It was calculated that 18.2 million people died as a result of the pandemic between 2020 and 2021 and numerous deaths were due to coronavirus disease 2019 (COVID-19) (Wang et al., 2022). The numbers were influenced by limited prevention and treatment measures aimed at other diseases. In the case of convalescents, persistent symptoms are common after the infection (Nasserie et al., 2021). To mention, one of the most important health consequences of COVID-19 infection are CT changes in the respiratory system, primarily lunges (Baratella et al., 2021). Although, physical activity prior COVID-19 might alleviate severity and reduce mortality range in COVID-19 patients. It is especially recommended to include physical activity in the amount of ≥150 min/week of moderate activity or ≥75 min/week of vigorous activity (Sittichai et al., 2022). Even full vaccination does not guarantee complete protection against long-lasting COVID-19 complications (Al-Aly et al., 2022).

The period of pandemic and the anxiety associated with it made a huge impact on the lifestyle of people around the world (Jodczyk et al., 2021; Jodczyk et al., 2022). Many areas of human life were negatively affected, including mental health (Gruba et al., 2021; Jodczyk et al., 2022). Depression, anxiety disorders, sleep disturbances, and perceived stress became more common compared to the period before COVID-19, both among healthy young individuals and those yet suffering from any diseases (Kasiak et al., 2022a). The shift in everyday routine also elicited reports of decreased physical activity in different populations (Wunsch et al., 2022).

New, pandemic conditions became a challenge for sportsmen. Although, athletes are not considered a high-risk group for high COVID-19 vaccine hesitancy (Ulaszewska et al., 2022). Moreover, about 94% of them, the course of the illness is asymptomatic or mild (Lemes et al., 2022). Nevertheless, the return to physical activity after the disease raised questions in terms of safety with myocardial involvement as the main concern (Rajpal et al., 2021; Modica et al., 2022). Health hazards other than cardiovascular sequelae include respiratory (Xia et al., 2020) or muscular (Seixas et al., 2022) complications. Although, these health complications do not always require hospitalization. Moreover, training in confined areas (Washif et al., 2022) and sports events (Sparrow et al., 2021) encountered significant organizational problems due to the risk of SARS-CoV-2 transmission.

The above-mentioned factors remain closely related to physical performance. There is evidence of reduced oxygen uptake (VO₂) and altered lactate metabolism in athletic convalescents after mild COVID-19. This issue is still not enough studied, but the physical capacity of the subjects tended to decline (Csulak et al., 2021; Komici et al., 2021). There is also a study whose results deviate from this assumption (Fikenzer et al., 2021). Cardiopulmonary exercise testing (CPET) has proven to be a useful tool in evaluating endurance athletes after COVID-19 and could be used to diagnose persisting symptoms (Moulson et al., 2022).

We stipulate that COVID-19 infection causes a deterioration in fitness performance with the aggravation of the body’s physiology and anthropometric measures (Ali and Kunugi, 2021), changes are noted especially in muscle building, the mechanism is not clear, but it may be due to e.g., the cytokine storm, malnutrition, prolonged inactivity (Zhou et al., 2020). This study aimed to: (1) assess COVID-19 consequences among endurance athletes with varied fitness level on exercise and body physiology and (2) adjust obtained results to time passed from COVID-19 infection. The main novelty and advantage of the present study is providing time adjusted results of CPET performance and somatic changes depending on the time that has passed since the COVID-19 infection. This approach enable to better understand possible consequences of the underwent disease. Moreover, we included EA with varied level of advancement, both professional and amateur ones, which facilitate more holistic and comprehensive assessment of COVID-19 infection on CPET performance.

The harmful influence of mild COVID-19 infection on CPET performance in a group of endurance athletes was demonstrated. The main findings are: (1) COVID-19 infection causes deterioration of exercise and body anthropometric parameters both on professional and amateur EA and (2) the impact of infection was the most significant for HR and VO₂, and (3) even the mild COVID-19 infection should be considered as a limiting factor for exercise performance. We noticed the weight and BMI increases. Interestingly, the percentage of BF and FATM is lower after contracting COVID-19.

Among our subjects, we noticed a difference in body composition before and after contracting COVID-19. Patients during the infection are at risk of weight loss due to weakness, fever, decreased appetite and taste, depending on the advancement of the disease (Yang et al., 2020). Factors that correlate with weight loss include: raised C-reactive protein levels, impaired kidney function, and long duration of illness (Anker et al., 2021). Among our patients, both weight and BMI increased, which is not typical for the immediate time of the disease. This may be due to the passage of time since the first test, as well as the lockdown and limited opportunities for physical activity and changes in eating habits such as overeating (Martinez-Ferran et al., 2020). A systematic review was created, which among respondents during the pandemic noted an increase in food consumption by 36.3%–59.6% of respondents and a decrease in physical activity by 67.4%–61.4% (Chew and Lopez, 2021). An unexpected decrease in BF and FATM with weight gain may be due to a specific diet aimed at minimizing the increase in body fat in athletes (Iraki et al., 2019) Adequate amount of protein in the diet and properly workouts support the burning of body fat and promote the retention of lean mass (Ruiz-Castellano et al., 2021).

Physical training allows one to exert higher intensities with lower lactate levels (San-Millan and Brooks, 2018) and increase VO_2max, which constitutes the basis for preparation to achieve optimal sports results (Seiler, 2010). Maintenance of physical fitness requires adherence to the exercise and training regimen. On the contrary, conditions of the COVID-19 pandemic presented many risks of reverse effects (Jodczyk et al., 2022). He disease itself may injure multiple organ systems, the more severe it is. However, even a relatively mild course of the illness has been associated with persistent health complications (van Voorthuizen et al., 2022). It is worth emphasizing because none of our study participants was hospitalized because of COVID-19 infection. One of the studies also showed that healthy patients who experienced mild COVID-19 have reduced efficiency compared to the control group, including reduction in peak VO2 associated with impaired systemic oxygen extraction, additionally, convalescents also showed greater respiratory failure (Singh et al., 2022). Convalescents report reduced daily physical activity, which in turn may lead to a decrease in efficiency (Delbressine et al., 2021). Despite the occurrence of side effects even in people with moderate to mild COVID-19, it is important to remember that these effects are potentially less persistent than in patients with severe COVID-19 (Jimeno-Almazan et al., 2022).

Evidence of lung fibrosis, although not common (in 1.1% of cases), was found even in mild-to-moderate cases of COVID-19 (Xia et al., 2020). Autonomic nervous system dysregulation in young patients, measured by HR variability and partly attenuated by physical activity, was also reported (Freire et al., 2022). A study of college athletes revealed signs of ongoing myocarditis or earlier myocardial injury in cardiac magnetic resonance performed up to 53 days after a period of quarantine (Rajpal et al., 2021). Muscle deconditioning was proposed to mainly limit CPET results in COVID-19 survivors (Rinaldo et al., 2021). Among possible mechanisms involved are direct cell invasion, ACE2 downregulation, and inflammation, as well as prolonged bedrest and hypoxia in severe cases (Seixas et al., 2022). Deconditioning due to hypoactivity occurs rapidly, e.g., just 5 days may be enough to negatively affect muscle strength or oxidative capacity, induce fiber atrophy and disrupt protein balance (Fovet et al., 2021). Given ordinary conditions, 8 weeks of a sedentary lifestyle was found to lead to wasted 6-week training results in terms of VO₂ (Fritzen et al., 2020). It is also noteworthy that a pandemic does not mean quarantine itself. Apart from COVID-19 illness, legal and social conditions promoted physical inactivity by encouraging social distancing and restricting access to sports facilities, sometimes even prohibiting unnecessary leaving residential buildings (Wunsch et al., 2022).

Our results demonstrate the cumulative negative effect of the pandemic on physical fitness among endurance athletes and show different infection consequences for both treadmill and cycle ergometry. These findings are in line with the previous research, as Price et al. confirmed that both modalities differed (Price et al., 2022), the choice of device was adapted to the leading discipline of the subject, differences may have occurred in VO₂ and HR at maximum exertion, at AT and at RCP between cycling and running CPET testing, in both males and females. In our participants, we found lower VO_2max and earlier lactate accumulation during CPET. Meanwhile, there are few studies that demonstrate the change in CPET performance after COVID-19 infection in endurance athletes and compare them with healthy controls. Similarly, to our results, in a sample of elite cross-country skiers—asymptomatic during evaluation, 4–6 weeks after COVID-19 diagnosis—CPET revealed lower VO_2max, VE, HR, and O_2pulse when compared to the control group. On the other hand, in contrast to our observations, there was no statistical difference in VO_2max between post-COVID-19 soccer players and controls at the age of 18–35. Although, the trend towards reduced VO₂ was visible. It is also worth noting that we recorded a decline in VO₂ of 2–4 mlkg·min⁻¹ over about 1.5 years between CPETs (exact interval was 591.67 ± 282.24 days; see Table 1). According to reference standards provided by Kaminsky et al. (2015) VO₂ should decrease by about 4–5 mlkg·min⁻¹ over 10 years. Our EAs experienced a much more dynamic decline in VO₂ than only based on age. Hence, it is worth considering the previous COVID-19 infection as a possible influencing variable. Resting and maximal HR, VE, and blood pressure were not changed compared to healthy controls, as well as other reported CPET parameters. The research excluded non-competitive athletes and required recovery from infection within the previous 30 days, which means earlier assessment than used in this study. Consequently, only 5 out of 24 examined post-COVID-19 soccer players showed no illness symptoms (Komici et al., 2021). Likewise, no deterioration of CPET performance was observed in elite swimmers from National Swim Team Hungary after a mild COVID-19 infection (Csulak et al., 2021). It is worth noting that physical performance declined with age to various degrees between participants, but age was also one of the contributing factors (Wiecha et al., 2022). So far, it has been postulated that training experience may reduce the decline in HR and VO₂ observed with age (Tanaka and Matsuura, 1984; Kaminsky et al., 2015). The less trained EAs usually noted a steeper decline. In our study, two most significantly different variables before and after infection, i.e., HR (at AT and RCP) and VO₂ (at AT, RCP and max) after adjusting for training experience did not show significant differences (except HR for subgroup with 6–10 years of training). Thus, underwent COVID-19 infection could be a contributing factor and cause a steeper deterioration of endurance capacity.

The maximal exercise capacity, formulated in metabolic equivalents of the task (MET) was linked with the risk of hospitalization due to COVID-19 with each 1 MET higher indicating odds lower by 13% (Brawner et al., 2021). In addition, age-related mortality of the disease remains in close contact with VO_2max declining over the course of the human lifespan. SARS-CoV-2 infection may unfavorably influence lipid metabolism pathways and cause mitochondrial dysfunction in a similar way to aging. Therefore, there could be a harmful cumulative effect on the athletes with age, as well as protection for physically fit people (Spedding et al., 2022). Despite COVID-19 infection, in our study, athletes’ muscle deconditioning due to pandemic-related lower physical activity might be enough to explain the results. What is more, lactate accumulation is known as an immunomodulatory factor because of its connection with IL-6 production, the activity of dendritic cells, or T-cell responses, which explains some of the anti-inflammatory characteristics of physical activity. Hence, it was even hypothesized that increasing the anaerobic threshold may protect against COVID-19 complications (AbdelMassih et al., 2021) and our findings may even indicate a pro-inflammatory aspect of the pandemic.

As sports performance, presented changes in CPET parameters need to be assessed in terms of the smallest worthwhile change (SWC). Most of them decreased by about 0.4 of baseline SD. The biggest post-COVID-19 declines were observed in VO₂ at RCP (0.46 SD) and absolute VO₂ at RCP (0.51 SD). This constitutes a visible factor during competitions if we define SWC as 0.2 of the baseline SD (Lindberg et al., 2022). Our other results are somewhat inconclusive. Division by measurement method (cycle ergometer or treadmill) indicated some statistical differences, yet they were most likely a matter of the small size of the groups distinguished. Besides, the time-dependence of BF percentage and FATM or power at the AT and HR at the RCP cannot be discriminated among the infection, detraining, and the passage of time.

A mild COVID-19 infection resulted in a decrease in professional and amateur EA’s CPET performance. The most significant changes were observed for VO_2, and HR. Medical Professionals and Training Specialists should be aware of the consequences of a mild COVID-19 infection in order to recommend optimal therapeutic methods and properly adjust the intensity of training. The results are a valuable addition to the current state of knowledge when preparing Return to Play clinical protocols and guidelines.