A Tale of Two Legs: Maintaining Dynamic Stability in A-P and M-L Directions in Persons with Unilateral Transtibial Limb Loss
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Falling remains a significant concern among individuals with lower limb loss. While recent years have seen major advances in prosthetic technology that have contributed to improvements in locomotor performance, gaps still remain to address impairments related to balance control. This is due to our limited understanding of the mechanisms involved in dynamic balance control among individuals with lower limb loss. The objective of this study was to characterize the dynamic balance control of individuals with transtibial limb loss (TILL) in response to unexpected support surface translations in 12 evenly distributed (0-3300) and randomly applied directions. We hypothesized that individuals with unilateral TILL would exhibit decreased dynamic balance control compared to matched controls. Dynamic balance control was quantified using stability margin, a metric that calculates the difference between peak center of pressure (COP) and peak center of mass (COM) displacements. A smaller stability margin is interpreted as an increased likelihood of losing balance. An increased stability margin (p=0.012) during lateral sway toward the prosthetic limb was identified for individuals with TILL (n=4) compared to controls (n=4). This increase was due to a larger cumulative COP displacement (p= 0.012) rather than a reduction in COM displacement. Additionally, while stability margin and cumulative COP displacement were the same between groups during forward sway; individual COP displacements in the intact leg of individuals with TILL were significantly larger than both the prosthetic leg (p<0.001) and control subjects (p=0.002). These findings demonstrate that the intact leg plays a substantial role in compensating for deficits in the dynamic response of the prosthetic limb during forward sway (posterior perturbations). During lateral perturbations that load the prosthetic limb, the results suggest that individuals with TILL are unable to modulate the COP response under that prosthetic foot. These findings may provide insight into prosthetic foot/ankle design and training strategies.