Abstract
Recent experimental findings support theoretical predictions that across walking conditions the motor system chooses foot placement to achieve a constant minimum "margin of stability" (MOSmin)--distance between the extrapolated center of mass and base of support. For example, while step width varies, similar average MOSmin exists between overground and treadmill walking and between overground and compliant/irregular surface walking. However, predictions regarding the invariance of MOSmin to step-by-step changes in foot placement cannot be verified by average values. The purpose of this study was to determine average changes in, and the sensitivity of MOSmin to varying step widths during two walking tasks. Eight young subjects walked on a dual-belt treadmill before and after receiving information that stepping on the physical gap between the belts causes no adverse effects. Information decreased step width by 17% (p = .01), whereas MOSmin was unaffected (p = .12). Regardless of information, subject-specific regressions between step-by-step values of step width and MOSmin explained, on average, only 5% of the shared variance (β = 0.11 ± 0.05). Thus, MOSmin appears to be insensitive to changing step width. Accordingly, during treadmill walking, step width is chosen to maintain MOSmin. If MOSmin remains insensitive to step width across other dynamic tasks, then assessing an individual's stability while performing these tasks could help describe the health of the motor system.
Highlights
Proper foot placement during gait, as quantified by step width, is thought to be important in the step-by-step maintenance of frontal plane stability (Bauby & Kuo, 2000; Hurt et al, 2010; Townsend, 1985)
Providing information regarding the between-belt gap was associated with a 17% reduction of step width (119 ± 40 mm vs. 99 ± 34 mm; p = .01)
While step width was significantly narrower after providing information regarding the effects of stepping on the physical gap between the treadmill belts, on average, MOSmin was not significantly affected
Summary
Proper foot placement during gait, as quantified by step width, is thought to be important in the step-by-step maintenance of frontal plane stability (Bauby & Kuo, 2000; Hurt et al, 2010; Townsend, 1985). Inverted pendulum–based models considering only center of mass position are poorer predictors of dynamic behaviors, including forward motion during gait termination and the need for compensatory steps after external perturbations, compared with models including position and velocity (states) of the center of mass (Pai & Patton, 1997; Pai et al, 1998) Using these states, a recent model defines the “margin of stability” as the distance between the base of support and extrapolated center of mass (XCOM; see Eq 1) (Hof, 2008). Theoretical predictions relate to step-by-step behavior whereas the previous investigations compared mean values of MOSmin across conditions It is unclear whether, across dynamic tasks, MOSmin is insensitive to naturally occurring step-by-step variations in step width. We hypothesized that on average, dynamic stability (MOSmin) would be invariant to changes in step width and, further, it would be insensitive to step-by-step changes in step width during each condition
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