Cognitive Function among Young Women's Football Players in the Summer Heat

Recently, there has been a growing focus on studies related to women's football. However, the cognitive function of female football players has not been extensively characterized. Thus, we explored how the cognitive function of female football players was altered during a series of matches in summer and examined day-to-day variations in cognitive function with regard to dehydration status. Resting cognitive function was assessed from 17 young women football players during the Japan Club Youth Women's football tournament, which spanned eight consecutive days. Cognitive function initially improved, with this improvement sustained throughout the tournament. It is worth noting that ten participants experienced symptoms of dehydration at least once during the tournament; however, these symptoms were not found to be linked to impaired cognitive function, suggesting that resting cognitive function remains unaffected during summer matches, even in the presence of dehydration symptoms.


Introduction
Growing attention has been devoted to studies on women's football [1][2][3].During football matches, players face the challenges of making quick and accurate decisions in a constantly changing environment.Decision-making is a cognitive process that involves choosing a course of action from among several alternatives to achieve a goal.Hence, cognitive function is a key determinant of football performance.Intriguingly, research has shown that female and male players recruit diferent cognitive processes during football-specifc cognitive tasks [4].However, the cognitive functions of female football players have not been well characterized in the literature.
To succeed in a tournament, football players must maintain a good performance in a series of competitive matches.Recently, the level of environmental heat stress, which signifcantly impacts exercise performance [5], has been continuously rising due to a combination of increased prevalence, intensity, and duration of bouts of extremely hot weather [6].Given that dehydration is frequently observed during exercise in hot environments [7], it is likely that football players will experience dehydration when playing consecutive matches in summer.Tis is a concern because dehydration may impair cognitive function [8].Terefore, unless adequate hydration is restored during a tournament, cognitive function may be affected.Furthermore, although there is currently no evidence that women have an inherent disadvantage in thermoregulation when exercising in heat compared to men of similar age and health status [9], some studies have classifed women as heat intolerant compared to men [10,11].In this context, it is particularly important to examine how the cognitive function of female football players is afected, especially when playing successive matches in the summer.Understanding the potential efects of dehydration on cognitive function will provide new insight into conditioning during consecutive summer matches.
In the present study, we investigated the impact of consecutive summer matches on the cognitive functions of female football players.Furthermore, we examined the dayto-day variations in cognitive function with respect to players' hydration status.Te fndings of this study provide practical information for women football players/coaches on how to preserve cognitive function during successive matches in hot summer conditions.

Materials and Methods
2.1.Participants.Tis study was conducted at the XF CUP 2021 Japan Club Youth (U18) women's football tournament.Participants were 17 young women football players (mean age, 17.2 ± 0.7 years).None of the participants had any cardiovascular, cerebrovascular, or respiratory diseases, and none were taking any prescribed or over-the-counter medications or supplements.Tis experimental protocol was approved by the Institutional Review Board of Toyo University (approval number: TU2021-022), and the participants (or parents/guardians) provided written informed consent prior to participation, in accordance with the principles of the Declaration of Helsinki, except for registration in a database.

Procedure.
Cognitive function was assessed during the tournament (from the day before the frst game (pregame) to the end of the ffth game) (Figure 1).Te day before premeasurement, participants were familiarized with the cognitive task.Cognitive function was assessed on a daily basis in a serene hotel room, approximately 1-2 hours after dinner.Following the game, participants promptly returned to the hotel for rest.Consequently, cognitive function assessments were conducted more than 6 hours after the game.To verify the presence of dehydration, body weight and urine-specifc gravity were measured.Importantly, the time elapsed between the assessment of cognitive function and body weight/urine measurements was consistently within 30 minutes.Te participants provided a urine sample (100 ml), which was analysed for the concentration of excreted particles using a refractometer (PAL-09S; Shiro Sangyo, Osaka, Japan).Clinical dehydration was confrmed if the USG was ≥1.030 [12].Based on these criteria, ten participants who experienced dehydration at least once during the tournament (once (n � 8), three times (n � 1), and six times (n � 1)) were classifed in the dehydration group, while seven were classifed into the normal group.

Cognitive
Tasks.We used the Go/No-Go task to assess cognitive function [13].Tis task is appropriate to evaluate decision-making in football players as it requires selective attention and response inhibition.Each trial began with a blank screen for 2.5 s, followed by a preparatory stimulus (green square) presentation at the centre of the computer screen for 1 s.Subsequently, one square of the four possible colours (red, blue, yellow, or purple) was presented for 1 s.One block of the Go/No-Go task consisted of 30 Go trials (red and blue) and 30 No-Go trials (yellow and purple).Participants responded to the Go signal by pressing the computer mouse with their right index fnger as quickly as possible.Cognitive function was assessed using the reaction time (RT, ms) and accuracy (%).Omissions of responses in Go trials or incorrect responses in No-Go trials were regarded as error trials.Accuracy was calculated as the number of correct responses divided by the total number of trials.

Statistical Analysis.
Statistical analyses were performed using JASP version 17.1 (JASP Team, Amsterdam, the Netherlands).Two-way repeated-measures analysis of variance was performed with day as the within-participant factor and group (dehydration vs. normal) as the between-participant factor.Te degrees of freedom were corrected using the Huynh-Feldt epsilon when the assumption of sphericity was violated.Since there were missing data from one participant (RT and USG on the day of 2nd game), the data from that participant were excluded from the ANOVA.Post hoc analyses using Bonferroni corrections for multiple comparisons were applied, as appropriate.Additionally, we calculated separate averages of RT for both nondehydrated and hydrated days for each participant in the dehydrated group.Subsequently, we compared RT between nondehydrated and dehydrated days using a Wilcoxon signed-rank test.Te efect sizes are presented as eta squares (η 2 ).Data are expressed as mean ± SD for normal distributions or median (interquartile range) for nonnormal distributions.Te signifcance level was set at p < 0.05.

Discussion
In the current study, ten female football players exhibited symptoms of clinical dehydration (as indicated by a USG value of ≥1.030) at least once during the tournament, and the dehydration group exhibited higher USG levels throughout the tournament.However, it is important to note that changes in USG during the tournament did not exhibit diferences between the two groups.A USG of 1.030 corresponds with a level of dehydration of 5% of body weight [14].A recent meta-analysis study indicated that dehydration (>2% of body weight) induced by exercise with/without heat stress may impair cognitive function [8].However, in the present study, cognitive function was not impaired throughout the tournament.Furthermore, within the dehydration group, no diferences in RT were observed between nondehydrated and dehydrated days.Tis indicates that the onset of dehydration did not have detrimental efects on cognitive function.Tese results suggest that the resting cognitive function was well maintained under these experimental conditions.Rather, RT decreased on the days after the 2nd game as compared with prevalue, and the RT improvements persisted until the end of the tournament.Tus, the cognitive improvement is primarily attributed to the learning efect associated with repeat testing, which is inherent in prospective studies [15].Although whether exercise-induced dehydration afects cognitive function remains controversial due to inconsistent methodological approaches [16,17], acute and long-term improvements in hydration status have been suggested to be linked with better cognitive function [16].Hence, our fndings do not negate the need to prevent dehydration in soccer players to maintain cognitive function during summer matches.In addition, we assessed only resting cognitive function, and it remains unclear the extent to which the cognitive function of female football players is afected during football matches.Additional studies are needed to examine whether female football players' cognitive functions are affected during football matches in summer.
We acknowledge that there are several limitations inherent to the feld study.First, we recognize that our sample size is relatively small and future studies with more participants are encouraged to draw frmer conclusions.Second, we matched the timing of cognitive assessment and body weight/urine measurements.However, all measurements were conducted at night.Hence, the timing of measurements could potentially infuence an individual's level of alertness/arousal, which might, in turn, afect cognitive function.Tird, it has been well-established that cognitive function is improved by acute exercise [18,19].In the present study, however, cognitive function assessments took place more than 6 hours after exercise.Meta-analysis studies have suggested that acute efects on cognitive function typically last for less than 30 minutes [20].Terefore, it is less likely that acute exercise signifcantly afected cognitive function in the present study.Nevertheless, we acknowledge that there is a possibility of confounding factors that might have infuenced the results.Finally, we did not verify the menstrual cycle phase of female football players.However, given that cognitive function was well-preserved throughout the tournament, these efects were presumably negligible.

Table 1 :
7.105 � Results of body weight, urine specifc gravity (USG), and accuracy of the Go/No-Go task evaluation.
Figure 1: Reaction time in the Go trial of the Go/No-Go task during the tournament.White bars represent the dehydration group, and dark grey bars represent the normal group.Data are expressed as mean ± SD or median (interquartile range).** * p < 0.001 vs. pre.Note that the number of participants was six on the day of 2nd game in the normal group.