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Research Papers

Human Foot Placement and Balance in the Sagittal Plane

[+] Author and Article Information
Matthew Millard, Derek Wight, John McPhee, Eric Kubica, David Wang

University of Waterloo, 200 University Avenue, West Waterloo, ON, N2L 3G1, Canada

J Biomech Eng 131(12), 121001 (Oct 29, 2009) (7 pages) doi:10.1115/1.4000193 History: Received September 03, 2008; Revised August 25, 2009; Posted September 10, 2009; Published October 29, 2009

Abstract

Foot placement has long been recognized as the primary mechanism that humans use to restore balance. Many biomechanists have examined where humans place their feet during gait, perturbations, and athletic events. Roboticists have also used foot placement as a means of control but with limited success. Recently, Wight (2008, “Introduction of the Foot Placement Estimator: A Dynamic Measure of Balance for Bipedal Robotics,” ASME J. Comput. Nonlinear Dyn., 3, p. 011009) introduced a planar foot placement estimator (FPE) algorithm that will restore balance to a simplified biped that is falling. This study tested the FPE as a candidate function for sagittal plane human-foot-placement (HFP) by recording the kinematics of 14 healthy subjects while they performed ten walking trials at three speeds. The FPE was highly correlated with HFP $(ρ≥0.997)$ and its accuracy varied linearly from 2.6 cm to −8.3 cm as walking speed increased. A sensitivity analysis revealed that assumption violations of the FPE cannot account for the velocity-dependent changes in FPE-HFP error suggesting that this behavior is volitional.

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Figures

Figure 1

The simplified biped before and after foot contact, assuming the foot sticks to the ground and momentum is conserved (3)

Figure 2

The simplified biped stepping relative to the FPE (3): (a) stepping closer than the FPE results in falling forward, (b) stepping further than the FPE causes the biped to fall back onto the swing leg, and (c) stepping precisely at the FPE will balance the COM above the standing foot

Figure 3

Box and whisker plots of the error between the FPE and each subject’s lateral malleolus at foot contact. Whiskers run between the fifth and the 95th percentiles; boxes between the 25th and the 75th percentiles with a hash at the 50th percentile. Natural paced trials align with the subject number; slow trials are immediately to the left and fast trials are to the right. Subjects are motivated by not only balance but pace and acceleration since they step further behind the FPE as they walk faster and with more variation when they initiate and terminate gait. We failed to collect constant cadence trials for subject 1, and fast constant cadence trials for subject 14.

Figure 4

Group ensemble plots of dimensionless normalized Ho, L, J, and T+V profiles between heel strike and the moment the COM passes over the LM. Means are drawn with a solid line while ±1 standard deviation is shown with a dotted line. The natural paced constant cadence trial is shown. The assumption that angular momentum is conserved is violated since it increases. The assumptions of constant L, J, and T+V are reasonable. Note that the sum of kinetic and potential energy (T+V) plot does not change appreciably because potential energy (V) dominates and remains relatively constant.

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