Understanding dynamic balance during walking using whole-body angular momentum
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Maintaining dynamic balance during walking is a major challenge in many patient populations including older adults and post-stroke hemiparetic subjects. To maintain dynamic balance, whole-body angular-momentum has to be regulated through proper foot placement and generation of the ground-reaction-forces. Thus, the overall goal of this research was to understand the mechanisms and adaptations used to maintain dynamic balance during walking by analyzing whole-body angular-momentum, foot placement and ground-reaction-forces in older adults and post-stroke subjects. The analysis of healthy older adults showed that they regulated their frontal-plane angular-momentum poorly compared to the younger adults. This was mainly related to the increased step width, which when combined with the dominant vertical ground-reaction-force, created a higher destabilizing external moment during single-leg stance. The results also suggested that exercise programs targeting appropriate foot placement and lower extremity muscle strengthening, particularly of the ankle plantarflexors and hip abductors, may enhance balance control in older adults. During post-stroke hemiparetic walking, ankle-foot-orthosis and locomotor therapy are used in an effort to improve the overall mobility. However, the analyses of healthy subjects walking with and without a solid ankle-foot-orthosis showed that they can restrict ankle plantarflexor output and limit the successful regulation of angular-momentum and generation of forward propulsion. Thus, the prescription of solid ankle-foot-orthosis should be carefully considered. The analysis of hemiparetic subjects walking pre- and post-therapy showed that locomotor training did not improve dynamic balance. However, for those subjects who achieved a clinically meaningful improvement in their self-selected walking speed, their change in speed was correlated with improved dynamic balance. Also, improved balance was associated with narrower mediolateral paretic foot placement, longer anterior nonparetic steps, higher braking ground-reaction-force peaks and impulses, higher (lower) propulsive ground-reaction-force peaks and impulses from the paretic (nonparetic) leg, and higher vertical ground-reaction-force impulses from both legs during the late stance. Further, simulation analyses of hemiparetic walking highlighted the importance of ankle plantarflexors, knee extensors and hip abductors in maintaining balance and revealed the existence of compensatory mechanisms due to the paretic leg muscle weakness. Collectively, these studies showed the importance of ankle plantarflexors and hip abductors in maintaining dynamic balance.