Coronavirus, a highly infectious disease currently termed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is generally accompanied by mild to moderate manifestations . However, a small proportion of patients develops a severe respiratory illness that may lead to death in some cases . The lungs are the organs primarily affected by COVID19 as the virus accesses host cells via the enzyme ACE2 (angiotensin-converting enzyme 2), which is most abundant in alveolar (Type II) cells of the lungs . Once the spike glycoproteins of SARS-CoV-2 connect to ACE2, the virus enters the host cell . Thus, even though the lungs are the most affected organs by COVID-19, the virus can spread to other organs and infect ACE2-expressing cells at local sites, causing multi-organ problems . Considering that ACE2 receptors are highly expressed in the heart, that may explain the acute myocardial injuries that have been noted as complications in patients infected with SARS-CoV-2 . It is worth noting that the exact pathophysiology of COVID-19 remains unclear, and cardiac injury is reported to result from direct or indirect mechanisms .
The infection rate among professional soccer players is consistent with that of the general population , and most of the time, COVID-19 positive athletes are asymptomatic. However, they may experience mild to moderate symptoms such as fever, cough, loss of taste or smell, headache, aches, muscle pain, sore throat, and tiredness [4,5,6]. Less common and more severe symptoms include shortness of breath, pain and pressure in the chest, or pneumonia . It has been recommended that symptomatic athletes with moderate manifestations should rest from exercise during the symptomatic phase and for at least 14 days after the complete resolution of symptoms, while asymptomatic patients should not resume physical activity for at least 14 days after diagnosis . Conclusively, COVID-19 positive professional soccer players find themselves in a unique situation in which they are not only obliged to be self-isolated but also, to abstain from any form of physical activity for at least 14 days after diagnosis.
Positive COVID-19 soccer players could potentially have even higher psychological and physiological strain than that reported during quarantine periods [7,8]. Notably, the lockdown alone has been indicated to evoke a negative effect on mental wellbeing  and emotional status, with a great proportion of individuals experiencing psychosocial and emotional disorders (7). Not surprisingly, an increase in sedentary behavior during leisure time was associated with poorer physical health, mental health, and subjective vitality . In addition to the psychological distress, a multilingual online survey of 5056 participants affirms that COVID-19 confinement led to impaired sleep quality that was related to sleep disturbances, daytime dysfunction, the use of sleep medications, and sleep latency (8). Additionally, although home-based training during lockdown effectively improved aerobic fitness [11,12], athletes' competitive power levels were not maintained . Therefore, a period of complete inactivity is expected to have more significant reductions in physical conditioning. Research affirms that detraining induces a rapid decline in aerobic capacity, resulting in an increased heart rate for any given workload . Furthermore, research demonstrated that even a short period of inactivity might have a significant detraining effect. In particular, 2-weeks of inactivity caused a marked reduction in endurance capacity and repeated sprint ability in semi-professional soccer players . Similarly, top-level road cyclists have been shown to produce significant reductions in aerobic capacity and power output after five weeks of detraining .
It is unknown whether soccer players who have been infected and recovered have residual cardiorespiratory complications, as there are no clinical data to indicate that. However, earlier reports in SARS patients (2003 outbreak) suggest the possibility of cardiorespiratory impairments in athletes even 24 months after SARS onset . Furthermore, considering that full-length genome sequencing has shown a close association between SARS-CoV-2 and SARS-CoV, monitoring for these impairments is paramount . Conclusively, the determination of the detrimental effects of SARS-CoV-2 on elite soccer players' cardiorespiratory system is of utmost importance. Therefore, the aim of this study was to assess the effect of COVID-19 on the aerobic capacity of professional soccer players who have tested positive.