Despite higher rates of incident or recurrent MI and fatal coronary heart disease (CHD) among women aged 85 and older (120/1000) than men (85/1000), and the disproportionate burden of disability and co-morbidity in aging women related to diagnosed and occult cardiovascular disease (CVD), prevention of CVD in this vulnerable population is vastly understudied. This application builds upon the now completed Objective Physical Activity and Cardiovascular Health (OPACH) Study (NHLBI R01 HL105065), ancillary study embedded in the Women’s Health Initiative Long Life Study led by Dr. LaCroix. The OPACH team studied accelerometer-measured physical activity (PA) and sedentary behavior (SB) in relation to incident CVD among 7048 women ages 63-99. Our primary results showed a statistically significant, dose responsive, 20% reduction in incident CHD, a 10% reduction in incident CVD, and a 24% reduction in all-cause mortality for every additional hour spent in light PA (1.6-2.9 METs) in older women; and a 12% increased risk of CVD for every additional hour spent sedentary. To classify light PA, the OPACH team conducted a laboratory calibration study (200 OPACH women) to determine accelerometer cutpoints for absolute intensity relevant to the usual daily PA habits of older women. Age-specific classification of PA intensity is critical because resting metabolic rate and maximal aerobic capacity decline with aging, whereas the energy cost of aerobic activity increases with age. Because of these age-related changes and the heterogeneity in how they occur in older adults, the 2018 PA Guidelines Evidence Report urged researchers to: “Conduct prospective cohort studies of PA and physical function in older adults that include objective measures (e.g., heart rate monitors) of relative intensity of PA.” The term “relative” in this context refers to the maximal exercise capacity of individuals. OPACH2 and the OPACH2 Calibration Study are designed specifically to derive validated accelerometer- measured PA relative intensity and determine its association with incident CVD (CHD, heart failure, stroke) and mortality through 2027. To do this, we will obtain a second accelerometer measurement of PA and SB with contemporaneous measurement of heart rate (a physiological measure of PA intensity) using a non-invasive cardiac monitor patch (Cardea SOLO) in 4200-5000 surviving OPACH women. We will also investigate cross- sectional associations at the second accelerometer wear (2022-2023) of relative and absolute PA intensity with indicators of underlying CV health including traditional risk factors (lipids, glycemia, inflammation, blood pressure, adiposity), novel biomarkers of CVD pathology (cardiac troponin, brain natriuretic peptide, galactin- 3), heart rate variability (from Cardea SOLO patch), and physical performance. Finally, we will determine the association of changes in accelerometer-measured PA and SB over ≈10 years with changes in traditional cardiovascular risk factors...