**1. Introduction**

The importance of physical fitness for the healthy development of children and the positive influence for a lifelong active lifestyle are well-known and documented [1,2]. The level of physical fitness predicts an individual's level of engaging in physical activity through given opportunities and limited capacities [3,4]. Physical fitness is the basis on which movement patterns are developed to be able to be physically active and, on the other hand, has a positive impact on psychosocial factors [5–8].

Nevertheless, the levels of physical activity and physical fitness in youth have declined over the last decades, along with other variables influencing an active lifestyle. Since the beginning of the twenty-first century, physical fitness has been stagnating at a low level worldwide. Overall, children are less fit than those of former generations [9–12]. Accordingly, the majority of children and adolescents do not meet the recommendations of the World Health Organization for daily physical activity [13,14]. Sedentary behavior of children and adolescents has increased and screen-time exceeds recommendations [15,16]. As a consequence, the prevalence of obesity and overweight has steadily increased in past years, especially in younger children [17].

With the beginning of the COVID-19 pandemic in December 2019, each country has developed a strategy to try to control the virus. In Germany, the first officially registered case of COVID-19 appeared at the end of January 2020. Nationwide school closures and contact

**Citation:** Eberhardt, T.; Bös, K.; Niessner, C. Changes in Physical Fitness during the COVID-19 Pandemic in German Children. *Int. J. Environ. Res. Public Health* **2022**, *19*, 9504. https://doi.org/10.3390/ ijerph19159504

Academic Editors: Clemens Drenowatz and Klaus Greier

Received: 31 May 2022 Accepted: 28 July 2022 Published: 2 August 2022

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restrictions were implemented for the first time in March 2020, and again in December 2020 until March 2021 [18]. These restrictions also affected sports clubs, fitness centers, and the cancellation of all sports in schools, i.e., physical education lessons, extracurricular sports groups, or being active during breaks in the schoolyard. The COVID-19 pandemic and the subsequent consequences therefore not only limited social life, but also the possibilities and structures for being physically active.

There are studies that have examined the influence of the COVID-19 pandemic on physical activity [19,20]. A meta-analysis revealed a slightly negative global change in physical activity for children and adolescents [19]. In Germany, the differentiated analysis of data from the Motorik–Modul (MoMo) study showed an increase of daily physical activity, such as playing outside or unstructured activities, during the first lockdown, but children could not maintain this level during the second lockdown [21]. In contrast, the time spent in organized sports and overall physical activity decreased over the study period [20–22].

There are also some studies that examined the influence of the COVID-19 pandemic and associated restrictions not only on physical activity, but also on the physical fitness construct [23–26]. Despite different measurement methods and study participants, the studies all reported a declining trend for endurance [23,24,27]. There appears to be less and inconclusive evidence for decreasing strength [24,25]. However, most of the studies have single measurement points before, during, or after the COVID-19 pandemic, but there is a lack of long-term monitoring. In our study, we conducted physical fitness testing in the same cohort over a period of eight years, plus 2020 and 2021, years in which the COVID-19 pandemic occurred. Therefore, these cohorts, which constitute the specific study population, provide the opportunity to draw conclusions based on a strong foundation of physical fitness data.

The aim of the study was to examine effects of the COVID-19 pandemic and changes in the different dimensions of physical fitness in a cohort that was investigated over an extended period.
