Nerve, muscle and adiposity: Associations with gait speed across adulthood in the Baltimore Longitudinal Study of Aging.

Walking is fundamental to human mobility, and slowing gait speed is a widely recognized indicator of aging-related mobility decline. Although mobility decline in older adults has been associated with aging-related neural and morphological changes in muscle, the specific neural and morphological correlates of gait speed remain incompletely characterized. Thus, our primary aim was to quantify how peripheral nerve function, muscle cross-sectional area (MCSA) and intramuscular fat (IMAT) are related to gait speed across adulthood. We studied 898 participants from the Baltimore Longitudinal Study of Aging (BLSA), aged ≥ 20 years, with complete data on gait speed, body composition, MCSA, IMAT, and peripheral nerve function. Multivariable regression explained 29 % and 40 % of the variance in usual and rapid gait speed, respectively. Peripheral nerve measures (signal amplitude and conduction velocity) showed modest associations with both usual and rapid gait speed after mutual adjustment. In contrast, age, height, body roundness index (BRI), MCSA, and IMAT were independently associated with both gait speeds. Structural equation modelling identified MCSA and IMAT as the primary mediators of age-related gait decline, while nerve function played a smaller, task-specific role. Multi-group analysis revealed that these physiological mechanisms were stable across adulthood; however, the direct effect of age on rapid gait attenuated in the oldest-old, indicating that functional decline in this cohort is fully mediated by neuromuscular and morphological factors. Together, these findings indicate that peripheral nerve function contributes to gait speed across adulthood but plays a comparatively limited role relative to muscle morphology (MCSA and IMAT), which emerge as dominant physiological contributors to age-related mobility decline.