For the first time, this study provides data on the impact of the coronavirus pandemic and its associated interruption of training and competition on the injury incidence in professional football. This study compared prospectively collected injury data on the first men’s professional football league in Germany (Bundesliga) obtained from media analysis before and after the lockdown. The nine remaining matches of the 2019–2020 season final played within 6 weeks in May and June 2020 showed significantly fewer injuries than the nine matches of the 2018–2019 season final. The unexpected break in the 2019–2020 season and the rapid return to competition of the first professional sports league worldwide , which was controversially discussed before and after the restart, had no negative effect on injury occurrence.
With regard to injury prevention, the time between the restart of team training and matches after the ease of the very strict lockdown restrictions was probably a highly relevant period. None of the 18 Bundesliga teams was allowed to participate in friendly matches with other teams—usually an integral part of pre-seasonal preparation—before the restart of the season. Therefore, the teams had less opportunity of preparing themselves for highly competitive, football-specific, on-field movements or tactics. In terms of injury prevention, practising football-specific movements in competition-like circumstances is deemed crucial [1, 21]. A further aspect to consider is the content of training sessions, and the very short time span between resuming full team training and the start of the nine remaining matches presented a challenge in this regard. According to previous research, short-term changes such as a sudden move to another club, replacement of the coach or relegation to a higher league  influence training contents; such influences or just an abrupt start of the season may increase the risk of sustaining some types of football injuries. However, this study does not provide indications for any significant increase in injury incidences after the lockdown and the rapid return to competition.
The unexpected reduction in the injury incidence gives rise to controversial discussions and speculations about the underlying reasons. The players did not completely stop training during the lockdown. Training continued on an individual basis and consisted of endurance, strength and conditioning elements. For some players, the lockdown period may have represented a chance to recover from a long-lasting overuse injury or to cure a minor injury, which may have subsequently led to a major injury. Also, preventive training may have been conducted more frequently and on a more individualised basis than in regular seasons. The decreased injury rate after the Bundesliga restart may have been also based on improved healing processes during the lockdown. This unique situation possibly gave injured tissue adequate time to heal and players the time for structured rehabilitation before their return-to-play [10, 25, 26], which is usually not possible amidst a professional football season. Therefore, previously injured players were able to start playing football with less overuse complaints. Such minor complaints are a risk factor for sustaining more severe injuries to the knees or the ankles , and the absence of such injuries led to high match availability after the restart and ultimately maybe to fitter players.
This prospective study showed that the injury incidence after the restart after a much longer mid-season absence from playing football than usual was not higher than in the control periods, i.e. the first few weeks of the 2019–2020 season or the weeks directly before the lockdown. This finding also holds true with regard to muscle injuries that tend to occur more often in case of insufficient fitness or during a tight match schedule [8, 9] such as after the restart with 9 matches in 6 weeks. The somewhat unprepared rapid restart of match play, which depended upon political regulations, made it impossible for the clubs to follow traditional seasonal preparation concepts. Because such short-term increases in psychological and physical stress are well-known factors for sustaining injuries, especially knee injuries [2, 23], this aspect was a major concern before and after the restart. Additionally, the control period of the final weeks of the previous 2018–2019 season showed a higher injury incidence than the 2019–2020 season final after the lockdown. Whether the lower injury incidence in head injuries after the restart was influenced by a lower rate of physical contacts due to a lower frequency of contact duels is speculative and cannot be answered finally. This question should be answered in further investigations based on the video analysis of players’ behaviour and the frequency of contacts or heading duels.
The findings of this study may have an important impact on injury primary, secondary and tertiary prevention strategies in professional football [10, 11]. In comparison to recent studies on injury epidemiology in national professional football in Germany [3, 6, 13, 22], this study shows a comparable injury prevalence with more than 80% of the players and with comparably high injury incidence, especially on muscle and tendon injuries and on joint injuries of the lower extremities [4, 16, 20]. Despite increasing evidence of the efficacy of injury prevention measures and an increasing number of prevention strategies, longitudinal injury surveillance does not show any significant decrease in injury incidences . One reason may be the slow transfer from research into practical routine, which is traditionally difficult to achieve in professional football  because time pressure and expectations on short-term success are more important than fundamental injury prevention requiring long-term commitment. Another aspect may be the lack of time for structured injury prevention measures and regeneration during regular seasons because of the tight match schedule or the pressure on teams, coaches and players. However, this study on the restart period after the corona lockdown may indicate that injury incidence can be reduced when football teams take the time for individual preparation strategies as part of primary injury prevention. Other debatable reasons for the lower injury incidence after the restart may be the increase in the number of substitutions from 3 to 5 per match implemented by the FIFA and a generally higher focus of football players on their professional activities because of the restrictions on social life during the pandemic.
One potential limitation of this investigation is the reporting of injuries at the restart of the Bundesliga. The period after the lockdown differed to other seasonal periods: on the one hand, sports journalists were not allowed to follow their routine of being present at the clubs’ training grounds on a daily basis. This absence may be a reason for a potential underreporting of injuries. On the other hand, professional football was the first professional sports to restart with political legitimation after the lockdown. Other professional sports were not allowed to restart, neither was the entertainment industry. This political legitimation increased the attention on Bundesliga football and subsequently on football injuries in the public and sports media and their daily reports about football clubs alike. Media-based injury registry may, therefore, be affected by the overreporting of injuries.
Media-based studies are associated with general limitations that may lead to imprecise injury statistics and debatable conclusions. However, injury reports provided by football clubs during the lockdown may also be invalid due to possible underreporting, but this lack is unlikely to have any impact because of the probably low number of injuries occurred during the lockdown. The current study population was evaluated by means of advanced national, regional and local media analyses with strict exclusion criteria. This media-based analysis was prospectively conducted over 5 years [22, 27], which improved injury recording because the most detailed information can be obtained immediately after the occurrence of an injury. Therefore, the strengths of our approach are the verification of the injury diagnosis by different public sources, its precisely defined methodology and our differentiation between valid and weak information (Table 1). These methods provide media-based data with high validity and represent a huge difference to other uncritically sampled and assessed media data about injuries also available in the scientific literature.
Because of the unexpected onset of the coronavirus pandemic, no other prospective study protocol could be implemented to compare injury occurrence before and after the lockdown. As the injury registration of this study was prospectively conducted for five seasons with constant methodology, the resulting injury incidence after the restart should be comparable to other periods of the present and previous seasons alike. The direct comparison of the 2019–2020 season final after the restart to the preceding 2018–2019 season final seems reasonable. Because of the lockdown break during the 2019–20 season, these two-time intervals differ significantly in physical preparation and also in a higher match frequency in a shorter period of time. Thus, the differences in the preparation before the last nine matches of the 2019–2020 season and the matches of the 2018–2019 season could be a reason for the significantly lower injury incidence. However, because the study period after the restart followed a longer break, we additionally used the periods after the summer and winter break as controls. Further investigations should follow based on our data. Especially the aspects fitness and training programmes during and after the lockdown may be important fields of research in the future.