Background: The older adult population is the least active age group in the U.S. by two distinct metrics: a lack of MVPA and a high rate of SED (sitting without any significant exertion). This represents a public health opportunity because engaging in 150 minutes of MVPA is known to improve the risk of morbidity, mortality, and physical function. SED has emerged as a risk factor for adverse outcomes, independent of MVPA, and observational studies indicate that more SED is linked to an increased risk of clinical outcomes (mortality, diabetes, and cardiovascular disease) and worse aging outcomes, e.g. physical function. However, there exists no experimental data comparing interventions to reduce SED vs. increase MVPA in older adults.