The individual aerobic generic fitness level limits the individual physical activity profile of every outfield female player during a soccer game [1]. Hence, aerobic generic fitness is a prerequisite for soccer-specific fitness [2]. Therefore, the supporting staffs of women's soccer squads need to be aware of the aerobic status of each player to adequately interpret the data registered during the monitoring of soccer trainings and competitions and adequately guide soccer-specific training. The in-season monitoring of the aerobic status of elite players is, however, challenging. Obtaining an optimal testing window is habitually hampered due to the imposed training and games demands, and according to some others [3], due to also the reluctance of the technical staff and players to perform maximal testing. The development of easy-utilizable functional tools to monitor the aerobic capacity in soccer is a topic of great interest for the sport scientific community and supporting practitioners of soccer squads. Accordingly, this issue has been widely investigated in male footballers [2], [3], [4]. However, literature regarding this topic in female footballers is scarce. Nevertheless, women's soccer is undergoing an exponential increase in economic support, participation and professionalization [5]. Investigations traditionally conducted in males aiming to develop easy-utilizable monitoring tools are nowadays of great need, interest and value in women's soccer.

Fixed Blood Lactate Concentration (FBLC) thresholds are well utilized to assess the aerobic capacity in soccer [3] because they are determinable by submaximal objective procedures avoiding exercising to volitional exhaustion. In addition, they are measurable in several players at the same time in field settings allowing assessment of an entire team (≈ 25 players) within 45 min [6], and are highly reliable [7]. Moreover, FBLC thresholds are better indicators of the total and the high-intensity running distance covered during competitive games in elite women soccer players in comparison with the Yo-Yo Intermittent Recovery Test and the maximal oxygen uptake [1]. Nevertheless, determination of FBLC thresholds at the time to assess a complete soccer team considerably increases the cost of blood sampling and requires participation of a number of qualified professionals at the same time [6]. To the best of our knowledge, there is only one scientific publication that has reported sex-specific operational regression equations for on-field practical determination of FBCL thresholds in semi-professional female soccer players [6]. The study reported promising results. However, to generalize the use of these gender-specific equations in semi-professional female soccer teams, equations must be cross-validated. Prior to the implementation of the predictions models in real on-field practice, it is a prerequisite to externally validate the models on a separate dataset [8]. Whether the models [6] are valid and accurate in a different semi-professional female soccer team has yet to be elucidated.

About two decades ago [9] the 30-15 Intermittent Fitness Test (30-15IFT) was introduced to assess the intermittent aerobic power in soccer. Since then, its popularity has dramatically increased [9], [10], [11]. The 30-15IFT does not exclusively evaluate the aerobic oxidative capacity (predominantly during rest periods) since the anaerobic glycolytic energy system (predominantly during exercise bouts periods) is greatly activated [10]. To date, however, it is still unknown the extent of the degree of commonality between the constant aerobic capacity measured by FBLC thresholds and the intermittent aerobic power measured by the 30-15IFT. To the best of our concern, whether FBLC thresholds are related to the 30-15IFT performance in semi-professional female footballers has not been hitherto investigated. Due to the tight training and competition schedules and constraints to obtain testing-days in elite soccer [3], prediction equations between these two aerobic performance markers would be of interest for the supporting staff of soccer teams to guide both, aerobic capacity and intermittent aerobic power training, by means of only one of the two abovementioned assessments.

The aims of this study were twofold:

to test (cross-validate), in real on-field practice, the predictive accuracy of the sex-specific operational regression equations to determine the FBLC thresholds previously [6] developed;

to examine the relationships, if any, between the FBLC thresholds and 30-15IFT performance in a semi-professional female soccer squad. The straightforward on-field applicability of these results might interest the supporting staff of female soccer squads.