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Abstract
Background While football injury and illness epidemiology surveillance at professional club level in Europe is available, epidemiological data from other continents are lacking.
Purpose Investigating injury and illness epidemiology in professional Asian football.
Study design Descriptive prospective study.
Methods Professional teams from the Asian Football Confederation (AFC) league were followed prospectively for three consecutive AFC seasons (2017 through 2019, 13 teams per season, 322 team months). Time-loss injuries and illnesses in addition to individual match and training exposure were recorded using standardised digital tools in accordance with international consensus procedures.
Results In total, 232 665 hours of exposure (88.6% training and 11.4% matches) and 1159 injuries were recorded; 496 (42.8%) occurred during matches, 610 (52.6%) during training; 32 (2.8%) were reported as ‘not applicable’ and for 21 injuries (1.8%) information was missing. Injury incidence was significantly greater during match play (19.2±8.6 injuries per 1000 hours) than training (2.8±1.4, p<0.0001), resulting in a low overall incidence of 5.1±2.2.
The injury burden for match injuries was greater than from training injuries (456±336 days per 1000 hours vs 54±34 days, p<0.0001). The two specific injuries causing the greatest burden were complete ACL ruptures (0.14 injuries (95% CI 0.9 to 0.19) and 29.8 days lost (29.1 to 30.5) per 1000 hours) and hamstring strains (0.86 injuries (0.74 to 0.99) and 17.5 days (17.0 to 18.1) lost per 1000 hours).
Reinjuries constituted 9.9% of all injuries. Index injuries caused 22.6±40.8 days of absence compared with 25.1±39 for reinjuries (p=0.62). The 175 illnesses recorded resulted in 1.4±2.9 days of time loss per team per month.
Conclusion Professional Asian football is characterised by an overall injury incidence similar to that reported from Europe, but with a high rate of ACL ruptures and hamstring injury, warranting further investigations.
- soccer
- epidemiology
- injury mechanism
- longitudinal study
Data availability statement
No data are available. Data are confidential and not available upon request.
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Introduction
Injuries in professional football do not only affect player health but also represent a significant threat to performance and financial burden for the clubs.1–3 According to the van Mechelen model,4 the first step of injury prevention is to accurately describe the injury epidemiology. This crucial step allows a better understanding of the injury risk and pattern (ie, mechanism, type, location and severity), permitting the initiation of appropriate prevention plans.
To date, different football populations5–7 have been investigated, yet most data on professional male footballers have been collected in Europe. However, since regional differences have been demonstrated even within Europe,8 it is important to extend studies to other continents. In Asia, data on injury epidemiology in senior men’s football are limited, typically from one team followed over a few seasons9 10 or some teams followed over one season or during a tournament.1 11 12 Only one published study has collected data from more than one team and over more than one season.13 However, in this study, only injuries with a time loss of minimum 7 days were recorded, which severely limited the ability to compare the data to other studies.14
This has led us to call for regional epidemiological studies using standardised methodology to assess potential differences between continents15 and to initiate the Asian Football Confederation (AFC) Injury Surveillance Programme, modelled after the UEFA Professional Club Injury Study and comprising Champions League level teams on the continent. Therefore, the aim of this study was to investigate the injury characteristics in professional Asian football, represented by 22 professional teams from nine countries, monitored for three seasons. We hypothesised that the incidence and pattern of injuries in Asia would differ from Europe.
Methods
We recorded individual time-loss injuries/illnesses and training/match exposure in Asian adult (≥18 years old) male professional football players through the 2017, 2018 and 2019 seasons. Twenty-two teams participating in the AFC Champions League or the AFC Cup at least one of these three seasons accepted our invitation to participate, representing the following countries: Australia (n=3), China (1), Hong Kong (1), Japan (3), Qatar (5), Thailand (1), United Arab Emirates (3), Kingdom of Saudi Arabia (1), India (1) and Iran (3). We followed these teams from January 2017 to December 2019 (included), throughout the domestic seasons as well as the periods of the AFC tournaments. For Qatar, United Arab Emirates and Kingdom of Saudi Arabia, the season is from the first week of July till the last week of May with 4 weeks winter break in December. The season starts in January and ends in November for teams from Australia, China, Hong Kong, Japan and Iran. In India, the season is from January to June. We included teams that provided at least three consecutive months of data. We included male, adult (≥18 years old) players being either a first-team squad member or training regularly with the first team. Players with pre-existing injuries at the start of the season were included in the study only after successful return to play from these conditions. Players newly recruited to a club were included from their recruitment date. Players transferred to another club were tracked until the date of transfer.
Data collection
A contact person among the medical staff (team doctor or physiotherapist) in each club was in charge of collecting the data, using standardised tools. A detailed study manual outlining the details of data collection was distributed to the contact person before the team’s enrollment into the study. Demonstration sessions were organised, depending on user needs (in person or online sessions). Data were recorded using a custom-made Microsoft Office Excel file for quick data entry, using pull-down menus to classify each injury (such as body part, injury type and diagnosis). Injury cards were also provided in Microsoft Office Word to assist clinicians in taking notes during daily clinical activity, prior to entry into the master file. A potential risk to data confidentiality was minimised as follows: the Excel files used for data collection were password protected and the data were deidentified (code instead of player names) before being sent to the study group. The password was known only the clinician and the member(s) of the study group in charge of data collection. The clubs were asked to submit their data every month by email. All players were verbally informed about the purposes and procedures of the study by the team doctor/physiotherapist and provided verbal consent before participating in the study.
Definitions
To facilitate comparison with previous studies, injury definitions and data collection procedures followed the 2006 consensus statement on epidemiological studies in football16 and the IOC model for recording illnesses.17 The results have been reported according to the 2020 IOC consensus statement on injury and illness epidemiology.14
All injuries and illnesses resulting in a player being unable to fully participate in training or match play (ie, time-loss injuries) were recorded, and the player was considered injured until the team medical staff allowed full participation in training and availability for match selection. Individual player exposure during training sessions and matches was reported (individual session duration for each player, in minutes). Injuries or illnesses that did not cause time off from football activities were not recorded in the database. Also, injuries occurring outside football activities were also not considered.
For each injury, the following characteristics were recorded: diagnosis, onset (sudden vs gradual), severity (number of days of time loss), injury type, body part, mechanism (sprinting, tackling, twisting/change of direction, kicking, jumping, stretching and other), index injury (first injury of one type recorded during the study) or reinjury (injury to the same body part and same structure type within 1 year from index injury), training or match (cases of gradual onset injuries, where the injury was not clearly attributed to a specific session, were classified as other). For the illnesses and diagnosis, number of days of time loss, affected system (from a predefined list), were recorded. We have classified cases as missing data when, despite all efforts to retrieve the data, the information could not be obtained.
Injury severity was classified according to the duration of time loss as follows: mild (0–3 days), minor (4–7), moderate (7–28) and severe (>28).
Statistical analyses
Descriptive data are presented as the mean with SD. For injury pattern severity, the median and IQR were used. Injury incidence was calculated as the number of time-loss injuries per 1000 player hours. Injury burden was calculated as the total number of days lost per 1000 player hours. Poisson distribution and test-based methods were used to construct CIs and compare rates. The significance level was set at p<0.05. Statistical analyses were carried out using Power Pivot (Microsoft Excel 2019) and SPSS V.22.0 programme for OS X (SPSS, Chicago, Illinois, USA).
Results
Population
The characteristics of teams and players over the three-season observation period are presented in table 1. Over the three seasons, 900 unique players participated to the study with 709, 163 and 28 players participated a least to one season, two seasons and three seasons, respectively.
Player population characteristics
Exposure
Over the three seasons, 108, 107 and 107 team months were covered, representing an observation period of 8.3±2.9, 8.5±2.7 and 8.2±2.9 months per team, respectively. In total, 232 665 hours (88.6% training and 11.4% match play) of exposure were recorded (72 431, 80 470 and 80 470 hours for the 2017, 2018 and 2019 seasons, respectively). This represents a total exposure of 5 480±705 hours per team per season. The mean overall exposure per player to football during one full season was 255±80 hours, 225±75 hours of training and 30±11 hours of match play. The training-to-match ratio was 7.5±2.4.
Injury incidence
In total, 1 159 injuries were registered, 496 (42.8%) occurred during matches and 610 (52.6%) during training; 32 (2.8%) were reported as ‘not applicable’ and 21 (1.8%) with missing information. A total of 846 injuries were reported as sudden onset (73%), 195 gradual onset (17%), while 19 were unsure (1.7%), and for 99 injuries (8.5%), information was missing. Reinjuries constituted 9.9% of all injuries. Index injuries caused 23±41 days of time loss compared with 25±39 days for reinjuries (p=0.62).
The overall injury incidence was 5.1±2.2 injuries/1000 hours (3.8±1.3 injuries per month per team), with greater incidence during matches (19.2±8.6 injuries/1000 hours) than training (2.8±1.4 injuries/1000 hours, p<0.0001).
Injury patterns
Thigh and knee injuries represented the greatest injury incidence and burden (table 2). The two specific injuries causing the greatest burden were complete ACL ruptures (0.1±0.09 injuries per team per month) and hamstring strains (0.6±0.4 injuries per team per month) (table 2).
Injury pattern by body region, injury type and diagnosis
Sprinting was the most common injury mechanism for a total of 231 injuries (20%), followed by other (206; 18%), tackling (185; 16%), twisting/change of direction (169; 15%), kicking (148; 13%), jumping (86; 7%) and stretching (53; 5%).
Figure 1 represents a risk matrix based on the incidence and severity of the most commonly reported body parts by mode of onset.
Risk matrix depicting the relationship between incidence (number of injuries per 1000 hours) and severity (average number of days lost with 95% CIs) for each body region that were split into sudden and acute onset mechanism of injury. The darker the colour, the greater the burden; isobars depict a burden of 5, 10, 20 and 30 days per 1000 hours, respectively. Only body regions with an injury burden (average number of days lost per 1000 hours) of >1 and a minimum number of >3 cases are included in the figure.
Injury burden and severity
The total injury burden was 112±56 days/1000 hours. The injury burden for match injuries was greater than the burden resulting from training injuries (456±336 days/1000 hours vs 54±34 days/1000 hours, p<0.0001). The majority of injuries was moderate (n=515; 44.4%) followed by minor (n=254; 21.9%), severe (n=229; 19.8%) and mild (n=161; 13.9%). Injury severity categories by body region are detailed in online supplemental appendix 1.
Supplemental material
Illnesses
In total, 175 illnesses were registered, which corresponds to an incidence of 0.9±0.9 illnesses/1000 hours of exposure (0.5±0.8 illnesses per month per team). The illness burden was 2.5±2.4 days of absence/1000 hours, corresponding to 1.4±2.9 days of time loss per month per team. The most commonly affected systems were the upper respiratory tract (n=23, 42.6%) and gastrointestinal tract (n=16, 29.6%).
Discussion
The principal finding of this study was that while incidence of injuries in this group of professional Asian players was low compared with previous studies from Europe, the burden of hamstring and ACL injuries was high.
Injury risk
The injury incidence observed during training and match play in the current study is consistent with a previous study performed at the club level in Qatar,12 both using the same consensus statement methodology. However, a prospective study in Iran reported a much greater total time-loss injury incidence.10 This may result from differences in the approach to data collection.
Despite the low injury incidence of the Asian clubs compared with UEFA clubs18 19 (5.1 vs 7.7–8.0 injuries/1000 hours, respectively, 65%), the injury burden observed in the present study was only slightly lower than the European clubs with 112 versus 130 days/1000 hours (86%), respectively.19 This apparent discrepancy results from greater injury severity in the AFC clubs. It should also be noted that the difference in injury burden was limited to match injuries (37 vs 53 days/1000 hours for the AFC vs UEFA, respectively); the training injury burden did not differ (455 vs 456 days/1000 hours, respectively).20 This suggests a more substantial impact from match injuries in European clubs, compared with their Asian counterparts. This greater burden from match injuries in the UEFA Professional Club Injury Study may be related to greater player match exposure in Europe (41 hours per season)18 compared with Asia (30 hours) or, simply, by larger squads of high-quality players, allowing clubs to rehab players longer after injury.
Injury types and locations
Figure 1 displays the relationship between incidence and severity, it is likely the most helpful tool to guide risk management and determine priorities for injury prevention.21 This risk matrix documents that ACL injuries and hamstring strains contribute the by far greatest, but similar, injury burden (29.8 and 17.5 days/1000 hours, respectively) although hamstring strains were more common and ACL tears were the most severe, as expected. These results are in accordance with other studies carried out at professional level using the consensus study methodology.18 22 Nevertheless, despite a similar median layoff time for ACL tear among Asian and the European players (197 vs 201 days, respectively), the ACL rupture rate (0.14 injuries/1000 hours) in the present study was two times that of the UEFA study (0.06 injuries/1000 hours)23 and our previous study in the Qatar Stars League (0.08 injuries/1000 hours).24 These differences could be due to the already described differences in injury patterns by geographical regions.25 26 Football teams playing in colder regions in Europe have higher injury rates compared with teams playing in warmer regions; conversely, the ACL injury incidence was lower in northern European teams, especially for non-contact ACL injuries.27 Factors such as pitch surfaces (natural/synthetic, type of grass),26 skill level of the players, the load from participation in the Asian Champions League and long travel distances could also impact injury patterns. In that regard, Asia is a vast continent covering 11 time zones, adding jet lag to long-haul travel fatigue when participating in the AFC competition. Even variation in temperatures from one match to the next over few days—playing in the snow on a Sunday to the heat on Wednesday, then back to cold, could impact injuries and illness patterns. We do not know whether these factors can explain some of the differences observed, but they certainly deserve future investigations.
Even though the median lay-off per ankle injury was similar in the UEFA and the AFC studies (7 vs 8 days, respectively), both ankle injuries and ankle sprain burdens were higher in Europe compared with Asia with 16.3 versus 10.6 and 9.6 versus 7.4 days /1000 hours, respectively.27 One potential reason is that the physical demand in Asian football is less than in Europe and may have facilitated earlier return to play. For instance, the Australian football league28 reported high speed runs and sprints match loads as 50% less than the English Premier League.29
Hip/groin injuries represented 10.3% of the total number of injuries, which is similar to the 12.5% in the UEFA study.22 However, the total incidence of hip/groin injuries was two times as high as in Europe (1.1 vs 0.5 injuries/1000 hours, respectively). It can be speculated that the high risk of hip/groin injuries reflects by the higher mean training-to-match ratio in Asia compared with the Europe18: 7.5 versus 5.2 (calculated from the data in the UEFA study,18 respectively). In this regard, a recent study has shown that the amount of sprinting during football matches had a harmful association with muscle injury occurrence.30
Further investigations are needed to inform the differences in injury patterns between the only available continental surveillance studies, the high ACL and hamstring injury incidence and burden in Asia and the greater impact of ankle and hip/groin injuries in Europe.
Reinjuries
The proportion of reinjuries was lower than that of the UEFA study (9.9% vs 12%, respectively).18 The mean absence following reinjuries was the same, 25 days versus 23 days for index injuries, and similar to Europe where reinjuries caused an average of 24 days of absence.18 In contrast, studies from Scandinavian professional football, using the same consensus statement definition as the AFC and the UEFA study, reported reinjury rates ranging from 22% to 30%.31–33 One possible explanation might be the difference in competitive level. One might speculate that top-level clubs in the European and Asian Champions League have more extensive medical support, providing for more personalised rehabilitation of injured players. This deserves more attention, as it is well known that the pressure to perform is probably greater in professional than subprofessional football, potentially increasing the risks taken by the triad, player, physician and coach, when deciding on return to play, especially when it comes to key players.
Sudden versus gradual onset
Sudden onset injuries represented 73% of all recorded injuries. The risk matrix clearly shows that the injury burden (days lost × incidence) from sudden onset was greater than gradual onset injuries, especially for thigh and knee injuries. Unfortunately, no similar representation of burden data is available in the literature allowing a comparison. We suggest that future studies present such data to help practitioners better appraise the burden of different types of injuries in their teams/leagues.14
Gradual onset injuries represented 16.8% of overall injuries, clearly representing a lower proportion than the UEFA study (28%).18 This was accompanied by a higher mean training-to-match ratio in the present study compared with the UEFA study.18 We cannot explain the lower proportion of gradual onset injuries, although the training-to-match ratio was high. Whether there are cultural differences between Europe and Asia in reporting physical complaints is not known. Nevertheless, using a time-loss definition of injuries only detects the tip of the iceberg for overuse injuries.34 35 This substantial underestimation has been clearly shown by Harøy et al 36 using the Oslo Sports Trauma Research Center Questionnaire to monitor groin problems in athletes.
Illnesses
The mean total illness incidence for all teams was 0.9±0.9 illness/1000 hours. Expressed differently, a team may expect one time-loss illness every 2 months (0.5±0.8 illnesses per month per team). Also, the burden from illness was limited, with an average of 1.4±2.9 days lost per month per team. This is similar to previous findings in European football37 and therefore we can conclude that, despite some specific differences in illness epidemiology between the European and Asian continents,38 illness incidence in professional footballers is low in both Asia and Europe. In accordance with the findings in the UEFA study, the most commonly affected systems were the upper respiratory and gastrointestinal tracts. Therefore, it seems that even when mild illnesses occur, allowing players to continue to train and/or compete as reported by the team physicians, the major cause for stopping football activity because of illness seems to be consistent across the Asian and European continents.
Study limitations
The present data could be subjected to some underreporting. Data were assessed on a monthly basis with regards to completeness, correctness, accuracy, plausibility and compliance with the definitions and categories, and the club doctor/physiotherapist was contacted to resolve any deficiencies. Most clubs adhered with this, but in some cases, the reporting frequency was lower; the longest period between two consecutive data submissions was 3 months. Cultural diversity and other factors represented challenges in engaging the same teams over three consecutive seasons. Geographical distance and language could potentially impact compliance with the present study protocol and resulting outcomes negatively. However, the study group has taken several measures to ensure data completeness and consistency: (1) providing a manual of the study (in English), with a detailed description of the study methodology, (2) an initial meeting with all club representatives prior to launching the study, explaining the study guidelines and procedures (in person and/or on-line meetings in English) and two subsequent group meetings for specific workshops, reinforcing collaboration and engagement with the study and (3) offering continuing technical support to the participating clubs, responding to any query within 24 hours. Also, the specifically designed digital tools provided, potentially with direct positive impact on daily practice, may have helped compliance.39 40 This has resulted in a relatively limited proportion of missing data—with negligible effect on the outcomes.
Factors such as manager changes could have impacted injury rates from season to season and within seasons. Finally, although this study is directly comparable to the UEFA study, readers should consider the differences in performance level and player quality between confederations when interpreting the results.
Conclusion
The present study shows that the injury and illness patterns at the professional Asian football level were characterised by an overall injury incidence similar to that reported from Europe, but with a high rate of ACL ruptures and hamstring injuries. This deserves further investigations and tailoring injury prevention programmes could be considered.
What are the new findings?
The injury and illness patterns at the professional Asian football level are consistent with the only other football confederation that has established systematic injury and illness surveillance to date (UEFA).
Professional Asian football is characterised by a high rate of ACL ruptures and hamstring injury.
Player match exposure in Asia (30 hours) is lower than Europe (41 hours).
How might it impact on clinical practice in the future?
These data will provide clinicians with prognostic information about the overall injury incidence and patterns in professional Asian football.
Data availability statement
No data are available. Data are confidential and not available upon request.
Ethics statements
Patient consent for publication
Ethics approval
This study protocol was approved and supported by the Medical Committee of the AFC and ethics approval was obtained from the Anti-Doping Lab Qatar Ethical Committee (E-2016000157).
Acknowledgments
The authors would like to express our gratitude to all the AFC team contact persons (club doctors and/or physiotherapists) for their engagement in the study. Our thanks also go to Dr Scott Gillogly, former CMO of Aspetar and his team, Dr Katharina Grimm and Dr Angus Burnett, who have supported the present study. We also thank Edna Carpio from the AFC Sports Medicine Department and Rima Tabanji from the Aspetar Sports Injury and Illness Prevention Program (ASPREV) for their help with organising the study.
References
Supplementary materials
Supplementary Data
This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.
Footnotes
Twitter @JanEkstrand, @ProfChamari
Contributors MT, CE and KC lead the project, were responsible for data collection, project management and for writing the initial version of the manuscript. MT was responsible for the statistical analysis. MT, CE, RB and KC were involved in the analysis and interpretation of the results. All authors have revised the manuscript critically for important intellectual content and approved the final version. In doing so, we agree to be accountable for all aspects of the work.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.
Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.