Rethinking Margin of Stability: Incorporating Step-To-Step Regulation to Resolve the Paradox

ABSTRACTMaintaining frontal-plane stability is a major objective of human walking. Derived from inverted pendulum dynamics, the mediolateral Margin of Stability (MoSML) is frequently used to measure people’s frontal-plane stability on average. However, typical MoSML-based analyses deliver paradoxical interpretations of stability status. To address mediolateral stability using MoSML, we must first resolve this paradox. Here, we developed a novel framework that unifies the well-established inverted pendulum model with Goal-Equivalent Manifold (GEM)-based analyses to assess how humans regulate step-to-step balance dynamics to maintain mediolateral stability. We quantified the extent to which people corrected fluctuations in mediolateral center-of-mass state relative to a MoSML-defined candidate stability GEM in the inverted pendulum phase plane. Participants’ variability and step-to-step correction of tangent and perpendicular deviations from the candidate stability GEM demonstrate that regulation of balance dynamics involves more than simply trying to execute a constant-MoSML balance control strategy. Participants adapted these step-to-step corrections to mediolateral sensory and mechanical perturbations. How participants regulated mediolateral foot placement strongly predicted how they regulated center-of-mass state fluctuations, suggesting that regulation of center-of-mass state occurs as a biomechanical consequence of foot placement regulation. We introduce the Probability of Instability (PoI), a convenient statistic that accounts for step-to-step variance to properly predict instability likelihood on any given future step. Participants increased lateral PoI when destabilized, as expected. These lateral PoI indicated an increased risk of lateral instability, despite larger (i.e., more stable) average MoSML. PoI thereby explicitly predicts instability risk to decisively resolve the existing paradox that arises from conventional MoSML implementations.

Using a stair horizontal-vertical illusion to increase foot clearance over an inconsistently taller stair-riser

Introduction Stair falls can be caused by inconsistent stair dimensions. During ascent, inconsistently taller stair risers lead to reduced foot clearances as the inconsistency goes unnoticed. A stair horizontal-vertical illusion increases perceived riser heights and foot clearance and could offset reduced foot clearances over inconsistently taller risers, though this might impact other stair safety measures. Method Twelve participants (age: 22 (3) years) ascended a seven-step staircase under three conditions: i) all steps consistent in riser height (consistent), ii) a 1cm increase in step 5 riser height (inconsistent) and iii) a 1cm increase in step 5 riser height, superimposed with a stair horizontal-vertical illusion (illusion). Vertical foot clearance, foot overhang, and margins of stability were assessed over step 4, 5 and 6. Perceived riser height due to the illusion was determined through a computer perception test. A One-Way Repeated Measures ANOVA compared biomechanical variables between conditions. A One Sample t test compared perceived riser height to the true height. Results Over the inconsistent step 5, foot clearance reduced by 0.8cm compared to consistent. Illusion increased foot clearance by 1.1cm and decreased foot overhang by 4% compared to inconsistent. On step 4 the illusion led to more anterior instability compared to inconsistent. Illusion and inconsistent led to more mediolateral stability compared to consistent. The illusion increased perceived riser height by 12%. Discussion Foot clearance reductions over inconsistently taller risers can be offset by a stair horizontal-vertical illusion. Additional benefits included a safer foot overhang and unaffected stability over the inconsistent riser. Changes to step 4 stability might have resulted from leaning forward to look at the step 5 illusion. The stair horizontal-vertical illusion could be a practical solution for inconsistently taller stair risers, where a rebuild is usually the only solution.

Static balance and proprioception evaluation in deaf national basketball players

Background: ‪Because of the proximity of anatomical structures involved in auditory and vestibular functions, complications concerning the inner ear may lead to problems involving both systems. This study investigated whether static balance and knee proprioception in deaf basketball players are different from those in individuals with normal hearing. Material and methods: ‪12 deaf national basketball players and 13 individuals with normal hearing were included. Static balance was evaluated using the Biodex-BioSway Balance System. Knee proprioception was assessed using a Dualer IQ Digital Inclinometer with the active angle repetition test. Results: ‪The dominant-side mediolateral stability index and non-dominant-side overall, anteroposterior, and mediolateral stability index scores of deaf basketball players in the postural stability test were different compared with those of the individuals with normal hearing (p<0.05). There was no statistically significant difference between the two groups in terms of knee proprioception (p>0.05). Conclusions: ‪Although there was no difference in knee proprioception in deaf basketball players, it is noteworthy that their postural stability was more impaired compared with that in individuals with normal hearing. Despite regular exercise, deaf basketball players had poorer balance than individuals with normal hearing. Therefore, balance and vestibular exercises should be added to the training programs of these athletes.