Introduction

The physical performance and tactical performance in football matches have been well documented recently, with numerous studies assessing players’ performance through various methods. Modern technology has also provided researchers and practitioners with increasingly detailed opportunities to analyze physical functions. With the increase in data capacity, there are more indicators available for evaluating football match performance, allowing for a more nuanced and accurate assessment of players’ performance.

The Chinese Super League (CSL) is the premier professional soccer league in mainland China. The Chinese Super League (CSL) is the top professional soccer league in mainland China. The CSL is the highest level of professional soccer league in mainland China, with its lower leagues being the Chinese Football Association First Division, the Chinese Football Association Second Division and the Chinese Football Association Member Association Championship. The CSL is one of the most competitive soccer leagues in Asia with the highest average attendance. COVID-19 emerged at the end of 2019 and rapidly spread worldwide, presenting unprecedented challenges to social life1. In response, many countries implemented restrictive measures to protect their citizens, the main ones being limitations on social distances and closure of places, including stadiums where large crowds gather, which forced the suspension of sporting events at all levels. This particular off-season has exerted multifaceted impacts on athletes across various sports disciplines, with these effects being far from uniform. Damir2 investigated the influence of COVID-19-induced lockdowns on the competitive performance of athletes in Croatia’s top-tier leagues. The pandemic-imposed restrictions curtailed the conventional training modalities available to athletes, which resumed to normalcy two months following the suspension of competitions. The study scrutinized the variations in match performance across different positions to assess the impact of the COVID-19 lockdown on the physical performance of players. The findings revealed a decline in physical performance across all positions, including Central Defenders (CD), Full-Backs (FB), Midfielders (MF), and Forwards (FW), compared to the pre-lockdown period. The author attributes this decline to the more congested fixture schedule post-resumption of official matches, which has constrained the athletes’ physical recovery. It has been shown that several top national competitions have experienced similar declines in athletes’ performance after the resumption of competition due to COVID blockades3,4,5. The results of another study on the impact of COVID on player performance in the Bundesliga were different6. The reference indicators mainly included the total running distance, sprint distance, high-intensity running distance and injury rate. And the results showed that in terms of the team as a whole, before and after the COVID suspension, the physical performance and the injury incidence of players remained basically the same. Injury incidence remained basically unchanged, which was also different from the results of athletes’ performance before and after previous Bundesliga suspensions. The authors suggest that the explanation for this phenomenon may be due to the changes in the rules for resumption of matches after the covid suspension, including the increase of substitution quotas from three to five, and in addition, the COVID also provided athletes with additional recovery time, thus offsetting the effects of COVID on the athletes. The impact of the special training pattern on the athletes during the period.

Many countries implemented restrictive measures to protect their citizens, including the Chinese Football Association (CFA), which was compelled to announce the suspension of the CSL. After a lengthy suspension, the CSL has been relaunched with rules and schedule changed. In the 2020 and 2021 seasons, the league’s format was shortened, and the rule was changed from the traditional home-and-away matches with live spectators to a centralized competition format without spectators.

And the changes in the rule due to COVID-19 affected match performance and results. Wing et al. studied the effect of the sudden change in rule (55% reduction in season length and 20% reduction in halftime) caused by COVID-19 on players’ running performance and injury rate, and the results revealed that the change in rule significantly decreased the running distance and increased the injury rate7. Before the outbreak of COVID-19, the Chinese Super League (CSL) followed a home-and-away match format. However, in 2020, the league implemented significant changes to this rule to ensure the continuation of the season. Considering international competitions and public health issues, the league could not complete 240 matches in 30 rounds as it did in the 2018–2019 season, so for the 2021–2022 season, a centralized home-and-away rule was implemented. According to the regulations, two cities were set as venues, and eight of the 16 teams were gathered in one city as their home and the remaining eight teams in the other city. Although the home and away concept was retained, the actual home match location was not the natural home of the team. Therefore, this study aimed to investigate the effect on rule change in CSL match performance.

This study analyzed the influence of COVID-19 on the performance of CSL matches by comparing the technical and physical performance of players between pre-COVID-19 (2018 and 2019 seasons) and post-COVID-19 (2020 and 2021 seasons).

Materials and methods

Samples

The data of this study collected from the 480 matches of CSL in 2018–2019 season and the data of match performance in 2020–2021 season. AMISCO provides us with the original competition data report. Previous studies have elaborated the workflow, accuracy, validity and reliability of the website in detail8,9. Our local university ethics committee approved the study, we also made a confirmation on CFA website (thecfa.cn) to ensure the reliability of the main data sources of home and away games related to the game results (home wins, draws, losses, goals and goals conceded).

Experimental methods

We collected and analyzed data before and after COVID-19, conducting analyses on them. Teams’ data were collected by AMISCO tracking system, all data collection schemes are implemented in accordance with relevant guidelines and regulations, and have been approved by the Chinese Super League Committee. The permission authorizes the data to be used only for academic research, and all athletes know and agree to this data collection. The study variables were selected as indicators related to the technical and physical performance in each match10,11,12,13. We categorized the indicators in the report into four categories according to the official data provided by Amisco: goal shot-related variables, offensive organization-related variables, defensive-related variables, and physical variable. In this study, we selected 16 technical indicators and 11 physical indicators as dependent variables, including goal scoring, passing and organizing, defending, physical variables. The operational definitions of these variables are presented in Table 1.

  • Shot an attempt to score a goal, made with any (legal) part of the body, either on or off target

  • Shot on target an attempt to goal which required intervention to stop it going in or resulted in a goal shot which would go in without being diverted

  • Pass an intentional played ball from one player to another

  • Pass accuracy (%) successful passes as a proportion of the total passes

  • Forward pass an intentional played ball from one player to another who is located closer to the opponents’ goal

  • Forward pass accuracy (%) successful forward passes as a proportion of the total forward passes

  • Cross a pass from the wide area of the pitch between the edge of the goal box and the sideline that travels into a more central area in the penalty box

  • Cross accuracy (%) successful crosses as a proportion of total crosses

  • Corner ball goes out of play for a corner kick

  • Offside being caught in an offside position, resulting in a free kick to the opposing team

  • Opponent 35 m entry number of times when the ball (possessed by the attacking team) enters the 35 m area (final third of the field) of the opponent’s half of the pitch; each time a player has made an individual possession in the final third of the field

  • Number of challenges number of times a player scrambles for control of the soccer ball

  • Number of aerial challenges aerial duels

  • Number of ground challenges ground duels

  • Foul committed any infringement that is penalized as foul play by a referee

  • Yellow card shown to a player by the referee for reasons of foul play, persistent infringement, handball, dangerous play, time wasting, etc.

  • Total distance (m) distance covered in a match by all the players of a team

  • Average speed (km/h) average running speed of players

  • Average speed in possession (km/h) average running speed of a team when receiving the ball from the wing without any obvious interruption

  • Average speed in opponent’s possession (km/h) average running speed of the opposing team when it receives the ball from the side without any noticeable interruption

  • Number of sprints number of sprints in a match by all the players of a team

  • Distance of sprint (m) distance covered at a speed of over 25.1 km/h in a match by all the players of a team

  • Average length of single sprint (m) average distance covered at a speed of over 25.1 km/h in a match by all the players of a team

  • Number of high-speed running number of high-speed-running in a match by all the players of a team

  • Distance of high-speed running (m) distance covered at a speed of over 19.8–25.1 km/h in a match by all the players of a team

  • Average length of single high-speed running (m) average distance covered at a speed of 19.8–25.1 km/h in a match by all the players of a team

  • Distance of moderate-speed running (m) distance covered at a speed of 14.4–19.7 km/h in a match by all the players of a team

Table 1 Parameters and operational definitions related to selected technical and physical properties.
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Statistical analysis

Uncertainty in the true effects of the predictors was evaluated using a non-clinical magnitude-based inference method, as implemented in the spreadsheet accompanying the package of materials for generalized mixed modelling with SAS Studio. The estimated magnitudes and their confidence limits were expressed in standardized units and assessed qualitatively with the following scale: < 0.2 trivial, 0.2–0.6 small, 0.6–1.2 moderate, 1.2–2.0 large, and > 2.0 very large. The effects were deemed clear if the 90% confidence interval did not simultaneously include positive and negative substantial values. Clear effects were reported with a qualitative likelihood that the true effect was either substantial or trivial (whichever probability was greater) using the following scale: < 0.5% most unlikely, 0.5–5% very unlikely, 5–25% implausible, 25–75% possibly, 75–95% likely, 95–99.5% very likely, and > 99.5% most likely.

Results

Descriptive statistics

The descriptive statistics for all variables are shown in Table 2.

Table 2 Match variables before and after COVID-19.
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Effect of COVID-19 on match performance

Figure 1 shows the effect of COVID-19 on the variables related to match performance. In terms of the technical indicators and among the variables related to offensive organization, pass accuracy (p < 0.001, ES = 0.23) and forward pass accuracy (p < 0.001, ES = 0.21) significantly decreased after COVID-19, while the number of crosses (p < 0.001, ES =  − 0.24) increased. Among the defensive-related variables, the number of challenges (p < 0.001, ES =  − 0.50) and ground challenges (p < 0.001, ES =  − 0.64) increased.

Fig. 1
figure 1

Effects of COVID-19 variables on the technical and physical performance of Chinese Soccer Super League (CSL) teams. Bars represent the 90% confidence intervals. Dotted lines represent the smallest worthwhile difference. Asterisks indicate the likelihood for the magnitude of the true effect as follows: * possible; ** likely; *** very likely; **** most likely. Asterisks located in the area between − 0.2 and 0.2 denote trivial differences.

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Among the physical performance indicators, total distance (p < 0.001, ES = 0.34), average speed (p < 0.001, ES = 0.35), number of sprints (p < 0.001, ES = 0.39), and sprint distance (p < 0.001, ES = 0.32) significantly decreased, while the average distance of single high-speed running increased (p < 0.001, ES =  − 0.66).

Discussion

The main purpose of the study was to understand the effect of changes in the rule of CSL and spectators on match performance before and after COVID-19. COVID-19 has had a negative impact on both the technical and physical performance of players, with physical performance being more significantly affected by COVID-19.

In our study, we found a significant decline in physical performance, including total distance, average speed, number of sprints, and sprint distance, our results are consistent with those of Radzimiński et al. and de Souza et al.14,15, who investigated the changes in the physical performance of professional soccer players in the Spanish First Division, German First Division, and Polish competitions after the COVID-19 blockade and found a clear trend of decreased physical performance, such as lower total distance, average speed, number of sprints, and distance of sprint. This may be because the start time of CSL was delayed for nearly 16 weeks due to the influence of COVID-19, which led to a crowded schedule and had a negative impact on the players’ performance. We suspect that this negative influence comes from the crowded schedule, which makes players lack enough physical recovery time, research has supported our hypothesis, Jamardo et al.16 studied the effect of different recovery times on physical performance among 4496 professional soccer players in different positions in the German Soccer League in the 2011–2012 season. The results showed that players with less than four days of recovery time showed a decrease in total distance, number of high-speed running, and sprint, while players in the center-back, wing-back, and midfield positions ran less total distance. Conversely, teams with less than three days of recovery time showed a decrease in total distance by center backs and wing backs compared with those with less than four days of recovery time. Thus, they concluded that physical performance was negatively affected when the recovery time was less than four days due to residual fatigue from the previous match. Folgado et al.17 investigated a professional football team’s physical and tactical performance during crowded and non-crowded schedules, this study analyzes the six home games of a British professional football team in the competitive season (the interval between three games and the previous game is three days, and the interval between three games and the previous game is six days or more). They found that tactical performance was negatively affected by congested matches due to increased fatigue and lack a of training time in the short term. Furthermore, during the COVID-19, all players were arranged in a specific area under simple training conditions. The volume of team pre-match training decreased, since there are no warm-up games before official matches, it is difficult for football players to prepare adequately.

Another possible explanation is that the lack of cheering from fans during matches led to a decline in enthusiasm for the match. Home advantage (HA) was first introduced by Koppet18 in 1972, has long been recognized in sports. In 1992, Courneya et al.19 found that the winning rate of the home team is over 50% when the home and away games are balanced. HA refers to the higher number of goals scored by the home team, attributed to various objective and subjective factors. These may include the support and motivation provided by the home audience, which can enhance the team’s performance on the field. Research by Terry et al.20 and Waters and Lovell has indicated that home players often exhibit higher self-esteem and self-efficacy. Additionally, numerous studies have identified factors such as rules, familiarity with the stadium environment, climate, referee bias, travel fatigue, and locality as significant contributors to HA21,22. For example, Pollard23 found that home teams performed worse in matches played on newly constructed soccer fields. However, the post-COVID-19 CSL adopted a centralized competition format where matches were not truly home or away games. This change meant that players competed in unfamiliar “home” environments. Such a shift is critical to team performance. Lago-Peñas et al.24 studied the impact of playing location (home vs. away) and team quality on 380 matches in the Spanish Men’s Professional League, revealing that the home team’s offensive metrics were significantly higher due to the advantages of familiarity and spectator presence.

It is worth noting that the average distance of single high-speed running increased compared with that pre-COVID-19, which is inconsistent with previous studies3,25,26, these research suggested that high-intensity running was comparable with that pre-COVID-19 or that the distance of high-speed running decreased. This change in our study might be due to the CSL’s adjustment in rules, increasing the number of substitutes from three to five, which helped maintain the players’ overall fitness levels. We believe that the reason for the increase in the average distance of single high-speed running after COVID-19 is that soccer is a complex team sport, players need to possess the ability to effectively combine skills, physical fitness, and tactics. Therefore, when players’ physical fitness declines, their acceleration decreases, and they cannot reach sprint speed. They must maintain the rhythm of the entire race so that the maximum effort can only reach a high-speed running speed. There is an increase in the average distance of single high-speed running, and this phenomenon provides evidence of a decrease in fitness in players’ match performance.

This study found a significant decrease in pass accuracy and forward pass accuracy. We believe that the possible explanation for this is that COVID-19 delayed the start of the CSL from March to July. And the geography of China is vast and complex. The temperature, humidity, and air quality were significantly different from those pre-COVID-19 due to the huge changes in the date of the CSL competition. As Benz27 showed in a comprehensive survey of home field advantage in American football, the study analyzed the drivers of changes in home field advantage from 2004 to 2023 across all levels of football leagues, showing that league level, changes in travel patterns, and the use of replay technology all work together to drive the formation of home field advantage and influence on-field player game performance. Benz28 also demonstrated the impact of COVID on home advantage in another study with a sample of data from 17 professional leagues analyzing changes in home advantage due to games played without fans, using yellow cards and goals scored as indicators of home advantage, and the results showed that both of these indicators of home advantage declined significantly overall after COVID, although the results were reversed in individual leagues. In conclusion while we all know the benefits of home advantage, it is not clear what home advantage encompasses in such complexity. This study is consistent with a previous study29, which showed a decrease in passing and organizing- related variables in the performance of European domestic leagues (German First Division, Spanish First Division, English Premier League, Portuguese Super League, and Italian First Division). Zhou et al.30 and Brocherie et al.31 found that situational factors, such as temperature and humidity, have important effects on players’ technical performance, with comfortable temperature environments promoting players’ soccer performance and higher or lower temperatures being unsuitable. Brocherie et al.31 investigated the influence of weather, ranking and HA on soccer results in the Gulf region, and determined the temperature difference as an important variable. In addition, when playing against non-Gulf Cooperation Council (GCC) opponents, every 1 unit increase in temperature difference increases the likelihood of a favorable outcome for a GCC team by 3 percent. Changes in temperature and humidity also had a significant impact on the performance of CSL players. Due to the impact of the epidemic, the traditional home and away system was canceled, and instead all matches were held in Dalian and Suzhou, which are a northern city and a southern city, respectively. The difference in temperature between the north and the south could not be ignored, and this had a significant impact on the foreign teams, and the team that suffered most from this was the Guangzhou team, with the majority of the team players being accustomed to the year-round hot and humid climate in Guangzhou, and being able to maintain their excellent game performance in the hot environment. Most of the players in the team are used to the hot and humid climate of Guangzhou all year round, and can maintain their excellent game performance in the hot environment, while they can’t show their advantages when the game venue is changed to Dalian to play against other teams, so they show a huge difference in their game performance before and after the epidemic, and after 2 years of unofficial home matches, Guangzhou was officially relegated to the first division of the Chinese League.

We generally think that excellent players who have the ability to participate, instruct, support and control the team to achieve the set goals and common vision can be called core players, the absence of core players from the team had a huge impact on the team’s match performance, with many players contracting COVID or having their movements restricted due to the policy, resulting in their inability to participate in matches. Many foreign aiders as core players have a leadership role in the team, and the game performance of other players will be driven. The travel restrictions caused foreign aiders to be unable to return to the team on time to participate in the match, which has a huge impact on the overall performance of the team. For example, the Guangzhou team’s foreign aid Paulinho, Talisca was stranded in Brazil and missed most of the Guangzhou team’s matches, which is also one of the reasons for Guangzhou’s overall performance decline and relegation. Psychological factors are also an important reason that affects players’ performance in competitions32,33. It was the first time for players to face a major health event and play without spectators, which would negatively affect athletes’ psychology. It will lead to mental fatigue of players, thus weakening the team’s physical strength, skills, decisions and tactics33.

The intensive CSL schedule caused by Covid-19 may also affect the match performance. Folgado et al.17 argued that tactical performance was negatively affected due to increased fatigue from congested matches and a lack of training time in the short term, while good tactics maximized technical play. Therefore, technical play will be negatively affected if teams lack the time to discuss tactics and summarize their experiences.

Another possible explanation is that COVID-19 has limited a team’s training conditions, resulting in players’ lack of training intensity and soccer-specific training, as well as friendly or warm-up matches before the official match. Players did not prepare a passing strategy during the possession of the ball, and ball possessions ended without realizing the pre-prepared tactical strategies, causing counterattacks from the opponent. Thus, players must make more challenges to recover ball possession during matches.

Our study created a unique natural experimental setting based on COVID-19. This environment features not only a lack of live spectator support and rule changes but also the effects of weather temperature, humidity differences, and the absence of travel fatigue and mental fatigue on match performance. However, the current discussion revolves more around the factors of spectator absence and rule changes; therefore, it is recommended to go further and examine the interaction of potential factors in match performance.

Conclusion

The technical and physical performance of the CSL was significantly affected by COVID-19, with a decrease in pass accuracy, forward pass accuracy, total distance, average speed, number of sprints, and distance of sprint, and an increase in the number of challenges, ground challenges, crosses, and average distance of single high-speed running. COVID has brought a more serious impact on the physical performance of CSL players. The coaching staff and managers should consider making different training programs for the players’ physical decline in special emergencies, to promote the recovery of CSL players’ physical fitness.

Limitations

Although this study presents new findings, we only studied observable outcomes, such as match performance, and we did not assess the psychological load of players after COVID-19. Therefore, there are still some limitations that should be considered in future studies.

This study does not reflect the difference of players’ performance in different positions. In fact, the players in each position have different physical qualities and skills, and the influence on COVID is also different. However, our statistical data lacks the data of individual players and only analyzes the changes of players’ performance at the team level, which limits the reference of formulating strategies. Therefore, future research needs to consider the differences of players’ performance in different positions. Ans in pursuit of more evidence, we suggest a comprehensive in-depth analysis using newer statistical methods and more variables in the future to contribute more to the development of the CSL.