# COVID-19 and Changes in the Model of Physical Fitness and Body Composition of Young Tennis Players
## 4 Discussion  
This study investigated the differences between selected physical fitness and body composition indicators during the COVID-19 pandemic (2020 and 2021) and their values as predicted using the pre-COVID-19 trend (2015-2019). Historical data from 227 female and 320 male tennis players collected before the pandemic were used to predict the expected age-adjusted evolution of body composition and physical fitness variables. This prediction was then compared to the age-adjusted measurements recorded during the COVID-19 pandemic. To our knowledge, this is the first study to analyse changes in body composition and physical fitness in young tennis players using this approach. In young male athletes, we observed significant changes in body composition relative to predicted values, which was not the case in female athletes. Specifically, body fat mass scores were significantly higher, and body muscle mass scores significantly lower, than the expected results based on the pre-COVID trend. These negative changes in body composition in boys persisted after the first lockdown and worsened after the second lockdown. The significant differences in body composition that we observed in young male tennis players as a result of the COVID-19 pandemic may indicate the negative effects of increased sedentary behaviour, as was also observed in a study of young badminton players in the COVID-19 period [30]. A significant increase in fat mass and a decrease in lean mass (measured by DXA) were also observed in male American football players, despite the fact that they reported strength training as their most frequent activity during the suspension period [21]. However, in a study by [PERSON] et al. [22], in which the body composition of fencing athletes was measured at three different time points (September 2019, June 2020, and September 2020), the authors reported an increase in fat mass only in women, which normalised at the third measurement. In another study [13], the body weight, fat mass, and muscle mass of university athletes were analysed using bioelectrical  
Figure 3: The difference between the age-adjusted effect in 2020 and 2021 and the age-adjusted predicted effect in 2020 and 2021 by sex with corresponding 95% confidence intervals and \(p\)-values. Note: The \(y\)-axis shows the estimated (age-adjusted) difference between the observed and predicted effect based on the estimated pre-COVID, non-linear trend, applying the square-root transformation to outcomes. Positive (negative) values of the contrast suggest that the age-adjusted outcome in a particular year (2020 or 2021) was larger (smaller) than that predicted by the age-adjusted, pre-COVID, non-linear trend. BFA—body fat mass; BMUSC—body muscle mass; OMBT—overhead medicine ball throw; BEEPVO2M—20 m shuttle run test; CJM—countermovement jump height; SJ—Squat jump height. impedance before suspension (January 2020) and shortly after the resumption of on-campus training (August/September 2020). A decrease in fat mass was observed in men, whereas fat mass increased in women. [PERSON] et al. [17] found no difference in body composition in elite futsal players when assessed twice across a three-month period using skinfold measurements. Body fat and muscle mass also remained unchanged in a group of female soccer players between August 2020 and February 2022 [31]. In our study, we also found no changes in body fat mass and muscle mass in female tennis players. This could be explained by behaviours relating to excessive concerns with weight and body image, as has been reported in young female tennis players [32]. We can also assume that the changes in body composition and physical fitness that occurred during the period of isolation disappeared or decreased by the time testing was conducted. For physical performance parameters, the only significant change observed in our study was an improvement in SJ height and a non-significant positive trend in CMJ values. [PERSON] et al.