|Anotation||Physical activity and a healthy lifestyle have a major influence on the length and quality of life (Arem et al., 2015; Moore et al., 2012). Sufficient level of physical activity is also directly related to mental health by positively influencing affect (Reed & buck, 2009) and reducing stress (Gerber & Puhse, 2009). Despite efforts to increase the level of physical activity, a large percentage of the public does not meet physical activity recommendations (WHO, 2018).
Thus, many studies have focused on factors that predict the initiation and level of physical activity (Bauman et al., 2012; Choi et al., 2017) as well as interventions to increase physical activity (Howlett, Trivedi, Troop, & Chater, 2018; King, Rejeski, & Buchner, 1998). Less attention has been devoted to the reasons why people stay physically active or why they drop-out of physical activity (Fraser-Thomas et al., 2008; Sazzazin, 2002). A related area which has been scarcely researched concerns factors affecting the fluctuations in the level of physical activity. Detailed examination of daily individual trends in physical activity is a novel approach enabled by rapid development of devices such as smartphones and fitness monitors. Detailed objective data about the level of physical activity can be further combined with intensive data about behavior, acute mental state, or perception (so-called ecological momentary assessment) providing insights into the dynamics of human behavior and its determinants.
Given the major importance of a stable level of physical activity on health and quality of life, it is important to address the determinants of fluctuations in physical activity, including injury as a specific type of fluctuation. At the moment, little is known about the psychological factors that determine these fluctuations - the influence of stable individual differences (e.g., self-efficacy, exercise identity, self-regulation), dynamic factors (e.g., affect, stress, fatigue), and their interplay.|