Key terms were identified based on the targeted population and examined using MeSH trees. Web of Science and NCBI Pubmed were used as the databases of choice. These data sources were used in order to generate as many relevant papers to this topic as possible as seen in Figure 1. The following key words were used:

- COVID-19:

2019 Novel Coronavirus Disease

2019 Novel Coronavirus Infection

2019-nCoV Disease

2019-nCoV Infection

COVID-19 Pandemic

COVID-19 Pandemics

COVID-19 Virus Disease

COVID-19 Virus Infection

COVID-19

Coronavirus Disease 2019

Coronavirus Disease-19

SARS Coronavirus 2 Infection

SARS-CoV-2 Infection COVID-19

- Athlete:

Elite Athletes

Professional Athletes

Para-Athletes

Sportsperson

Sportsman

Sportswomen

Sports Person

- Fibromyalgia

Diffuse Myofascial Pain Syndrome

Fibromyalgia, Primary

Fibromyalgia, Secondary

Fibromyalgia-Fibromyositis Syndrome

Fibromyositis-Fibromyalgia Syndrome

Fibrositis

Myofascial Pain Syndrome, Diffuse

Rheumatism, Muscular

Chronic Fatigue Syndrome

- Quality of Life:

HRQOL

Health-Related Quality Of Life

Life Quality

QOL

Health Status

- Patient Reported Outcome Measures:

Patient Reported Outcome

Patient Reported Outcome Measure

Patient Reported Outcomes

Patient-Reported Outcome

Patient-Reported Outcomes

Patient-Reported Questionnaires

We focused on athletes with a confirmed diagnosis (symptomatic/asymptomatic) of COVID-19 (mild/moderate/severe disease) with either confirmed or suspected fatigue/chronic fatigue syndrome. Athletes were defined as adults over the age of eighteen that are currently registered with the appropriate sporting body/club or receive a monetary reward for their physical performances. Studies without a comparator were not considered.

Papers identified were then included/excluded by selected criteria. Published works (full text papers, systematic reviews, literature reviews, RCTs, meta-analysis papers, clinical trials/experimental studies, case reports/series and cross-sectional studies) from 2019-2021 only in English were accepted. Athletes must be diagnosed with COVID-19 either via antigen testing, antibody testing or PCR test (asymptomatic or symptomatic). Papers which include athletes diagnosed by a clinician based on symptoms will be accepted if it is a minor number of the overall participants. College level athletes were considered if playing competitively. Adults defined as individuals of any sex over the age of 18, there is no upper age limit if they fulfil the other criteria (monetary reward/affiliations). Ex-professional athletes/retired athletes were not considered. Published works were excluded if they were abstract only, study protocols, conference pieces, poster presentations or opinion pieces. Studies that reported athletes presenting with COVID-19 symptoms, but not yet diagnosed/excluded Influenza or other upper respiratory tract infections were included if assessed by a registered clinician to have COVID-19. PRISMA guidelines were followed to identify papers for extraction.

The quality assessment tool utilised was the Hawker tool, papers were rated to assess credibility, dependability, confirmability and transferability. These ratings were used to grade the papers from low quality to high quality.

The type of PROM used, the time period covered and its implementation to measure the health of athletes was assessed.

Eight articles were selected for extraction following the eligibility criteria and were graded by quality using the Hawker quality assessment tool [14] - [21] . The majority of papers (N = 6) were published in the USA with no papers published from Asia or South America. They were published in medium-high impact journals and have been cited elsewhere. In terms of study design, there were three observational studies, three cross-sectional studies, one retroactive case review and one case report.

Four articles indirectly obtained information on athlete’s health from databank institutions [15] [17] [18] [19] . These databanks used routine medical screening questions to assess fatigue in athletes. The other articles (N = 4) directly obtained information from athletes via either a questionnaire or online survey [14] [16] [19] [21] .

Petracek et al., (2021) used the unidimensional wellness score which rated physical health from 0 (lowest) to 100 (highest) [19] . Schwellnus et al., (2021) used the Wisconsin respiratory symptom survey which is a standardised and validated method used to evaluate patient quality of life post-upper respiratory tract infections [21] . Moulson et al., (2021) and Petek et al., (2021) used the Updated Lake Louise imaging criteria which is recommended prior to cardiac magnetic resonance for suspicion of myocarditis as a cause of systemic symptoms such as fatigue [18] [19] . Martinez et al., (2021) used the ACC RTP cardiac screening protocol while also using databanks for patient symptoms [17] .

Persistent fatigue and CFS currently have no standardised validated PROMs, with large discrepancies between clinician and patient standards. The majority of PROMs focus on physical symptoms and exclude patient emotional or mental well-being which affects patients QOL the most [22] [23] [24] . A lack of knowledge on the underlying pathophysiology makes it difficult to create specific PROMs. Roberts in 2017 determined that the WHOQoL-Bref26 can offer an accurate snapshot of patient condition during the course of treatment but offers little in terms of long-term value or symptom severity [24] . New frameworks have been proposed by Parslow et al., in 2020 to assess CFS but it is limited to a children population [25] . Each group used a different PROM tool to assess fatigue in athletes (Table 1). This lack of standardisation makes it difficult to cross-reference results from different athlete populations to accurately assess the effect of COVID-19 on fatigue or CFS.

PROMS were used by all papers over the course of one to six months to obtain information regarding athlete physical condition post-infection (Table 1). Mental state of athletes was not assessed despite brain fog commonly reported in patients with long-COVID or diagnosed with COVID-19 associated CFS [26] [27] . Brito et al., (2021), Petraceck et al., (2021) and Schwellnus et al., (2021) used a questionnaire format to obtain information about athlete quality of life directly [14] [20] [21] . Krzywanski et al., (2021) carried out routine medical screenings and the rest of the papers obtained data indirectly via institutional data banks [16] .

Associations between ethnicity and CFS have been previously reported in literature [28] [29] . Of the papers reviewed, little attempt was made to link athlete ethnicity and fatigue although Petek et al., (2021) reported that black athletes were five times more likely to have exertional symptoms than white and white-Hispanic athletes (Table 2) [19] . Moulson et al., (2021) reported higher rates of myocardial disease in the same population [18] . 50% of the papers reviewed did not disclose patient ethnicity (N = 4). The relationship between

COVID-19 severity and type of sport on fatigue level was also unclear. Krzywanksi et al., (2021) categorised the different types of sports but there was no cross-analysis to determine if fatigue varied between the different sporting populations [16] . Hull et al., (2021) carried out a similar approach but divided their athlete population by summer and winter sports [15] . Additionally, they were the only group to clarify about the type of athlete with twenty-five para-athletes included in the study population. Para-athletes and were found to have a greater risk ratio of symptoms impacting more than twenty-eight days when compared to athletes (1.2 to 0.8).

Age has been reported to be one of the greatest independent risk factors for adverse COVID-19 outcomes [30] [31] . Papers reviewed opted to either use mean or median ages rather than sort symptoms by age groups (Table 3). Hull et al., (2021) and Petek et al., (2021) both reported little difference of age on symptom duration including fatigue [15] [19] . Males have a higher unadjusted COVID-19 mortality rate than females [32] . Furthermore, men are more likely to be hospitalised and develop long-term sequalae including fatigue [33] . All studies reviewed, bar one, included a mix of genders, but none assessed the effect of athlete gender on COVID-19 outcomes. Hull et al., (2021) demonstrated that males were almost twice as likely to suffer from short term fatigue symptoms post-infection, however, there was no statistical difference after twenty-eight days [15] . Petek et al., (2021) however reported that male athletes were 10% more likely to have persistent symptoms including fatigue [19] . Kryzywanski

et al., (2021) also found that females were more likely to have asymptomatic cases across all sporting brackets [16] .

A large proportion of athletes screened suffered from fatigue post-COVID-19 infection (Table 4). The majority of papers assessed fatigue spanning a time period of several months post-infection. Brito et al., (2021) demonstrated that 25% of athletes with symptomatic COVID-19 symptoms had fatigue [14] . A higher figure of 56% was reported by Krzywanski et al., (2021) [16] . A similar figure was reported by Hull et al., (2021) with 47% reporting fatigue post-infection and 10% showing signs of persistent fatigue after 28 days [15] . Schwellnus et al., (2021) reported the highest fatigue rates but included athletes presumed to have COVID-19 adding a confounding bias [21] . A large cohort study including 3597 participants by Petek et al., (2021) demonstrated that 9% of athletes had exertion fatigue, which decreased to 4% after three weeks [19] . A cross-sectional study by Martinez et al., (2021) assessed fatigue in thirty athletes with abnormal heart conditions with five reporting fatigue [17] . These studies indicate that persistent

fatigue is a relatively common long-term complication of COVID-19 in athletes. A case review by Petraceck et al., (2021) highlighted a case of CFS in young athlete but other studies assessed fatigue on a shorter time frame [20] .

Developing persistent fatigue is thought to be independent of COVID-19 severity across all age groups and genders [34] [35] . Most papers excluded asymptomatic patients whilst some omitted severity. Brito et al., (2021) found no significant difference in cardiovascular activity between asymptomatic and symptomatic athletes [14] . The majority of papers used either lateral flow testing or PCR swabs to determine COVID-19 status. Petek et al., (2021) and Schwellnus et al., (2021) used an LFT, PCR swab and antibody test [19] [21] . Schwellnus et al., (2021) assessed a large number of symptoms including fatigue along with median duration but included athletes assumed to have COVID-19 without a formal diagnosis [21] . Hull et al., (2021) also included seventy-six athletes clinically assumed to be positive [15] .

Despite the clear association between COVID-19 and symptoms of long term COVID-19 sequela, the absence of standardised and validated PROMs to assess CFS or persistent fatigue results in a large heterogeneity within literature regarding this topic. The effect of lockdowns on sporting cancellations can also have a compounding factor on athlete fatigue due to decreased fitness levels. The effects of factors such as the type of athlete, type of sport and ethnicity of athlete on fatigue was equally unclear as the studies assessed either omitted or did not differentiate inter-variably. It was difficult to assess the impact of age as competitive athletes are generally younger rendering a selection bias. There is also an inherent sparse-data bias due to the difficulty assessing CFS as it takes six months to diagnose, thus requiring a large number of participants over a prolonged period. This study excluded non-English papers as well as non-competitive athletes, such as amateur bodybuilders, which could have yielded further insight. The emergence of new variants was also not assessed despite the different effects they confer.

Sparse-data bias meant that further follow-up studies are required to assess if the prevalence of CFS in athletes is similar to the general population. The potential impact on the commercial and marketing value of the players was not assessed and further studies are required to investigate this.