Maintaining adequate levels of physical activity (PA) and limiting screen time (ST) are crucial for the physical, psychosocial, and mental well-being of children and adolescents. The World Health Organization (WHO) recommends at least 60 minutes of moderate-to-vigorous PA daily and limits on recreational ST. Many countries achieve these goals through organized PA opportunities like school sports. The COVID-19 pandemic, however, disrupted these routines by closing schools and leisure facilities, creating a natural experiment to study the impact on children's PA and ST. Germany's lockdown, which closed kindergartens, schools, and sports clubs, provided a unique setting to examine how children adapted their PA and ST behaviors. Prior research suggests that structured days (school days) support healthier behaviors compared to unstructured days. Therefore, the lockdown's impact on children's PA and ST is a significant research question, particularly considering the potential for a shift from organized SA to either other forms of self-determined PA or increased ST and sedentary behaviors. Existing studies on the impact of COVID-19 on PA and ST lack methodological consistency and representative samples, limiting generalizability. This study utilizes data from the German Motorik-Modul (MoMo) cohort study to investigate changes in SA, HA, and recreational ST before and during the strictest phase of the first German COVID-19 lockdown, offering a nationally representative perspective.

Several studies have examined the impact of COVID-19 lockdowns on children's PA and ST. A study in Shanghai showed a drastic reduction in overall PA and a significant increase in ST among children (6–17 years). Similar findings were reported in Croatia, with a stronger decrease in PA observed in urban adolescents. Other studies using step counts in Canada and PA data in Italy also indicated a reduction in PA during lockdown. Studies investigating guideline adherence showed a significant drop in PA guideline compliance and a reduction in ST guideline adherence during the pandemic across multiple countries. However, these studies vary considerably in methodology and sample representativeness, making comparisons challenging. The diverse policy responses to the pandemic across countries created a heterogeneous set of natural experiments, presenting an opportunity to gain insights into human behavioral responses to such disruptions.

This study used data from the German Motorik-Modul (MoMo) cohort study, which has been tracking the PA, fitness, and health of children and adolescents in Germany since 2003. The study employed a longitudinal design, collecting data at two time points: before (pre) and during (peri) the strictest period of the first COVID-19 lockdown in Germany (April 20th to May 3rd, 2020). A representative sample of 1711 children and adolescents aged 4–17 years participated in both measurement points. The MoMo Physical Activity Questionnaire (MoMo-PAQ) was used to assess PA, differentiating between organized sports activity (SA), habitual activities (HA), and recreational screen time (ST). The MoMo-PAQ included items on frequency, duration, type, intensity, and setting of activities. For children under 11, parents completed the questionnaires with their children. During the lockdown, the questionnaire was administered online. Statistical analyses included repeated-measures ANOVA to assess changes in PA and ST, with time representing the lockdown effect. 95% confidence intervals, effect sizes (Cohen's d), means, and standard deviations were calculated. Sociodemographic data, including age, sex, and socioeconomic variables, were also collected.

The study found a significant decrease in organized sports activity (SA) due to the lockdown. However, there was a significant increase in non-organized sports activity and habitual physical activity (HA). This increase in HA, combined with the increase in non-organized sports, resulted in an overall significant increase in total physical activity and an increase in the percentage of participants meeting WHO PA guidelines. The increase in PA was more pronounced in younger children. Conversely, recreational screen time (ST) significantly increased across all age groups, resulting in a decrease in the percentage of participants meeting ST guidelines. This increase was more pronounced in older adolescents. Analysis of guideline adherence showed an 11.1% overall increase in PA guideline adherence and a 17.5% decrease in ST guideline adherence. The effects varied in size depending on age, with younger children showing larger increases in PA and smaller increases in ST compared to older adolescents. The correlation between non-organized sports activity and other PA measures was low, indicating that the increase in PA during the lockdown was primarily driven by HA rather than a simple substitution of organized sports with other forms of sports activity. The Gini coefficient, used to assess inequality in PA distribution, decreased for SA and ST but increased for HA, suggesting a potential equalization effect of lockdown.

The findings challenge the assumption that PA and ST are functionally opposite behaviors. While the lockdown led to a decrease in organized SA, the substantial increase in HA compensated for this, resulting in an overall increase in PA. This suggests that the impact of lockdowns on PA is highly context-dependent and that factors such as access to outdoor spaces and the ability to engage in self-directed activities are crucial. The increase in recreational ST is consistent with findings from other studies during lockdowns in Spain and China. The lack of structured activities likely contributed to this increase. The difference in response to the lockdown between countries could be due to differing policies or methodological variations in tracking PA. For instance, the MoMo-PAQ's detailed assessment of PA across various settings provided a comprehensive understanding of behavioral shifts. The more pronounced negative impact on adolescents' SA compared to younger children could relate to developmental changes in PA behaviors. The decrease in the Gini coefficient for SA and ST, along with the increased focus on a healthy lifestyle during the pandemic, suggests policies should promote access to non-organized PA.

This study demonstrates that PA in children and adolescents is highly context-dependent, and that the impact of lockdowns on PA and ST is complex. While organized SA decreased, substantial increases in HA resulted in an overall increase in PA. Recreational ST also increased. These findings highlight the need for policies that ensure access to non-organized PA opportunities for all children and adolescents, particularly during periods of restricted social life. Future research should explore innovative virtual PA solutions to address inequalities in PA access and explore the long-term effects of the pandemic on PA and ST behaviors.

The study relies on self-reported data, which may be subject to recall bias. The lack of a control group limits the ability to definitively attribute observed changes solely to the lockdown. The age of participants increased slightly between the pre and peri measurements, which may slightly underestimate the increase in PA due to age-related decline in non-organized PA. The representativeness of the sample may be slightly affected by the interruption of data collection due to the unexpected pandemic.