The Effect of COVID-19 Infection on the Aerobic Capacity of Professional Soccer Players.

This investigation assessed the effect of COVID-19 on the aerobic capacity of professional soccer players who have tested positive. Twenty-one division-1 elite soccer players (age 24.24±5.75 years, height 178.21±5.44 cm, weight 74.12±5.21 kg) participated in this study. This observational study compared the same players' aerobic capacity pre-, and 60-days post COVID-19 recovery. The statistical analysis demonstrated that the infected players had signicantly lower VO2max values [t (20) =5.17, p<0.01, d=0.613 (medium effect)], and signicantly lower VO2 values at RC [t (20) = 2.97, p<0.05, d= 0.39 (small effect)] after recovery. Furthermore, results indicated a signicantly lower running time (RT) on the treadmill [t (20) =4.84, p<0.01, d=0.46 (small effect)] when compared to the results that were obtained before they got infected. In addition, velocity at VO2max ( V VO2 max ) was signicantly lower [t (20) =2.34, p<0.05, d=0.41 (small effect)] and the heart rate values at LT [t (20) = -2.79, p<0.01, d= 0.55 (medium effect)] and RC [t (20) = -3.72, p<0.01, d= 0.52 (medium effect)] were signicantly higher post recovery. The aforementioned ndings indicate that post COVID-19 soccer players may not reach full recovery at two months. Therefore, our results should alert practitioners and tness coaches of the risk of longer-duration silent symptoms even in athletes that experience mild to moderate manifestations.


Introduction
Coronavirus, a highly infectious disease currently termed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is generally accompanied by mild to moderate manifestations [1]. However, a small proportion of patients develops a severe respiratory illness that may lead to death in some cases [1]. 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 [2]. Once the spike glycoproteins of SARS-CoV-2 connect to ACE2, the virus enters the host cell [2]. 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 [2]. 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 [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 [3].
The infection rate among professional soccer players is consistent with that of the general population [4], 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 [5]. 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 [5].
Conclusively, COVID-19 positive professional soccer players nd 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 [9] 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 [10]. In addition to the psychological distress, a multilingual online survey of 5056 participants a rms that COVID-19 con nement 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 tness [11,12], athletes' competitive power levels were not maintained [11]. Therefore, a period of complete inactivity is expected to have more signi cant reductions in physical conditioning. Research a rms that detraining induces a rapid decline in aerobic capacity, resulting in an increased heart rate for any given workload [13].
Furthermore, research demonstrated that even a short period of inactivity might have a signi cant detraining effect.
In particular, 2-weeks of inactivity caused a marked reduction in endurance capacity and repeated sprint ability in semi-professional soccer players [14]. Similarly, top-level road cyclists have been shown to produce signi cant reductions in aerobic capacity and power output after ve weeks of detraining [15].
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 [32]. 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 [16]. 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.

Subjects
Twenty-one division-1 elite soccer players (age 24.24±5.75 years, height 178.21±5.44 cm, weight 74.12±5.21 kg) participated in this study. All the players were tested at the end of April (pre-testing) as part of their teams' assessment (Table 1). Those who underwent the initial testing and have tested positive for COVID-19 during the following months were recruited for this study. However, not everyone that was initially recruited participated in the study. Asymptomatic players were excluded, and only those that reported two or more mild to moderate symptoms of body discomfort were included in the study. Of note, none of the athletes have had COVID-19 severe enough to be hospitalized.
Furthermore, players who have tested positive after being vaccinated were also excluded from the study. Finally, goalkeepers were excluded from the study as they did not follow the same re-training and adaptation program to the in-eld players after they recovered. Conclusively, those players who met the inclusion criteria were tested after 60 days of their return to normal activities. As reported by the team's staff and medical group, prior to the resumption of vigorous training, the players had a minimum of two consecutive negative PCR tests and a con rmed IgM negative test in addition to the speci c health evaluation that included a cardiology assessment.
Furthermore, before the post-testing, the players followed a 2-week re-training program based on the safe return to sport activities guidelines [17]. In addition, they followed a 10-days speci c adaptation program and a 20-days game adaptation program based on the guidelines [17] ( Table 2). Application of speci c recovery protocols post games and the day after Thus, this observational study compared the same players' aerobic capacity measurements that were obtained at the end of April following the 2nd lockdown to those obtained 60-days post COVID-19 recovery. All the participants had medical clearance and a negative COVID-19 polymerase chain reaction (PCR) test within 48-72 hours before the testing despite recovering from COVID. Players' participation in this study was completely voluntary, and each player was briefed on the procedures before they signed an institutionally approved written informed consent form.
The study was carried out in accordance with the Declaration of Helsinki and was approved by the University's ethics committee board and the National Committee on Bioethics. The researchers were required to wear a face mask, face shield, gown, and gloves during data collection and adhered to the guidelines for patients and testing personnel during aerosolizing procedures as indicated by the Centers for Disease Control and Prevention [18].

Procedures
Anthropometric measurements were recorded using a wall stadiometer (Leicester; Tanita, Tokyo, Japan) to measure the players' stature and a leg-to-leg bioelectrical impedance analyzer (BC418MA; Tanita) to measure body composition.

Cardiopulmonary exercise testing
The players completed an incremental maximal cardiopulmonary exercise testing (CPET) until they reached exhaustion on a treadmill (h/p/Cosmos Quasar med, H-P-Cosmos Sports & Medical GmbH, Nussdorf-Traunstein, Germany). A breath-by-breath analysis was performed on the Cosmed Quark CPET (Rome, Italy) system. Laboratory conditions were kept constant, with the temperature at 22±1ºC and relative humidity at 50%. The players were tested utilizing the modi ed Heck incremental maximal protocol, which was previously validated for its reliability to test soccer players [19]. The test came to an end when the participant reached volitional fatigue or when there was no variation among the VO2 levels while the workload increased. The VO2max was detected after having the results ltered to identify the highest value for an average of 10 seconds.

Determination of Ventilatory Threshold and Respiratory Compensation Point
The ventilatory threshold (VT) and respiratory compensation point (RC) were determined using different criteria.
The VT was determined through the V-Slope method, the point at which the increase in the rate of elimination of carbon dioxide (V̇ CO2) is greater than the increase in V̇ O2.  (20) =4.84, p<0.01, d=0.46 (small effect)], when compared to the results that were obtained before they got infected (Table 3). In addition, the running velocities at LT ( V LT) and RC ( V RC) were not signi cantly different post-COVID-19 recovery while the velocity at VO2max ( V VO2 max ) was signi cantly lower [t (20) =2.34, p<0.05, d=0.41 (small effect)] (

Discussion
To the best of our knowledge, this is the rst study to examine the effect of COVID-19 infection on the aerobic capacity of professional soccer players. Considering that the average intensity in a 90-min soccer game is close to that of the lactate threshold [23], it is evident that aerobic performance in soccer is not only dependent on VO2max but also on lactate thresholds and the associated running velocities [24]. Research a rms that enhancement of aerobic capacity leads to improved soccer performance, as it increases the total distance covered by the players, the level of work intensity [34] as well as the number of sprints during competitive games [25]. This study demonstrated a signi cant reduction in VO2max and running times on the treadmill about 60 days post COVID-19 recovery. Furthermore, the VO2 at RC was signi cantly lower while the VO2 at LT was reduced but not signi cantly.
Studies highlight the importance of using thresholds to indicate and monitor the improvements in aerobic performance indices during each period of a typical soccer season [26]. More speci cally, research indicated that 8- week preseason training caused signi cant increases in the RC point and VT [26]. Furthermore, research indicated that RC point is inversely correlated with hypoxic ventilatory response and that 40 to 50% of the variance of RC is accounted for by hypoxic ventilatory response and delta slope (rate of lactic acid increase during exercise) [21]. The reduction in the RC point in this study may indicate an increased hypoxic ventilatory response which could be compensated by the cardiovascular system in soccer players. While "silent" hypoxemia has been reported in COVID-19 [27], it is unclear if it persists after recovery in athletes with mild or moderate manifestations. Additionally, prior to the RC point, the cardiorespiratory challenge was not associated with the expiration of the amount of CO2, and the need for oxygen was met primarily through the increase in tidal volume and not the respiratory rate. While that is true for VT, the RC point is associated with hyperventilation, that is, the loss of linearity in a plot between VE and V CO2. In this study, the earlier occurrence of the RC point, which is associated with hyperventilation due to the failure of bicarbonate buffering and the consecutive fall in blood pH [28], should be taken into consideration as the signi cant reductions in aerobic performance indices infer a decline in physical performance during the competitive games of the players. Concurrently, these reductions cannot be solely attributed to the detraining period as the athletes followed a 2-week re-training program, a 10-days speci c adaptation program, and a 20-days game adaptation program following their recovery (Table 1). Therefore, our ndings may raise essential concerns regarding the players' preparedness as we demonstrate that the players have reduced exercise capacity even 60 days post COVID-19 recovery.
Despite the signi cant reductions in VO2max, RT, and VO2 at RC, the velocities at both TL and RC remained the same. With regards to the velocities, the only signi cant reduction was indicated on V VO2max. Research demonstrated that running speeds at maximum lactate steady state are directly related to the ability to use oxygen and, subsequently, to enhance metabolite removal [29]. Additionally, the velocity at which VO2max is reached ( V VO2max) in soccer has positively correlated with the distance covered and the running intensity of professional players [30]. Even though V VO2max was signi cantly lower (from 17.43 to 16.86 km/h) following COVID-19 recovery, similar running velocities were reported by others after the preseason training [26].
With respect to heart rate changes, this study demonstrated signi cantly higher heart rates at both LT and RC while maximal HR was increased but not signi cantly following COVID-19 recovery. Considering the reduction of RC point, it could be assumed that increased heart rates at RC and LT may be associated with an increased cardiovascular response due to hypoxemia [27]. While increases in submaximal and maximal heart rates have been associated with detraining, the mechanisms involved in the elevation of heart rates at LT and RC in our study need further investigation. To date, there are no published studies on the assessment of maximal functional capacity using direct measurement of oxygen uptake in athletes post COVID-19. A study of SARS survivors six months after discharge indicated that 75% of the survivors had abnormal tests, out of which 43% had reduced work due to deconditioning, 19% had cardiovascular limitations, and 6% had pulmonary limitations [31]. Another study reported that survivors of ALI (acute lung injury) experience frequent and often distressing pulmonary and psychological symptoms long after their lung injury [32].
To sum up, all the aforementioned ndings indicate that post COVID-19 soccer players may not reach full recovery at two months; therefore, a more detailed evaluation should be conducted before they return to o cial games. Our results should alert practitioners, tness coaches, and soccer players of the risk of longer-duration silent symptoms even in athletes that experience mild to moderate manifestations. Practitioners need to be able to adjust the training intensities to gradually enhance the tness levels of the infected players. Consideration should be given not only to the VO2max and running time on the treadmill but also to other performance indices such as the VT, RC, heart rates, and running velocities at those thresholds. Furthermore, it is suggested that maximal cardiorespiratory testing should be utilized to assess the aerobic capacity of the players before the o cial games. Since there is the possibility that athletes who contracted COVID-19 have late cardiorespiratory complications, the cardiorespiratory testing can help differentiate low conditioning from cardiorespiratory ine ciency.

Conclusions
This study demonstrated that a successful return to play necessitates the development of more speci c protocols for the safe return of athletes that were infected even if they experienced mild to moderate manifestations. Ideally, those protocols should be determined on a case-by-case basis considering the severity of COVID-19, period of inactivity, and pre-infection tness level. In addition, consideration should be given to the psychological status [7], playing position [33], age, and division in which each player participates, as all the above will contribute to the safe and successful return to training and competition. Lastly, further investigation is needed using GPS technology to evaluate to what extend the external load of the infected players was affected.

Declarations
Author contributions: KP and MM contributed equally to the design of the work; the data acquisition, analysis, interpretation of data and writing of the manuscript. Both KP and MM approved the submitted version.
Con ict of interest: The authors declare no competing interests.