Dynamic margins of stability during human walking in destabilizing environments

Abstract

Understanding how humans maintain stability when walking, particularly when exposed to perturbations, is key to preventing falls. Here, we quantified how imposing continuous, pseudorandom anterior-posterior (AP) and mediolateral (ML) oscillations affected the control of dynamic walking stability. Twelve subjects completed five 3-minute walking trials in the Computer Assisted Rehabilitation ENvironment (CAREN) system under each of 5 conditions: no perturbation (NOP), AP platform (APP) or visual (APV) or ML platform (MLP) or visual (MLV) oscillations. We computed AP and ML margins of stability (MOS) for each trial. Mean MOS(ml) were consistently slightly larger during all perturbation conditions than during NOP (p≤0.038). Mean MOS(ap) for the APP, MLP and MLV oscillations were significantly smaller than during NOP (p<0.0005). Variability of both MOS(ap) and MOS(ml) was significantly greater during the MLP and MLV oscillations than during NOP (p<0.0005). We also directly quantified how the MOS on any given step affected the MOS on the following step using first-return plots. There were significant changes in step-to-step MOS(ml) dynamics between experimental conditions (p<0.0005). These changes suggested that subjects may have been trying to control foot placement, and consequently stability, during the perturbation conditions. Quantifying step-to-step changes in margins of dynamic stability may be more useful than mean MOS in assessing how individuals control walking stability.

Figure 1

A) MOS_ap was defined as the distance between the anterior boundary of the BOS, defined by the leading toe marker (either LTOE or RTOE for the left and right foot, respectively) and the XcoM. MOS_ml was defined as the distance between the lateral boundary of the BOS and the XcoM. The lateral boundary of the BOS was defined by the lateral heel marker (LLHL and RLHL for the left and right foot, respectively) of the lead foot. Here, the left foot is shown leading. B) Quadrants of the MOS_i vs. MOS_i-1 plane (i.e., first-return map) were defined to compare step-to-step changes in MOS. Points in Q1 indicate stable steps followed by stable steps. Points in Q2 indicate stable steps followed by unstable steps. Points in Q3 indicate unstable steps followed by unstable steps. Points in Q4 indicate unstable steps followed by stable steps.

Figure 2

A) Mean MOS in the mediolateral (ML) direction. B) Mean MOS in the anterior-posterior (AP) direction. C) Variability of ML MOS. D) Variability of AP MOS. Error bars indicate between-subject standard deviations. * indicate significant (p < 0.05) differences from NOP.

Figure 3

First-return plots for AP MOS showing MOS_ap,i for step i versus MOS_{ap,i- 1} for the previous step, i-1. Columns (A) and (B) show data from 2 representative subjects. Red ‘o’ indicate MOS_ap,i-1 was at left heelstrike (LHS). Black ‘x’ indicate MOS_ap,i-1 was at right heelstrike (RHS).

Figure 4

First-return plots for ML MOS showing MOS_ml,i for step i versus MOS_ml,i-1 for the previous step, i-1. Columns (A) and (B) show data from 2 representative subjects who exhibited different stepping responses to the perturbations. Red ‘o’ indicate MOS_ml,i-1 was at left heelstrike (LHS). Black ‘x’ indicate MOS_ml,i-1 was at right heelstrike (RHS). Note that Subject 12 (B) is more asymmetrical than Subject 2 (A), but is not more unstable.

Figure 5

A) MOS_ml by right and left foot for all test conditions for a typical subject (Subject 6). B) MOS_ap by right and left foot for all test conditions for the same subject as in (A). LHS indicates MOS at left heelstrike. RHS indicates MOS at right heelstrike. This subject clearly shows the strong medilateral right-left asymmetry, where right steps were associated with greater lateral stability (A). A less pronounced (but still apparent) asymmetry, also in favor of the right leg, was seen in the anterior-posterior MOS (B).