Female soccer is growing yearly and the number of female players is increasing. Thus, several studies have now quantified the physical demands of training and matches. For example, during a match, female soccer players cover a total distance of 5.3–11 km, including 420–766 m high-speed running, and also perform 158–423 acceleration, 430 deceleration, and 14–40 sprints [1], [2], [3], [4], [5], [6] Female players also perform locomotor activities more than 1300 times every ∼4 seconds during the match [1]. These results show that numerous factors might affect match performance in female soccer. In particular, effective match performance in female soccer depends on increasing players’ ability to maintain intense levels of activity and limit fatigue simultaneously, which means that players require well-developed endurance.

Running-based training is generally used for the development of endurance performance. For example, Helgerud et al. [7] stated that running-based interval training performed four times 4 min at 90–95% of maximal heart rate, two days per week for eight weeks which caused an improvement of approximately 10.7% VO2max values of the players. In addition, game-based training is another training method used for endurance performance development. Small-sided games (SSG) are among the most used game-based training methods to improve endurance performance in soccer. For example, Radziminski et al. [8] revealed that 3 vs.3 with a neutral player or 3 vs. 3 SSGs performed five times 4 min, two days per week for eight weeks, which caused an improvement of approximately 8.0% VO2max values of the players.

SSGs are match-specific training that includes game-specific movements. However, it is more difficult to reach the targeted exercise intensity to develop endurance performance in players during SSGs than in running-based training. To reach the targeted exercise intensity during SSGs, coaches should consider various modifications such as the number of players, game format, bout duration, pitch size, team formations, game rules, and playing with or without a goalkeeper [9], [10], [11], [12], [13]. For example, Halouani et al. [11] showed that the game rule change (stop-ball vs. small-goal rules) in the 2 vs. 2, 3 vs. 3, and 4 vs. 4 SSGs affects the soccer players’ physiological responses.

While the previous studies on SSGs generally examined male soccer players’ internal and external load performance, few studies examine the SSG performance of female soccer players [14], [15], [16], [17]. In one of these studies, Mara et al. [14] stated that more appropriate physical and physiological responses were observed for developing endurance performance in SSGs (4 vs. 4 and 5 vs. 5) compared to medium (6 vs. 6 and 7 vs. 7) and large-size (8 vs. 8 and 9 vs. 9) games in female soccer players. In another study, López-Fernández et al. [17] stated that 4 vs. 4 games played at 400 m2 pitch size caused more internal load response than 200 m2 and 800 m2 pitch size in female soccer players. However, there is no study examining the effect of keeping the size of the playing field per player constant and increasing the number of players on the internal and external load responses in young female soccer players. Consequently, this study aimed to compare the acute physiological responses and covered distances at different running speed zones to 2 vs. 2, 3 vs. 3, and 4 vs. 4 SSGs in female soccer players. It was hypothesized that keeping pitch size per player constant at 100 m2 and increasing the number of players would change internal and external responses during the SSGs.