Background: During the COVID-19 pandemic, free movement, leisure activities and sports, both competitive and elite, were restricted by measures against the spread of the infection. The study aimed to evaluate the impact of the restricttions on the performance of Czech adolescent competitive athletes. Methods: The sample of the retrospective study comprised 125 participants (41 girls, 84 boys). The initial examination, the mean age of the entire sample was 15.5 ± 1.2 (median 16.0) years; 16.0 ± 1.2 in girls and 15.5 ± 1.2 in boys, respectively. All participants were examined during regular preventive sports health checks from September to November 2019 and a year later. The examination consisted of a complete medical history and physical examination, anthropometric measurements (height, weight and body mass index [BMI] calculations) and maximal exercise testing on a leg cycle ergometer, ECG and blood pressure recordings. The response variables in regression models were power output (W/kg) and MET. Results: In the entire sample, as well as in the boy and girl subgroups, body height, weight, BMI, BMI percentile and power output were significantly increased in 2020 compared to 2019, yet with a statistically significant reduction in both power output per kilogram of body weight and metabolic equivalent (MET). In 2020, the relative power output dropped by 0.13 W/kg and MET by 0.6. There was no statistically significant predictor of changes in relative power output and MET identified in regression models, including no significant impact of the sport type. There were no significant differences in results between genders. Conclusions: Performance and anthropometric parameters, specifically power output per kilogram of body weight and MET, worsened in the sample of adolescent male and female athletes.