Aging amplifies context-dependent variability in postural long-latency reflexes without enhancing reflex selectivity

Abstract

Falls are the leading cause of injury-related death in older adults, yet neural mechanisms underlying age-related balance decline remain poorly understood. Long-latency reflexes (LLRs; 50−100 ms) are particularly vulnerable to aging and represent a promising target for fall prevention. We examined whether wobble-board standing exposes age-related changes in these mechanisms in 96 healthy adults (48 younger, 48 older) performing a Trail Making Task with or without a mediolateral wobble board. Center-of-pressure trajectories were analyzed using multiscale probability density function analysis to estimate non-Gaussianity (λ̂2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\hat{\lambda }}^{2}$$\end{document})—a statistical signature of intermittent, burst-like fluctuations—across timescales putatively aligned with short-latency reflexes (SLRs; 20−50 ms), LLRs (50−100 ms), and compensatory postural adjustments (CPAs; 100−1000 ms). The wobble board selectively increased λ̂2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\hat{\lambda }}^{2}$$\end{document} within LLR-band timescales and reduced it within CPA-band timescales, consistent with faster intermittent corrections and reduced reliance on slower compensatory dynamics. Older adults showed greater absolute λ̂2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\hat{\lambda }}^{2}$$\end{document} under instability driven by elevated baseline variability rather than enhanced mechanism-specific modulation; their responses spread across LLR- and SLR-band timescales, consistent with reduced timescale selectivity. Instability shifted postural dominance from anteroposterior to mediolateral, with older adults showing reduced directional differentiation. Phase-randomized surrogates confirmed nonlinear temporal organization. Wobble-board instability may serve as both a diagnostic probe and a candidate exercise-as-medicine context for probing timescale selectivity in aging—a hypothesis that requires testing in future intervention studies.