## 4 Discussion  
To the best of our knowledge, this is the first study that has investigated the impact of new substitution regulations on the running activity of professional football players. The main results of this study were as follows: (1) the possibility of making up to five substitutions did not lead to advantages in improving the running performance of full-time players; (2) During the first half, regardless of the number of substitutions made, a greater running distance was covered in all the analyzed variables except for the DC \(>\) 25.2 km\(\cdot\)h\({}^{-1}\); (3) Despite the higher number of substitutions made in Group B (4.4 vs. 2.8) during the second half, no statistical differences in running performance were observed between groups. The findings of this article are in line with the previously published literature, in which no differences were found in total running distance relating to the number of substitutions [22; 25]. A study conducted during the Brazilian National Championships, comparing the 2019 season (with up to three substitutions per match) and the 2020 season (with up to five substitutions per match), found no significant differences in the absolute and relative running performance of female players when the number of substitutions was different [25]. These authors suggested that the number of substitutions made during matches did not affect the total distance covered, total sprint distance (\(>\)18 km\(\cdot\)h\({}^{-1}\)), or accelerations and decelerations \(>\) 3 m\(\cdot\)s\({}^{-2}\)[25]. Despite some differences in parameters, such as the sprint speed threshold used (\(>\)18 km\(\cdot\)h\({}^{-1}\) vs. \(>\)19.8 km\(\cdot\)h\({}^{-1}\)) and the different gender and level of the participants, their results were comparable to those of our study. Likewise, [PERSON] et al. [22] analyzed how the introduction of the five-substitution option affected football teams' running performance. Although they demonstrated that the five-substitution option allowed increased running performance at DC \(\geq\) 21.0 km\(\cdot\)h\({}^{-1}\) during the matches, it was the starting players who covered higher running distances at high speed  
Figure 2: Relative running activity during the second halves for each group (full-time players). Group A = players who play matches with \(\leq\)3 substitutions; Group B = players who play matches with 4 or 5 substitutions; TD = total distance covered; DC 19.8-25.2 km\(\cdot\)h\({}^{-1}\) = distance covered between 19.8 and 25.2 km\(\cdot\)h\({}^{-1}\); DC \(>\) 25.2 km\(\cdot\)h\({}^{-1}\) = distance covered above 25.2 km\(\cdot\)h\({}^{-1}\); DC \(>\) 19.8 km\(\cdot\)h\({}^{-1}\) = distance covered above \(>\)19.8 km\(\cdot\)h\({}^{-1}\); SPR = sprint distance covered; #SPR = number of sprints; #ACC = number of accelerations above 3 m\(\cdot\)s\({}^{-2}\); #DEC = number of decelerations above 3 m\(\cdot\)s\({}^{-2}\). during the matches. Although our study did not directly compare running metrics between substitutes and starters, an analysis of the relative running performance per minute played among full-time players in Group A and Group B revealed no differences in running performance. This suggests that starters, despite the higher number of substitutions and the different number of players involved, did not exhibit higher performance. Differences between these studies could be attributed to the limited sample size, the varying leagues and quality of players, and the specific speed metrics examined. Another study conducted by [PERSON] et al. (2022), which examined all teams across the 2019, 2020, and 2021 seasons in the J-League (Japan), showed that increasing the maximum number of substitutions from three to five resulted in a significant increase in total distance covered and number of sprints (\(>\)24 km\(\cdot\)h\({}^{-1}\)) during professional football matches ([PERSON] et al., 2019).