## 4 Discussion
Additionally, PL values observed in this study, which are associated with the intensity of the game (namely accelerations and decelerations), supported the suggestion that smaller formats seem better for increasing exercise intensity and the number of accelerations and decelerations by soccer players [19], nonetheless, the use of GK and goal objective seems to be a good strategy if the purpose is to decrease the intensity during the SSG since often in SSGs these factors are not associated with the necessity of very intense actions such as changes in direction, sprints, accelerations and decelerations. Furthermore, it has been suggested in the literature that fatigue mechanisms affect decision-making skills [34] and the higher physiological strain typically observed in SSGs on larger pitches is likely to be due to the possibility of making longer offensive and defensive runs [35]. This methodology could add to a congested training schedule, causing excessive physical stress which, in turn, can result in burnout and/or injury in players [36]. Addressing this topic, [PERSON] et al. [37] stressed that the 3 vs. 3 format is perhaps less structured than the 6 vs. 6 format in terms of the tactical approach to the match, allowing each player to assume more than only one role (e.g., defender or attacker) and to participate in all moments of the match. This indication was in line with previous findings that revealed a tendency of players to be more conservative in the 6 vs. 6 and aggressive in the 3 vs. 3 format, probably based on the space management and the fewer interactions between teammates [38]. In a comparative study of two intermittent regimens (4 vs. 4 min and 2 vs. 8 min), it was found that both the first four-minute periods of exertion had greater values of total distance [39], thus suggesting that the time of recovery was not enough to enable players to achieve similar levels as they did in the absence of fatigue. In the 4 vs. 4 format tested by [PERSON] et al. [40] a progressive decrease in total distance was found from the first to the fourth set (4 \(\times\) 4 min with 3 min passive recovery, 30 \(\times\) 20 m, SSG-P with the help of four support players--out of the playing area and not allowed to perform more than 1 touch for each involvement), and was justified by the possible accumulation of potassium in the muscle interstitium and the subsequent depolarization of the muscle membrane potential, which reduced the force development during these intensity drills [3]. Our results indicate that in sub-elite U-23 soccer players, the 3 min exercise with 3 min time recovery provides the possibility of balance in fatigue throughout the SSGs, since contrary to the study of [PERSON] et al. [40], also with international soccer players, in our study, the distance covered always increased from SSG1 to SSG4, except when involving GK participation. We should, however, underline that based on our study results and comparing with previous studies, fatigue accumulation and significant influence in internal and external training load seem to be related to more than one stimulus strategy, the time of practice vs. rest interval time, pitch size and also GK participation. It was previously indicated that it is likely that SSGs are associated with the limitation of producing high-speed activities [41], because they are played on smaller pitch areas where the players do not have enough space to reach their maximal sprinting speed [42,43]. Our results support this hypothesis since the five performed SSGs were predominantly associated with distances covered at 0-6, 6-12 and 12-18 km\(\cdot\)h\({}^{-1}\) running speeds, when compared to 18-21 km\(\cdot\)h\({}^{-1}\), and fundamentally 21-24 km\(\cdot\)h\({}^{-1}\) and 24-50 km\(\cdot\)h\({}^{-1}\). The large number of significant changes throughout the SSGs in running speed, as well as distance covered and PL, were more evident when compared to HR, which means that external load changes differ comparatively to internal load in sub-elite soccer players.