Rethinking Margin of Stability: Using STEP-To-STEP REGULATION TO RESOLVE THE PARADOX
Meghan Kazanski a,*, Joseph P. Cusumano b, Jonathan B. Dingwell a
a Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802 USA
b Department of Engineering Science & Mechanics, The Pennsylvania State University, University Park, PA 16802 USA
Derived from inverted pendulum dynamics, mediolateral Margin of Stability (MoSML) is a mechanically-grounded measure of instantaneous frontal-plane stability. However, average MoSML measures yield paradoxical results. Gait pathologies or perturbations often induce larger (supposedly “more stable”) average MoSML, despite clearly destabilizing factors. However, people do not walk “on average” – they walk (and sometimes lose balance) one step at a time. We assert the paradox arises because averaging MoSML discards crucial step-to-step dynamics. We present a framework unifying the inverted pendulum with Goal-Equivalent Manifold (GEM) analyses. We identify in the pendulum’s center-of-mass dynamics constant-MoSML manifolds, including one candidate “stability GEM” signifying the goal to maintain some constant. We used this framework to assess step-to-step MoSML dynamics of humans walking in destabilizing environments. While goal-relevant deviations were readily corrected, people did not exploit equifinality by allowing deviations to persist along this GEM. Thus, maintaining a constant is inconsistent with observed step-to-step fluctuations in center-of-mass states. Conversely, the extent to which participants regulated fluctuations in mediolateral foot placements strongly predicted their regulation of center-of-mass fluctuations. Thus, center-of-mass dynamics may arise indirectly as a consequence of regulating mediolateral foot placements. To help resolve the paradox caused by averaging MoSML, we present a new statistic, Probability of Instability (PoIL), used here to predict lateral instability likelihood. Participants exhibited increased PoIL when destabilized (p = 9.45 × 10^−34), despite exhibiting larger (“more stable”) average MoSML (p = 1.70 × 10^−15). Thus, PoIL correctly captured people’s increased risk of losing lateral balance, whereas average MoSML did not. PoIL also helps explain why people’s average MoSML increased in destabilizing contexts.
Figure: (A): Mediolateral Margin of Stability (MoSML) predicts distinct stability regions in the center-of-mass phase plane. (B): A candidate stability goal-equivalent manifold (GEM) exists in this plane and defines all combinations of CoM states that achieve the same constant MoSML. (C): PoIL measures lateral instability risk, computed as the percent likelihood that CoM dynamics from any future step will fall in the Lateral Instability region (A).
Highlights
Current implementations of A.L. Hof's Margin of Stability (MoS) (used in hundreds of studies evaluating balance) often deliver paradoxical indications of stability status.
We present the Probability of Instability (PoI) as a novel statistic that appropriately characterizes instability likelihood.
A goal-equivalent manifold (GEM) exists in the mediolateral center-of-mass phase plane.
This GEM mathematically defines a feasible potential mediolateral stability strategy.
Variability structure but NOT temporal structure of center-of-mass state fluctuations is consistent with a GEM-aware stabilizing strategy.