Most children in the United States currently do not meet physical activity (PA) guidelines, and perceived stress is a known barrier against PA. However, most studies have solely focused on the between-subject (BS) association of stress and PA, whereas the within-subject (WS) relationship remains under-studied in children. This limits our understanding of day-level psychosocial factors that influence children's PA engagement. This study assessed the same-day associations of the Morning Cortisol after Awakening (MCA) and morning perceived stress with moderate-to-vigorous physical activity (MVPA) in children using Ecological Momentary Assessment (EMA) methodologies. The analytic sample consisted of 143 children from Los Angeles in the Mothers and Their Children's Health (MATCH) study. Children collected four saliva samples and wore accelerometers to assess MVPA minutes across four days (two weekdays and two weekend days) during each data collection wave. On weekend days, they also answered a mobile phone EMA survey on perceived stress within 1 h of awakening. Data from four assessment waves, each approximately six months apart, were combined and analyzed cross-sectionally. Multilevel modeling, which adjusts for the nested data structure, was used to test the same-day associations of MCA and weekend morning perceived stress with MVPA. On weekend days when morning perceived stress was higher than usual, participants engaged in more MVPA on the same day (β = 0.79, SE = 0.32, p = 0.02), but no association was found between MCA and MVPA. These findings elucidate same-day factors that predict MVPA in children and inform future WS studies on stress and PA.HighlightIt is unknown how stress influences children's daily activity levels. We tested if two types of stress (self-reported and saliva-measured) are related to children's daily activity. We found that self-reported stress is associated with physical activity, but saliva-measured stress is not.
Keywords: HPA axis; MVPA; children; exercise; salivary cortisol; stress appraisal.