The COVID-19 epidemic adversely affects the lifestyle of people. This study aimed to examine the impacts of social isolation on physical activity (PA) levels and mood states of children and adolescents and to explore the correlation between them during the COVID-19 epidemic. A total of 9979 children and adolescents (11.63 ± 1.23 years old) from Yan’an, China volunteered to participate in this study and completed online questionnaires. PA and mood states were measured by International Physical Activity Questionnaire Short Form (IPAQ-SF) and Profile of Mood States (POMS), respectively. The results showed that the mean of students’ moderate-to-vigorous PA (MVPA) was 23.19 min per day. The total mood disturbance in the moderate and high-level PA groups were significantly lower than those in the low-level PA group (p < 0.05). Additionally, boys and girls had significant differences in PA levels (p < 0.01), and the PA levels of students in different grades were also significantly different (p < 0.01). Meanwhile, boys’ mood states were worse than girls’. The Grade 4 in primary schools had the worst mood states while Grade 5 had the best mood states. The results suggested that the MVPA of students has dropped badly, compared with the results of previous studies investigated in normal times. In addition, the PA level had a significantly positive impact on the mood states of children and adolescents during the COVID-19 epidemic. Sex and grades were factors which affected the PA levels and mood states. This study can help policymakers and healthcare professionals understand PA and mood states of Chinese children and adolescents during the epidemic. We should pay attention to the changes in PA levels and mood states of children and adolescents.

Background: Restricted outdoor activities and online learning during COVID-19 pandemic may pose threats on the physical and mental health of Chinese adolescents in middle schools. The study was to explore the relationship of Physical Activity (PA) and Sedentary Time (SED) with mood states among Chinese adolescents during the pandemic. Methods: A total of 4898 adolescents (16.3 ± 1.3 years old) from 49 middle schools in 13 urban and rural areas in Yan’an, China volunteered to participate in this study and completed online questionnaires. International Physical Activity Questionnaire (IPAQ) were used to measure PA and SED, and Profile of Mood States were used to evaluate mood states. We used Mann–Whitney U -test and Kruskal–Wallis test to assess differences in PA and SED in different sex and grade group, respectively. Mixed linear regression was used to analyze the relationship between PA/SED and mood states. Results: Participants in our study accumulated 23.4 ± 52.5 min of moderate-to-vigorous PA and 363.6 ± 148.4 min of SED per day. The mood states of boys were better than that of girls ( p < 0.01). The mood states of the students in Senior High School Grade 3 were at the highest level of mood disturbance. Higher levels of PA were significantly associated with lower levels of total mood disturbance in this population (High PA group according to IPAQ: B = −3.22, SE = 0.40, p < 0.001; Moderate: B = −1.47, SE = 0.37, p < 0.001, compared to Low PA group). Conclusion: Chinese adolescents maintained a sedentary lifestyle during the COVID-19 pandemic. Girls and students in Grade 3 Senior High School had higher level of mood disturbance. More PA was associate with improving mood state among adolescents in the pandemic. The present study may assist policymakers and healthcare professionals understand PA and mood states of Chinese adolescents during the epidemic and take targeted actions in case of emergencies in future.