The purpose of this study was to explore the impact of removing HA on the performance of CSL teams. This research found that after moving from home stadium to neutral venues in 2020 season, CSL teams’ performance reduced, especially in goals scoring and offence performance indicators. Comparing with away matches in 2019 season, teams playing in the neutral venues in 2020 season showed similar performance with visiting teams.
The COVID-19 pandemic affected professional football leagues in almost all countries around the world in 2020. Thus, the competition system of CSL in the 2020 season was changed to a new system due to the pandemic and the games were played without fans in neutral venues. The change of the competition system enabled all teams in the 2020 season to avoid crowd support and long-distance travel and all teams were identically familiar to the stadiums. From the perspective of scientific research, this is a more ideal experimental environment without any HA effect. The results of research on the German Bundesliga, English Premier League, Spanish La Liga, French Primeira Liga, Italian Serie A and other leagues during the pandemic showed that the home teams’ shots and shots on target significantly reduced and the number of fouls and yellow cards increased significantly [16, 44,45,46,47,48,49,50]. These findings are consistent with the results of current study. Some studies found that the supporters’ shouts and cheers from the stands of the stadium can increase the home players’ testosterone level, so that the players would be more aggressive and motivated at home . The COVID-19 pandemic resulted in empty stadium play, perhaps due to lack of support from home fans in the stadium, the players' behaviours and actions related to goals scoring declined and the home team players’ desire for offensive performance decreased [31, 50]. Combining with the results of multiple linear regression, the results found that all teams of CSL were equivalent to playing “away games” in the 2020 season (in neutral venues).
The away teams pass accuracy (%) improved greatly in the neutral venues performed in the 2020 season in the CSL. This indicated the away teams had more time and space for making successful passes, which enhanced pass accuracy (%). Meanwhile, the away team's defensive indicators declined, for instance, tackles, clearances and block shots. This might be related to the home teams’ decline in offensive performance. Thus, if the home team's shots and shots on target decreased significantly, then the away team's block shots would probably fall as well. Before the pandemic, some studies found that when teams played at home, their number of passes and pass accuracy (%) were higher [25, 46]. After that, the game data in empty stadiums showed that lack of support from the fans did not cause the decrease of home teams' pass accuracy (%) [31, 50]. Perhaps in the empty stadiums, the players' mentality in the attack might be more stable and they would prefer to make safer and low- risk passes. At the same time, it may be related to the fact that coaches could provide more effective verbal guidance and feedback to players on the field in the quiet stadium, which would make players' behaviours more rational [31, 51].
Compared with the 2019 season, both home and away teams’ free kicks in the 2020 season were on the rise. Before the pandemic, Liu, García-de-Alcaraz  found that the teams were awarded less free kicks at home than away. Current study found that the number of free kicks given to the home team increased significantly in the 2020 season. However, there was no significant increase in the number of free kicks in the front field. This implied that home teams in neutral venues got more free kicks in their own half field. This could be probably caused by the opponent's offside, or it might be a result from the opponent's aggressive high press. At the same time, the away teams’ free kicks in the front field increased significantly in the neutral stadiums. This may be due to the rise of away teams’ offensive performance in the neutral stadiums, which caused more threats to the home teams and the home teams committed more fouls .
Previous study found that away teams of CSL were given more yellow cards , which was also consistent with the results of current study. In the 2020 season, when HA disappeared, the home teams' yellow cards increased by 0.48 per game. However, it is worth noting that the away teams’ yellow cards also increased slightly in the 2020 season. Leitner and Richlan  compared the leagues of eight European countries during the pandemic and found that in empty stadiums, the home teams’ yellow cards also increased significantly. The reasons for yellow cards could be criticism, unfair sportsmanship and fouls. After comparing the three categories of yellow cards, it is found that in empty stadiums, yellow cards awarded for fouls increased strongly for home teams by 26.2% but only slightly for away teams (+ 2.8%). A study of the Bundesliga discovered that the number of yellow cards without fans was significantly higher than when there were fans in the stadiums before the pandemic [49, 52,53,54]. This might be because the referees were under less pressure from the supporters in an empty stadium, they were more determined when giving a yellow card.
Last but not the least, current study did not find the contribution of long travel and familiarity to HA. In the 2020 season, teams were playing in one city and didn’t need to travel long distance. It was believed that long distance travel was significantly related to goals scored (r = − 0.08, p < 0.001), which means that teams scored fewer goals as they travel longer distance . This result is not consistent with current study because the away teams’ goals scored in the 2020 season CSL did not increase although they did not need to travel. Therefore, it seems that the travel fatigue in CSL had little influence on away teams’ goals scored. Moreover, regarding the relations between facility familiarity and HA, previous research found that “special” fields (larger, smaller or artificial turf) may bring very subtle advantage to home team . In current study, no evidence could be discovered to support that stadium familiarity would contribute to HA in CSL and familiarity remains a plausible but unsubstantiated factor affecting HA. In addition, this study has limitations in using transfermarket data to measure team strength, because players’ value is influenced by many factors, such as current age, future skill, value of previous years and so on. Thus, teams that appear to have more value may not actually have better team strength for the current season. In future research, models from other researchers could be considered to better solve this problem. This study sheds light on the HA from a new perspective, exploring the changes of technical and tactical indicators when the HA is completely removed, however, HA is a multifactorial phenomenon, various data and factors should be put into consideration, such as fitness data and the modification of football law (number of substitutions, yellow or red cards to the team officials and etc.). Further studies may seek to detect the influence of removing HA in different countries and collect more diversified data to analyse their relations to HA.