces accompanied by inappropriate changes to muscle activity patterns when performing a mechanically controlled locomotor task, and would be exaggerated with increased postural loading. Sixteen individuals with chronic poststroke hemipa-resis and 14 age-similar nonimpaired controls pedaled on a cycle ergometer under 1) seated supported and 2) nonseated postural loaded pedaling conditions, generating matched pedal force outputs of two effort levels. When we compared postural influence with seated pedaling, we observed increased magnitudes of forward-directed shear forces in the paretic legs associated with increased magnitude of leg extensor muscle activity, but not in controls. These findings provide evidence to support a model that describes independent controllers for posture and locomotion, but that the interaction between the two controllers is impaired poststroke.