Background The coronavirus disease (COVID-19) and universal mitigation strategies have fundamentally affected peoples' lives worldwide, particularly during the first two years of the pandemic. Reductions in physical activity (PA) and increased mental health (MH) problems among children and youth have been observed. This systematic review and meta-analysis investigated the relationship between physical activity (PA) and mental health (MH) among children and youth during the COVID-19 pandemic.

Methods Four electronic databases (EMBASE, PsycINFO, PubMed, and Web of Science) were systematically searched to identify studies that (1) examined the relationship between PA and MH among children and youth (aged 2-24 years old) and (2) were published in peer-reviewed journals in English between January 2020 and December 2021. Relationships between PA and two MH aspects (i.e., negative and positive psychological responses) among children and youth at different age ranges and those with disabilities or chronic conditions (DCC) were synthesized. Meta-analyses were also performed for eligible studies to determine the pooled effect size.

Results A total of 58 studies were eventually included for variable categorization, with 32 eligible for meta-analyses. Our synthesis results showed that greater PA participation was strongly related to lower negative psychological responses (i.e., anxiety, depression, stress, insomnia, fatigue, and mental health problems) and higher positive psychological responses (i.e., general well-being and vigor) in children and youth during COVID-19. The pattern and strength of relations between PA and MH outcomes varied across age ranges and health conditions, with preschoolers and those with DCC receiving less attention in the existing research. Meta-analysis results showed that the magnitude of associations of PA with negative (Fisher's z = - 0.198, p < 0.001) and positive (Fisher's z = 0.170, p < 0.001) psychological responses among children and youth was weak. These results were linked to age of participants, study quality, and reporting of PA-related information.

Conclusions PA participation and MH among children and youth deteriorated during the COVID-19 pandemic and were closely associated with each other. For the post-COVID-19 era, additional research on age- and health condition-specific relationships between PA and MH outcomes from a comprehensive perspective is warranted. (Word count: 344 words).