Abstract
BACKGROUND: Few studies have analyzed sensor-derived metrics of mobility abilities and total daily physical activity (TDPA). We tested whether sensor-derived mobility metrics and TDPA indices are independently associated with mobility disabilities. METHODS: This cohort study derived mobility abilities from a belt-worn sensor that recorded annual supervised gait testing. TDPA indices were obtained from a wrist-worn activity monitor. Mobility disability was determined by self-report and inability to perform an 8-feet walk task. Baseline associations of mobility metrics and TDPA (separately and together) were examined with logistic regressions and incident associations (average 7 years follow-up) with Cox models. Mediation analysis quantified the extent mobility metrics mediate the association of TDPA with mobility disability. RESULTS: 724 ambulatory older adults (mean age 82 years, 77.4% female) were studied. In separate models, mobility abilities (eg, step time variability, turning angular velocity) and TDPA were related to mobility disabilities. Examined together in a single model, mobility abilities remained associated with mobility disabilities, while TDPA was attenuated. This attenuation of TDPA could be explained by mediation analysis that showed about 50% of TDPA associations with mobility disabilities is mediated via mobility abilities (prevalent mobility disability 54%, incident mobility disability 40%, incident loss of ambulation 50%; all p's < .001). CONCLUSIONS: Sensor-derived mobility metrics assess more diverse facets of mobility. These metrics mediate approximately half of the association of higher levels of daily physical activity with reduced mobility disability in older adults. Findings may inform the design of targeted interventions to reduce mobility disability in late life.