Lower-limb motor function transitions and subsequent cognitive function among older Chinese adults: a longitudinal study using CHARLS 2011–2020

Lower-limb motor function and cognitive function are closely linked in later life, but most prior studies have focused on single-time-point measures. Whether transitions in combined lower-limb motor function status are associated with subsequent cognitive outcomes remains unclear. This longitudinal study used data from the 2011–2020 CHARLS waves. Participants were aged ≥ 60 years with classifiable lower-limb motor function in both 2011 and 2015 and non-missing cognition scores in 2015 and 2020. Lower-limb motor impairment was operationally defined as a relative performance-based classification within each wave, based on low gait speed or poor chair-stand performance using sex-specific quartile cut-offs. Participants were classified into four transition groups: persistent normal, improved, incident impairment, and persistent impairment. The primary outcome was total cognition score in 2020 adjusted for 2015 score. Random-intercept mixed-effects models evaluated cognitive trajectories across 2011, 2013, 2015, and 2020. The final sample included 1729 participants: 770 persistent normal, 270 improved, 361 incident impairment, and 328 persistent impairment. In the primary model, incident impairment was associated with lower total cognition in 2020 versus persistent normal (β = -0.64, 95% CI -1.14 to -0.13, P = 0.014). The improved group did not differ from persistent normal (β = -0.05, 95% CI -0.60 to 0.51, P = 0.866), and persistent impairment showed a non-significant negative association (β = -0.48, 95% CI -1.03 to 0.07, P = 0.087). Among cognitive domains, attention showed the strongest signal (β = -0.29, 95% CI -0.50 to -0.07, P = 0.009), followed by orientation (β = -0.14, 95% CI -0.27 to -0.01, P = 0.038). Trajectory models showed visual separation in predicted cognitive levels, but interaction terms between follow-up time and transition group did not provide clear statistical evidence of different slopes. Membership in the incident impairment group was was associated with poorer subsequent global cognitive function, particularly attention. This operational transition represents movement into a wave-specific, sex-specific low-performance category rather than clinically defined incident impairment.