Rethinking margin of stability: Incorporating step-to-step regulation to resolve the paradox.
biomechanical · Level V
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- Record sourced from PubMed, PMID 36244320.
- Also identified by DOI 10.1016/j.jbiomech.2022.111334 and PMC identifier 10474615.
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Abstract
Derived from inverted pendulum dynamics, mediolateral Margin of Stability (MoS<sub>ML</sub>) is a mechanically-grounded measure of instantaneous frontal-plane stability. However, average MoS<sub>ML</sub> measures yield paradoxical results. Gait pathologies or perturbations often induce larger (supposedly "more stable") average MoS<sub>ML</sub>, 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 MoS<sub>ML</sub> 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-MoS<sub>ML</sub> manifolds, including one candidate "stability GEM" signifying the goal to maintain some constant MoS<sub>ML</sub><sup>∗</sup>. We used this framework to assess step-to-step MoS<sub>ML</sub> 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 MoS<sub>ML</sub><sup>∗</sup> 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 MoS<sub>ML</sub>, we present a new statistic, Probability of Instability (PoI<sub>L</sub>), used here to predict lateral instability likelihood. Participants exhibited increased PoI<sub>L</sub> when destabilized (p = 9.45 × 10<sup>-34</sup>), despite exhibiting larger ("more stable") average MoS<sub>ML</sub> (p = 1.70 × 10<sup>-15</sup>). Thus, PoI<sub>L</sub> correctly captured people's increased risk of losing lateral balance, whereas average MoS<sub>ML</sub> did not. PoI<sub>L</sub> also helps explain why people's average MoS<sub>ML</sub> increased in destabilizing contexts.
Medical subject headings
- Gait
- Postural Balance