PURPOSE: Physical activity could be associated with psychological symptoms in young people with major depressive disorder (MDD). Using actigraphy and ecological momentary assessment (EMA), we investigated the associations between physical activity and stress, anxiety and positive and negative affect in young people with MDD. METHODS: Actigraphy and EMA were collected daily in 40 young participants (aged 16-25 years) with MDD over 8 weeks. Multi-level linear mixed models were used to examine within- and between-person daily associations between physical activity and symptoms of stress, depression and positive and negative affect. RESULTS: Participants with at least 14 days of complete data were included in the analysis (n participants = 22
total days = 598). Typical (defined as average across the assessment period) vigorous physical activity was significantly associated with lower daily stress (β[95% CI] = -0.152 [-0.298, -0.007], p = 0.041) and higher daily positive affect (0.526 [0.061, 0.992], p = 0.028). Variability in daily light (0.004 [0.001, 0.006], p = 0.010) and moderate physical activity (0.004 [0.001, 0.007], p = 0.009) were positively associated with daily stress. Variability in daily light (0.003 [0.001, 0.006], p = 0.018) and moderate physical activity (0.004 [0.001, 0.007], p = 0.011) were positively associated with daily anxiety. CONCLUSIONS: Various intensities of daily physical activities were associated with symptoms of stress, anxiety and positive affect in young people with MDD. Future research on larger samples should validate the causal and daily associations between physical activity and psychological symptoms to promote evidence-based behavioural strategies to improve psychological symptoms in young people with MDD.