From inactivity to initiation: moderate-to-vigorous physical activity and subsequent functional outcomes in older adults—a sequential target trial emulation

Background Most studies of physical activity and functional ageing examine static activity levels or long-term trajectories, leaving the implications of an episode-specific transition to moderate-to-vigorous physical activity (MVPA) uncertain. Methods Health and Retirement Study data (2004–2018) were used to emulate four sequential trials among adults aged 60 years or older who were below the MVPA threshold at the preceding wave. MVPA initiation was defined from self-reported HRS frequency items as moderate- or vigorous-intensity activity at least once per week at the trial baseline among participants below that threshold at the preceding wave; the items assessed frequency rather than session duration or weekly minutes. The sample included 7,463 participants and 12,421 person-trial records. Multiple imputation, inverse probability weighting, and weighted generalized estimating equations assessed functional limitation scores among survivors and the composite outcome of incident ADL limitation or death over 2–6 years. Results MVPA initiation was associated with lower functional limitation scores at 2 years (mean difference, − 0.408 points; 95% CI, − 0.509 to − 0.307), 4 years (− 0.396 points; 95% CI, − 0.513 to − 0.278), and 6 years (− 0.352 points; 95% CI, − 0.493 to − 0.211). Risk ratios for incident ADL limitation or death were 0.844 (95% CI, 0.776 to 0.919), 0.869 (95% CI, 0.809 to 0.933), and 0.908 (95% CI, 0.854 to 0.965), respectively. Findings were directionally consistent under a stricter definition of more than once-weekly moderate or vigorous activity. Conclusions Episode-specific MVPA initiation was associated with subsequent functional outcomes in older adults. These estimates may represent a potentially relevant behavioral transition, although residual confounding and selection related to survival cannot be excluded.