Age and Cognitive Load Alter Joint Work Distribution and Postural Stability During Single Limb Support

Age‑related declines in neuromuscular control increase fall risk during single‑limb stance, especially under cognitive load. Altered joint‑work distribution and reduced coordination precision may contribute to this instability, yet dual‑task effects on these mechanisms remain unclear. PURPOSE: To examine age‑related differences in joint eccentric work, inter‑joint coordination (FWHM), and postural stability during single‑limb support under normal and cognitive dual‑task conditions. METHODS: Eight adults (Young: n=4, 22 ± 2 y; Older: n=4, 73 ± 5 y) performed ~3 s of single‑limb support on a force plate following a crossover step under (1) Normal and (2) Cognitive conditions. Under the Cognitive condition, participants were given a randomized starting number and instructed to subtract by serial 7s. Outcomes included knee and hip eccentric work (J/kg), inter‑joint coordination, full width at half maximum (FWHM), for ankle–knee, knee–hip, and hip–ankle pairs, center‑of‑pressure (CoP) velocity, and RMS shear force (% body weight). Due to the pilot sample size, descriptive statistics (mean ± SD) were reported. RESULTS: Joint work (J/kg): Young showed greater knee eccentric work (Normal: 0.308 ± 0.148; Cognitive: 0.462 ± 0.328) than Older (Normal: 0.205 ± 0.131; Cognitive: 1.679 ± 0.393 [n=3]). Older exhibited markedly greater hip eccentric work (Normal: 4.392 ± 4.972; Cognitive: 10.588 ± 13.457) than Young (Normal: 1.226 ± 0.807; Cognitive: 1.921 ± 1.086). Postural stability: CoP velocity increased with cognitive load in both groups (Older AP: 0.236→0.356 m/s; Younger AP: 0.139→0.146 m/s). Representative RMS shear forces rose with dual‑tasking in Young (AP% 7.036 ± 0.525 → 7.207 ± 0.962), while Older showed lower magnitudes but higher variability (Normal AP% 4.326 ± 5.028; Cognitive 0.020 ± 0.011 [n=3]). Inter‑joint coordination (FWHM): Older demonstrated broader FWHM than Young under Normal conditions (e.g., ankle–knee representative range: Young ~0.17–0.35 s; Older ~0.32–0.33 s), indicating less temporally precise coupling; cognitive load further widened FWHM in both groups, with the largest increases in Older. CONCLUSION: Older adults relied more on hip‑dominant stabilization, whereas Young produced greater knee extensor eccentric control. Cognitive load reduced postural stability and coordination precision in both groups, with Older showing the largest increases in hip work and FWHM. Lower RMS shear in older adults likely reflects stiffening: reduced ankle motion with corrections shifted to the hip/trunk, leading to lower foot shear yet greater variability and poorer stability. These pilot findings support age‑related distal‑to‑proximal control shifts and heightened dual‑task vulnerability during balance tasks.