Responsiveness to change of the psychological determinants and outcomes of physical activity and sedentary behavior

Part of the intrigue – and difficulty – in studying physical activity and sedentary behavior is their dynamic nature. People's movement can vary as a result of slow changing processes such as habit formation (Gardner & Rebar, 2018) or rapid fluctuations such as day-to-day contextual or motivational circumstances (Conroy, Maher, Elavsky, Hyde, & Doerksen, 2013; Dunton et al., 2014). Notably, our ability to investigate the dynamics of these behaviors and the associated determinants and outcomes is dependent on responsiveness to change of our use of measures (Hays & Hadorn, 1992). Typically, little evidence is provided that change in a measure validly reflects true change in the underlying construct; it can be easy to take this aspect of measurement validity for granted. The conception of this special issue was the result of our surprise (and concern regarding some of our own work) when the legendary Adrian Bauman casually mentioned during a presentation that the very popular short-form International Physical Activity Questionnaire (IPAQ; Craig et al., 2003) is not an appropriate measure for within-person change in behavior. The IPAQ is only valid for population-level surveillance (see Bauman et al., 2009). This special issue is designed to highlight two aspects of responsiveness of change amongst the field. First, this issue demonstrates researchers' responsiveness to investigating change in these behaviors and the associated determinants and outcomes thereof. Second, this issue represents a call to researchers to determine whether our measures truly are responsive to change. Within this special issue, there is a wide variety of represented populations, constructs, study designs, and practical implications; all tethered by the theme that they capture and/or describe change in physical activity and sedentary behavior and their associated psychological determinants or outcomes. The contributions to this issue highlight how dynamically rich the psychology of physical activity and sedentary behavior truly is and what advances are needed in our field to ensure our theories, measures, and studies of these processes truly are responsive to change.