Evaluation of Light Physical Activity Measured by Accelerometry and Mobility Disability During a 6-Year Follow-up in Older Women.

Importance: Almost 1 in 4 women older than 65 years is unable to walk 2 to 3 blocks, and mobility disability is a key factor associated with loss of independence. Lack of moderate to vigorous-intensity physical activity is associated with mobility disability, but whether lighter physical activity is associated with mobility disability is unknown. Objective: To determine the association of light-intensity physical activity and incident mobility disability among older women. Design, Setting, and Participants: This prospective cohort study included women enrolled in the Objectively Measured Physical Activity and Cardiovascular Health study, an ancillary study of the Women's Health Initiative, between March 2012 and April 2014, with follow-up through March 31, 2018. The Women's Health Initiative was a population-based, multisite study that recruited from 40 clinical sites across the US. Participants in the present analysis included 5735 of 7058 ambulatory, community-dwelling women aged 63 years and older who returned an accelerometer with usable data, were free of mobility disability, and had follow-up data on mobility status. Data were analyzed from August 2018 to May 2019. Exposures: Light-intensity physical activity, defined as movement requiring energy expenditure between 1.6 and 2.9 metabolic equivalents, captured using an accelerometer over 7 days. Main Outcomes and Measures: Incident mobility disability, defined as the first self-reported inability to walk 1 block or up a flight of stairs at annual follow-up, and persistent incident mobility disability, defined as incident mobility loss that persisted through the end of follow-up. Results: A total of 5735 participants were included for primary analysis of all incident mobility disability (mean [SD] age, 78.5 [6.6] years [range, 63-97 years]; 2811 [49.0%] White participants). Compared with women in the lowest quartile of light-intensity physical activity, lower risk of incident mobility disability was observed in quartile 2 (multivariable hazard ratio [HR], 0.78; 95% CI, 0.67-0.90), quartile 3 (HR, 0.60; 95% CI, 0.51-0.71), and quartile 4 (HR, 0.60; 95% CI, 0.51-0.71) (P < .001). This beneficial association was stronger for persistent mobility disability in quartile 2 (multivariable HR, 0.72; 95% CI, 0.60-0.85), quartile 3 (HR, 0.55; 95% CI, 0.46-0.67), and quartile 4 (HR, 0.52; 95% CI, 0.42-0.63) (P < .001). Stratified analyses showed the association was stronger among women with a body mass index of less than 30.0 (HR, 0.73; 95% CI, 0.66-0.82) compared with women with a body mass index of 30.0 or higher (HR, 0.91; 95% CI; 0.79-1.04; P = .04 for interaction). Conclusions and Relevance: In this cohort study, increased time spent in light-intensity physical activity was associated with reduced incident mobility disability. These findings support placing greater emphasis on promoting light-intensity physical activity for preserving mobility in later life.