# COVID-19 and Changes in the Model of Physical Fitness and Body Composition of Young Tennis Players
## 1 Introduction  
COVID-19, caused by the SARS-CoV-2 virus, is a multisystem disease that can affect the pulmonary and cardiovascular systems. The virus affects the ability of many people to exercise and benefit from sports participation [1]. In particular, the Slovenian Government implemented a strict first lockdown from 12 March 2020 to 15 May 2020 as a preventive measure to limit the spread of the disease. The second lockdown was introduced during the second wave of COVID-19, from 18 October 2020 to 15 May 2021. All schools were required to close during both lockdown periods; therefore, lessons were carried out online.  
The strict measures adopted to stabilize the healthcare system had some direct and some indirect societal consequences, particularly for young athletes. Specifically, youth athletes are characterized by developing intrinsic dual careers (i.e., a combination of academic and athletic activities), and, therefore, simultaneously face several athletic, academic, psychological, psychosocial, financial, and legal challenges [2]. These individuals were strongly affected by COVID-19 due to numerous factors, including new academic realities,the loss of practice and competition, and social distance from teammates. Isolation during the pandemic led to radical changes in athletes' lifestyles, which affected both their physical activity levels and dietary habits. [3]. The recurring, widespread suspensions (travel restrictions, social distancing, and closure of training facilities) also prevented athletes from continuing their usual training programmes [4], and competitions were suspended at all competitive levels.  
While each sport was affected differently, depending on its particular characteristics, the global outbreak of COVID-19 certainly had a considerable impact on tennis [5]. Importantly, tennis is a highly demanding sport characterized by repeated high-intensity efforts during a variable match time. This requires highly competitive players to excel across a range of different fitness components, such as power, speed, agility, and endurance [6; 7]. These components are influenced by body composition parameters [8; 9; 10], which are often used to evaluate an athlete's potential to compete at the highest level [11; 12].  
The observed changes in physical activity and dietary habits induced by COVID-19 may have influenced the body composition of tennis players. Physical activity and dietary patterns are known to change during extended breaks (e.g., summer breaks, off-season, injuries, etc.), especially if they occur unexpectedly, as in the case of COVID-19 lockdowns [13]. Although attempts were made to compensate for these changes with alternative strategies such as training at home [14; 15], it is important to assess whether these strategies provided sufficient stimuli for competitive athletes [16] to prevent negative changes in body composition. Many studies that have examined the effects of COVID-19 restrictions on physical fitness [17; 18; 19; 20] or body composition have been conducted on professional athletes and have shown conflicting results [21; 22; 23]. As body composition fluctuates and develops in young athletes on a regular basis, it is difficult to identify the extent to which changes can be attributed to the effects of COVID-19. The aim of this retrospective study was, therefore, to estimate the differences in the selected physical fitness and body composition indicators during the COVID-19 pandemic (2020 and 2021) and to compare these values to those predicted using the pre-COVID-19 trend (2015-2019).