Adiposity Predicts Postural Sway in Older but Not Younger Adults: A Novel Age-Dependent Relationship

Background Aging is associated with increases in fat mass (FM) and decreases in fat-free mass (FFM), yet whether these indices independently predict force plate-derived postural sway and low-frequency postural oscillations remains unknown. Methods Eighty-seven participants: 44 younger adults (YA; 26 ± 5 years) and 43 older adults (OA; 69 ± 6 years), completed body composition assessment using bioimpedance spectroscopy and postural sway assessment using a force plate. Fat mass index (FMI), fat-free mass index (FFMI), and the FM/FFM ratio were derived from FM and FFM. Wavelet analysis extracted low-frequency oscillations (0–1 Hz) from the COP signal. Multiple linear regressions analyzed body composition indices as predictors of COP sway displacement and 0–1 Hz power separately for YA and OA. Results OA exhibited significantly greater COP sway displacement and 0–1 Hz power when compared to YA, despite no significant group differences in body composition indices. FM/FFM ratio and FMI significantly predicted COP sway displacement (R² = 0.195, p  = 0.016; R² = 0.218, p  = 0.009) and low-frequency oscillations (R² = 0.235, p  = 0.006; R² = 0.236, p  = 0.006) in OA, but not YA. FFMI did not significantly predict postural sway in either group. Conclusions FMI, but not FFMI, predicts postural sway and low-frequency postural oscillations specifically in OA, despite comparable body composition profiles between groups. This age-dependent relationship suggests that increases in YA FM do not meaningfully affect balance but do create postural instability in OA. These findings highlight the importance of body composition assessment when evaluating fall risk in OA and suggest that interventions targeting FM reduction may be more effective than lean mass preservation alone for improving postural stability in aging populations.