Objective: We examined the acute bidirectional relationships between affective states and moderate-to-vigorous physical activity (MVPA) or sedentary behavior (SB) in children, and whether perceived stress moderates these associations.

Method: A total of 180 children (mean age = 9.6 years, 51.7% female, 53.9% Hispanic) completed a 7-day ecological momentary assessment (EMA) study, where they received 3-7 random prompts per day asking about their current affective states. MVPA and SB during this period were measured by waist-worn accelerometers. Children's and mothers' perceived stress were measured by paper questionnaires. Multilevel models tested the within-person (WP) and between-person (BP) associations of (a) MVPA and SB 30 and 60 min before an EMA prompt with subsequent affective states at the prompt, and (b) affective states at the prompt with MVPA and SB in the subsequent 30 and 60 min after the prompt. Interaction terms were used to assess whether children's and mothers' perceived stress moderated these associations.

Results: Children reported a higher positive affect after engaging in more MVPA than usual (WP; β = 0.04, SE = 0.02, p < .05) and a lower positive affect after spending more SB than usual (WP; β = -0.02, SE = 0.01, p < .05) in the previous 30 min. Children's affective states were unrelated to time in MVPA and SB within the subsequent 30 min. Parent's perceived stress level attenuated the relationship between children's time spent in MVPA 60 min before a prompt and self-reported positive affect at that prompt (β = -0.01, SE = 0.01, p < .05).

Conclusions: MVPA and SB acutely impacted children's psychological well-being, with the benefits of MVPA on positive affect across longer intervals attenuated among children whose mothers had higher perceived stress. (PsycINFO Database Record