I. Introduction
A variety of serious health conditions, among them breast and colon cancer, obesity, diabetes, and cardiovascular disease, are linked to physical inactivity [1]. For breast cancer in particular, estimates suggest that the risk of developing this disease can be reduced by 20-30% by engaging in 30–60 minutes of moderate-intensity physical activity (PA) per day [2]. Mobile health technologies (mHealth) are well suited for delivering cost-effective, scalable, and adaptive interventions for promoting PA; smartphones play an important role because they can sense users' activities and context, and enable the implementation of advanced applications. However, most researcher-developed mHealth PA interventions have only been evaluated in short-term pilots (e.g., 4 weeks to 6 months) with modest effects [3] and require an assessment of their effectiveness under real-world conditions.