The outbreak of the coronavirus disease 2019 (Covid-19; severe acute respiratory syndrome coronavirus 2, SARS-CoV-2) significantly changed everyday life. In March of 2020, it was recognized as a pandemic (World Health Organization, 2020b) . By September 30, 2020, there were more than 33.71 million confirmed infections and more than 1,008,000 deaths worldwide (see https://github.com/owid/covid-19-data/tree/master/public/data). To slow down the spread of the pandemic, many governments introduced extraordinary measures that reduced physical contact among the population (so-called -social distancing‖) (World Health Organization, 2020a) . The measures varied between and within countries, but mainly included bans on non-family gatherings and travel, cancellation of mass events, temporary closure of public institutions, non-essential businesses, and entertainment venues; a -stay-at-home‖ was requested or ordered by many governments and authorities (Sohrabi et al., 2020) . Behavioral measures, such as the wearing of face masks and maintaining of distance to other people in public places, became mandatory in many countries (Tso and Cowling, 2020) . that typically results in a negative emotional response characterized by frustration, anxiety, and uncertainty. As a consequence, they have significant problems to manage everyday life and to fulfill their obligations under the new conditions (Taylor et al., 2020) . Other people experience the current situation as less burdensome and react in an adaptive way. They try to maintain a daily routine as far as possible (Brailovskaia and Margraf, 2020b) . The way people perceive and deal with the current Covid-19 situation significantly influences their mental and physical health (Taylor et al., 2020) . Moreover, it impacts their adherence to actively participate in the governmental measures such as wearing of face masks (Gandhi and Rutherford, 2020) . The level of adherence and cooperation among the population determines the success of the governmental measures in fighting the pandemic (Galea et al., 2020) .
Against this background, it is urgent to investigate factors that can predict the level of psychological burden caused by the current Covid-19 situation. This knowledge can be used to identify persons at risk for a high burden experience, to reduce it, and thus to longitudinally protect mental and physical health and to enhance adherence to the anti-Covid-19 measures.
Previous research described an increase of depression symptoms in the last decade, especially among the younger population (Brailovskaia and Margraf, 2020a; Twenge et al., 2019) . Recent studies predicted a further increase of the symptoms during the Covid-19 pandemic (Bueno-Notivol et al., in press.; Galea et al., 2020) . Individuals with enhanced levels of depression symptoms are characterized by feelings of control loss, hopelessness, and helplessness (Buysse et al., 2008; Lei et al., 2016) . They tend to ruminate and are overwhelmed by the requirement to adapt to new uncertain situations (Spasojević and Alloy, 2001) . Low self-efficacy and the lack of adequate coping strategies often contribute to maladaptive reactions such as social withdrawal, excessive worries, or suicide ideation and behavior (Gorday et al., 2018; Schönfeld et al., 2016) . Based on this background, it can be assumed that depression symptoms might belong to factors that foster the experience of psychological burden by COVID-19 BURDEN, DEPRESSION & PHYSICAL ACTIVITY 5 the Covid-19 situation (Bueno-Notivol et al., in press.) . This could be true especially for patients with diagnosed affective disorders as well as individuals from the general population with enhanced depression symptoms.
Following the available research (Camacho et al., 1991; Eime et al., 2013; Velten et al., 2014) , physical activity could be a factor that reduces the experience of high psychological burden by Covid-19. In accordance with the recommendations of the World Health Organization, about 30 minutes of cumulative moderate physical activity daily can provide significant health benefits (World Health Organization, 2003) . Regularly engagement in activities like jogging, cycling, or swimming not only improves the physical health state, but also fosters mental health (Rebar et al., 2015) . It buffers the negative impact of traumatic experiences and daily stressors. Moreover, it can reduce depression symptoms and addictive tendencies (Brailovskaia et al., 2018; Harris et al., 2006) . Physical activity can contribute to the experience of positive emotions and enhance the individual level of sense of control, selfefficacy, self-esteem, and positive mental health (Richards et al., 2015) . It can also foster the individual resilience level and contribute to adaptive coping strategies in stressful and uncertain situations (McAuley et al., 2000; Wunsch et al., 2017) . Considering the presented framework, it can be hypothesized that physical activity may be a significant protective factor in the current Covid-19 situation. Individuals who frequently engage in physical activity could experience less psychological burden by Covid-19. Moreover, physical activity could buffer the association between depression symptoms and experienced burden.
Based on the presented background, the main aim of the current study was to investigate the relationship between depression symptoms, psychological burden by Covid-19, and physical activity. It was expected that depression symptoms are positively associated with psychological burden by Covid-19 (Hypothesis 1a); physical activity is negatively linked to depression symptoms (Hypothesis 1b) and psychological burden (Hypothesis 1c); and physi-COVID-19 BURDEN, DEPRESSION & PHYSICAL ACTIVITY 6 cal activity buffers the link between depression symptoms and psychological burden (Hypothesis 2).
Considering that Covid-19 is a global problem that impacts life in many countries, it is urgent to gain a broad cross-national picture of the burden caused by the pandemic and potential factors that can influence its extent. Therefore, the present study investigated whether similar result patterns occur in samples from four countries where the impact of the pandemic varies in its extent: Germany, Italy, Russia, and Spain (see https://github.com/owid/covid-19-data/tree/master/public/data). In all four countries, national lockdowns started in March of 2020 that included bans on gatherings and closing of different establishments; -stay-at-home‖ was ordered at least in some regions and wearing of face masks in public places is still mandatory (Bundesministerium für Gesundheit, 2020;
Ministerio de Sanidad, 2020b; Ministero della Salute, 2020; The Russian Government, 2020).
The hypotheses were examined in the overall sample that includes data from all four countries. Furthermore, in order to identify possible country-specific differences, the hypotheses were examined separately in the respective country samples.
The overall sample was comprised of 1,931 participants from four countries: Germany: N=625, Italy: N=936, Russia: N=230, and Spain: N=140. Demographics of the overall sample and of the country-specific samples are shown in Table 1 . Data were collected simultaneously in Germany, Italy, Russia, and Spain by means of online surveys in the respective national languages of the four countries between late March of 2020 and early September of 2020. At the moment of the data collection, all participants were geographically located in the appropriate country (i.e., German participants were located in Germany, Italian participants were located in Italy, Russian participants were located in Russia, and Spanish participants COVID-19 BURDEN, DEPRESSION & PHYSICAL ACTIVITY were located in Spain). This information was provided by a self-declaration at the beginning of the survey. The present study is based on an ad-hoc cooperation project between researchers from the four countries. Due to the Covid-19 situation, the research conditions and possibilities of data collection were different in the individual countries. This explains the different size and composition of the convenience samples and the different methods of recruitment.
In Germany, a participation invitation that included a link leading to the online survey was emailed to overall 700 persons. They all are current or former students of a large university in the Ruhr region and had agreed to be contacted for research investigations when they enrolled at the university. Overall the survey was completed by 98.6% of the 634 individuals who activated the survey link.
In Italy, the link to the survey was sent to all institutional email addresses of the students of the University of Florence together with an invitation to disseminate the link via email or social media among friends and acquaintances. Overall the survey was completed by 98.3% of the 952 individuals who activated the survey link.
In Russia, the link to the online survey was sent to overall 200 professors and student course mentors of a large university in Moscow. The contact details were available from the responsible authorities of the university. All participants were invited to share the link with their colleagues and fellow students. The survey was completed by all individuals who activated the survey link.
In Spain, the study was advertised on different social networking sites (i.e., Facebook, Twitter, Instagram, and WhatsApp). The link to the online survey was included in the advertisement to facilitate the response to the questionnaires. Moreover, the dissemination of the study among friends, co-workers and relatives was encouraged. Overall the survey was completed by 90.9% of the 154 individuals who activated the survey link.
There were no specific requirements for participation, which was voluntary and not compensated in Italy, Russia and Spain. In Germany, participation was compensated by course credits for students. All participants were provided instruction and gave informed consent to participate via an online form. The present study was coordinated in Germany. Therefore, the responsible Ethics Committee of a large university in the German Ruhr region approved the implementation of the international study. Additionally, an approval was received from the Ethics Committee of the University of Florence.
[insert Table 1 Frequency of physical activity. The frequency of physical activity was assessed using the item -How frequently did you engage in physical activity (e.g., jogging, cycling) in the last 12 months?‖ rated on a 5-point Likert-type scale (0=never, 4=four times a week or more).
Available research described it to be a reliable and valid instrument to measure physical activity (Brailovskaia et al., 2018; Milton et al., 2011) .
Depression symptoms. Depression symptoms were assessed with the depression subscale of the Depression Anxiety Stress Scales 21 (DASS-21; Lovibond and Lovibond, 1995) .
The DASS-21 is an international well-established instrument with good psychometric properties (Scholten et al., 2017) . The seven items (e.g., -I felt that life was meaningless‖) are rated on a 4-point Likert-type scale (0=did not apply to me at all; 3=applied to me very much or 
Statistical analyses were conducted using SPSS 26 and the macro Process version 3.5 (www.processmacro.org/index.html; Hayes, 2013) . After descriptive analyses, the relationship between psychological burden caused by Covid-19, depression symptoms, and physical activity was assessed by zero-order bivariate correlation analyses. Next, a mediation model was calculated (Process: model 4) that included depression symptoms as predictor, physical activity as mediator, and psychological burden as outcome; age and gender were included as covariates. The relationship between depression symptoms and physical activity was denoted by path a; path b denoted the association between physical activity and psychological burden.
The indirect effect (ab) was represented by the combined effect of path a and path b. The basic association between depression symptoms and psychological burden was denoted by path c (the total effect), while the relationship between depression symptoms and psychological burden after the inclusion of physical activity in the model was denoted by path c' (the direct effect). The mediation effect was assessed by the bootstrapping procedure (10,000 samples) that provides percentile bootstrap confidence intervals (CI 95%). Figure 1 illustrates the model. All calculations were conducted in the overall sample and in each country-specific sample separately. There were no missing data. Power analyses (G*Power program, version 3.1) revealed that the sample sizes are sufficient for valid results (power >.80, α=.05, effect size: f 2 =.15; cf., Mayr et al., 2007) .
[insert Figure 1 near here] Table 1 and Table 2 present the descriptive statistics of the investigated variables for the overall sample and for each participating country. Furthermore, Table 2 shows the correlations between psychological burden, depression symptoms, and physical activity. The results revealed the same correlation pattern in all samples. Psychological burden was significantly positively correlated with depression symptoms. Physical activity was significantly negatively correlated with psychological burden and depression symptoms (see Table 2 ).
[insert Table 2 near here]
The mediation analysis also showed the same result pattern in all samples (see Table   3 ). Physical activity partly mediated the positive relationship between depression symptoms and psychological burden by Covid-19. The basic relationship between depression symptoms (predictor) and psychological burden (outcome) was significant (see Table 3 , total effect, c).
The association between depression symptoms and physical activity (mediator) (path a: all samples: p<.001), and the relationship between physical activity and psychological burden (path b: Germany and Italy: p<.05, Russia p<.01, Spain: p<.001) were also significant. The association between depression symptoms and psychological burden remained significant after the inclusion of physical activity in the model. However, the total effect was higher than the direct effect (see Table 3 , direct effect, c'). The indirect effect was significant (see Table   3 , indirect effect, ab).
[insert Table 3 near here]
The level of psychological burden caused by the current Covid-19 situation can impact the individual health state and the willingness to adhere to the governmental measures to fight the pandemic (Brailovskaia and Margraf, 2020b ). The present cross-national results revealed two potential factors that can influence the level of psychological burden and explain how they can interact.
As expected, depression symptoms were positively associated with psychological burden caused by Covid-19 (confirmation of Hypothesis 1a). Note, the sudden outbreak of Covid-19 required many people to (partly radically) change their everyday life (Lewnard and Lo, 2020) . For instance, planned trips, visits and in-person meetings had to be cancelled. The introduction of measures such as home-schooling and home-office required rapid adaptation of the daily routine and implied new obligations (Galea et al., 2020) . The requirements are challenging, especially for people with enhanced levels of depression symptoms (Bueno-Notivol et al., in press.) . Their low self-efficacy and tendency to ruminate and to worry can contribute to the experience of psychological burden when facing the need to manage the new uncertain Covid-19 situation.
Available literature emphasized that physical activity is an important protective factor of mental and physical health (Vuillemin et al., 2005) . It buffers the negative impact of stressful experiences and confers resilience to manage challenges (Brailovskaia et al., 2018; Richards et al., 2015) . Correspondingly, in the present study, physical activity was negatively associated with depression symptoms (confirmation of Hypothesis 1b) and psychological burden caused by Covid-19 (confirmation of Hypothesis 1c). Moreover, physical activity buffered the relationship between both variables (confirmation of Hypothesis 2). The present findings can be explained as following. It seems that individuals who regularly engage in physical activity, for example jogging or cycling, experience the current Covid-19 situation as less burdensome. They are more resilient and have more resources to manage and to adapt to the new extraordinary requirements. Following the current results, especially people with enhanced levels of depression symptoms, who are characterized by problems to adapt to new situations (Andrews and Wilding, 2004) , can benefit from physical activity. The engagement in physical activity reduces the time of rumination and can contribute to the experience of positive emotions (Rebar et al., 2015) that are often missed by individuals with increased depression symptoms (Freudenstein et al., 2012) . Improvement of one's performance and the achievement of small self-imposed physical goals such as an increase in speed when jogging are rewarding and can enhance enjoyment and happiness (Bailey et al., 2013) . Moreover, physical activity fosters the individual sense of control and self-efficacy (Eime et al., 2013) .
Both are urgent to experience the current Covid-19 situation as less burdensome and to manage the new requirements of everyday life (Allington et al., 2020) .
The present results from four countries emphasize that while depression symptoms may be a significant risk factor that fosters the experience of psychological burden caused by Covid-19, physical activity can contribute to less burden experience and a more adaptive response to the current situation. The protective effect of physical activity could be used to buffer the negative impact of enhanced depression symptoms. Considering that the global course of the pandemic remains unclear, it is important for governments and authorities to consider present findings when working on specific steps to longitudinally protect mental and physical health during the pandemic. Public governmental communication should explicitly address the positive effect of physical activity especially for individuals with enhanced levels of depression symptoms and encourage the participation in the activities. It should be emphasized that physical activity does not necessary imply competitive sport or the use of specific expensive equipment. Everyone can perform physical activity in the own rhythm and according to the own physical conditions at home or outdoors (Eime et al., 2013) .
Despite the timeliness of the present study, the following limitations should be taken into account. First, due to the cross-sectional design, only hypothetical causality considerations are possible. Therefore, future studies are suggested to replicate the present findings by using a longitudinal experimental design that includes several measurement time points, and for instance the manipulation of the potential buffer (physical activity). Second, the mostly female and rather young composition of the convenience samples limits the generalizability of the current findings. Note that women are at a higher risk than men to develop depression symptoms (Parker and Brotchie, 2010) . Moreover, recent research described a significant increase of depression symptoms specifically in adolescents and young adults in the last years (Brailovskaia and Margraf, 2020a; Twenge et al., 2019) . Against this background, it can be assumed that the found protective effect is especially relevant for young female individuals.
However, the present findings should not be overstated because the composition of the investigated samples does not allow gender-and age-related conclusions. To partly tackle this limitation, gender and age were included as control variables in the mediation analysis. Nonetheless, the present samples are convenience. Therefore, the current findings should be interpreted with caution. Their replication in population representative samples is desirable. Third, due to the Covid-19 situation, the research conditions and possibilities differed between the four countries. This resulted in varied samples sizes. Specifically, the samples in Spain and Russia are remarkably smaller than the samples in Italy and Germany. Interestingly, the largest buffering effect of physical activity was found in Spain, followed by Russia. The smallest effect was found in Italy, followed by Germany (see Table 3 ). A possible explanation for the largest effect in Spain and Russia may be at least partly related to the similarities of the strict confinement Covid-19 measures introduced in both countries. It can be assumed that the strict limitations for engaging in physical activity during the confinement (i.e., the impossibility of engaging in outdoor activities such as jogging) limited to a greater extent individuals with enhanced depression symptoms who are characterized by an inability to adapt to new uncertain situations. This could reinforce the negative relationship between depression symptoms and physical activity, and consequently, the negative relationship between physical activity and psychological burden. On the contrary, individuals with lower depression symptoms could be rather able to find ways to engage in indoor physical activity and therefore to reduce the psychological burden caused by the Covid-19 situation. The small effect in Italy might be at least partly due to the low reliability of the psychological burden measure in this country.
Fourth, in order not to overload the participants, only self-reported frequency of physical activity in the last twelve months was assessed. Earlier research reported this 1-Item instrument to be a reliable and valid measure (Milton et al., 2011) . Nevertheless, in order to obtain a more comprehensive picture of physical activity as buffer between depression symptoms and psychological burden caused by Covid-19, future studies should include further (more objective) measures of physical activity (e.g., exact daily time spent on physical activity, which activity is performed and its power level). Furthermore, future longitudinal research is suggested to assess potential changes of the frequency of physical activity since the beginning of the pandemic and the lockdowns introduced to reduce its spread. Fifth, the mediation analysis revealed a partial mediation effect in all samples. Future studies should investigate further factors than can buffer the association between depression symptoms and burden caused by Covid-19, such as for instance sense of control, self-efficacy, and perceived social support (e.g., Gorday et al., 2018; Lei et al., 2016) . Sixth, the main focus of the present study was on psychological burden that implies a strong feeling of being generally overwhelmed by the Covid-19 situation. However, burden can also be associated with specific domains (e.g., home, work, financial, health). The importance of such domains can interpersonally differ.
Negative events in domains that are of specific importance for the individual are experienced as more burdensome and have a stronger impact on mental and physical health than negative events in less important domains (Keyes et al., 2002) . Therefore, future research is suggested to assess more specifically the domains of the burden and their individual relevance. Additionally, even though similar result patterns were found in the overall sample and in the coun-try-specific samples, individual culture and country-specific interpretation differences considering the meaning of burden cannot be excluded.
To conclude, it is still unclear how long and to which extent the pandemic will spread around the globe. Knowledge gained by the present findings can contribute to the development of programs to reduce negative consequences of Covid-19 for mental health and physical health, and to strengthen an adaptive response and adherence to the introduced measures.
The present cross-national study shows that across four countries (Germany, Italy, Russia, and Spain), individuals with enhanced depression symptoms are at risk to experience psychological burden by the current Covid-19 situation. However, physical activity can buffer this negative effect. This issue should be addressed in governmental steps of health protection to successful manage the Covid-19 pandemic and its longitudinal consequences. 
Conflicts of interest: none.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. N=10.000; c=relationship between depression symptoms and burden (total effect);
c'=relationship between depression symptoms and burden after inclusion of physical activity in the model (direct effect); ab=combined effect of path a (relationship between depression symptoms and physical activity) and path b (relationship between physical activity and burden). Notes. c=path of predictor to outcome, without inclusion of mediator (total effect); a=path of predictor to mediator; b=path of mediator to outcome; c'=path of predictor to outcome including mediator (direct effect).
Depression Symptoms a Physical Activity b c /c '