1. Center for Wireless and Population Health Systems, University of California, San Diego, La Jolla, CA, USA
2. Exercise and Physical Activity Resource Center, University of California, San Diego, La Jolla, CA, USA
3. Family Medicine and Public Health, University of California, San Diego, La Jolla, CA, USA
4. Department of Behavioural Science and Health, University College London, Torrington Place, London, UK