Influence of Follicular vs. Luteal Phases on Sweat Rate and Estimated Sodium Loss in University Female Football Players: A Field-Based Within-Subject Study

Abstract

This study examined the influence of the follicular (FP) and luteal phases (LP) of the menstrual cycle on sweat rate, estimated sweat sodium concentration, heart rate, hydration status, fluid intake, and perceived exertion in first-division university female football players. A small sample of eight athletes completed two monitored training sessions, one in each estimated-menstrual phase, following a repeated-measures field-based design under habitual training conditions. Sweat rate was determined using pre- to post-exercise body mass changes and microfluidic sweat patches, while estimated sweat sodium concentration was obtained via wearable colorimetric sensors. Heart rate was continuously monitored, hydration status was assessed using urine specific gravity, fluid intake was recorded, and perceived exertion was evaluated using the Borg CR-10 scale. Sweat rate was significantly higher during LP compared with FP (0.83 ± 0.20 vs. 0.55 ± 0.25 L·h⁻¹, p = 0.026), alongside greater estimated sweat sodium concentration (695 ± 305 vs. 404 ± 159 mg·L⁻¹, p = 0.031) and higher perceived exertion (4.63 ± 1.41 vs. 3.13 ± 0.83, p = 0.021). Fluid intake was also significantly greater during LP (0.99 ± 0.19 vs. 0.49 ± 0.25 L, p < 0.001). No significant differences were observed for urine specific gravity, mean heart rate, or total body mass change (p > 0.05). These findings suggest that the luteal phase may be associated with higher thermoregulatory and perceptual responses during football training, highlighting the potential importance of menstrual cycle-informed hydration and training management strategies in female athletes.

1. Introduction

Women’s football has experienced a remarkable increase in participation, visibility, and professionalization over the last decade, leading to growing scientific interest in its physiological and performance demands [1]. As a high-intensity intermittent sport, football requires repeated bouts of aerobic and anaerobic efforts, including accelerations, decelerations, sprints, jumps, and frequent changes in direction [2], all of which must be sustained over approximately 90 min of play and, therefore, fitness and physiological state are crucial [2]. Elite female football players typically cover between 8 and 13 km per match, with a substantial proportion of the distance performed at high intensities (>18 km·h⁻¹), depending on playing position and competitive level [3,4]. These demands highlight the importance of optimal physical conditioning, thermoregulation, and hydration strategies to maintain performance and delay fatigue [1].

In female athletes, sport performance is influenced not only by training status and environmental conditions, but also by sex-specific physiological factors [5]. Among these, the menstrual cycle (MC) has emerged as a relevant yet historically underexplored variable [6]. The MC is characterized by cyclical fluctuations in ovarian hormones, primarily estrogen and progesterone, which can influence thermoregulation, cardiovascular function, substrate metabolism, fluid–electrolyte balance, and perceived exertion [6]. Conventionally, the cycle is divided into the follicular phase (FP), marked by relatively low concentrations of progesterone and increasing concentrations of estrogen, and the luteal phase (LP), characterized by elevated progesterone levels and decreasing levels of estrogen following ovulation [7]. Mechanistically, progesterone elevates the hypothalamic thermoregulatory set-point (~0.3–0.7 °C), shifting the thresholds for sweating and cutaneous vasodilation, whereas estrogen tends to facilitate heat dissipation [8,9]. In parallel, luteal progesterone has been linked to higher aldosterone under high-sodium balance, suggesting interactions within the renin–angiotensin–aldosterone system that may alter sodium handling and contribute to greater electrolyte loss during exercise in this phase [10,11]. Autonomic control also varies across the cycle. Several studies report higher sympathetic activation and phase-related changes in sudomotor and vasomotor activity, with increased sweating and skin blood flow in LP during heat stress [12,13,14]. Together, these endocrine–autonomic interactions may increase thermoregulatory strain and perceived effort in LP [15], particularly in warm environments [5] while during the FP, lower basal body temperature and reduced progesterone concentrations may facilitate heat dissipation and aerobic efficiency, thereby influencing key components of football performance, including endurance capacity [6], fatigue perception [4], and physiological responses to exercise [7].

Thermoregulation and hydration are especially critical in football due to prolonged exercise duration and substantial sweat losses. Sweat rate and sweat sodium concentration are key determinants of fluid and electrolyte balance, yet both variables show considerable interindividual variability and may be modulated by hormonal status [16,17]. Previous studies indicate that progesterone may alter sweating responses and sodium conservation mechanisms, potentially increasing sweat rate and electrolyte loss during the LP [18]. These alterations may elevate the risk of dehydration, negatively affect exercise tolerance, and compromise performance if hydration strategies are not individualized [18].

Despite the growing recognition of the importance of MC, informed training and nutrition, research focusing on female football players, particularly at the university level, remains limited [6,19]. Moreover, practical and field-applicable tools for monitoring sweat rate and electrolyte loss in female athletes have only recently become available [20]. Wearable technologies, such as microfluidic sweat patches, offer a non-invasive and accessible approach to estimating sweat rate and sweat sodium concentration in real training environments, potentially supporting personalized hydration strategies [21].

Therefore, the aim of the present study was to examine the effect of the follicular and luteal phases of the MC on sweat rate, estimated sweat sodium concentration, heart rate, hydration status, and perceived exertion in first-division university female football players. By integrating MC monitoring with field-based physiological assessments, this study seeks to contribute to the growing body of evidence supporting sex-specific and cycle-informed approaches to performance optimization and hydration management in female athletes.

We hypothesized that, in this applied field setting, athletes would exhibit higher sweat rate, greater estimated sweat sodium concentration, and higher perceived exertion during the estimated LP compared with the estimated FP.

2. Materials and Methods

2.1. Study Design

A field-based repeated-measures longitudinal design was employed to examine physiological and perceptual responses across two phases of the MC: the FP and LP. Each participant completed one monitored training session in each phase, allowing within-subject comparisons. This design enabled the assessment of phase-related responses while maintaining high ecological validity by evaluating athletes under habitual football training conditions.

2.2. Participants

Eight university female football players aged from 18 till 22 years (mean age: 20.0 ± 1.5 years) from a first-division representative university team participated in the study. Given that data collection was conducted during routine team training within a single squad, the sample reflects the available and eligible athlete population under these field constraints, which is common in applied sport research. Importantly, the repeated-measures design (each player tested in both phases) focuses on within-player differences, reducing between-athlete variability and improving statistical efficiency compared with between-group designs. Accordingly, results are presented as preliminary estimates and interpreted cautiously. Given the applied nature of the study, no prior power calculation was performed [22,23]. Participants were recruited using non-probabilistic convenience sampling, including players who met the eligibility criteria and voluntarily agreed to participate. The following inclusion criteria were considered: (i) regular MC (26–32 days) during the previous twelve months; (ii) active participation in regular team training; and (iii) absence of injury or illness at the time of data collection. Moreover, the exclusion criteria included: (i) use of pharmacological treatments that could affect thermoregulation or fluid balance; and (ii) diagnosed menstrual or endocrine disorders. All participants provided written informed consent prior to participation. The study was conducted in accordance with the Declaration of Helsinki and was approved by the Research Ethics Committee of Universidad Europea (approval code: 2025-322). Additionally, ethical authorization was obtained in accordance with Mexican regulations for research involving human participants.

2.3. Menstrual Cycle Phase Determination

MC phases were determined using self-reported menstrual calendars and mobile application tracking, a common practice in applied sport settings when laboratory verification is impractical [24]. Nevertheless, recent systematic reviews [25] show that using only a calendar can lead to phase misclassification. Therefore, findings should be interpreted cautiously. The FP was estimated as days 1–14 of the cycle, beginning with menstruation, while the LP was estimated as days 15–28 following ovulation. These operational windows were anchored to reported menses and did not verify individual ovulation timing. Although hormonal concentrations were not directly measured, calendar-based phase classification is frequently reported as a pragmatic, field-feasible proxy in athletic contexts [24], yet carries a meaningful risk of phase misclassification. Therefore, best practice is to incorporate ovulation testing and/or biochemical verification of ovarian hormones whenever feasible [25]. Because hormone measurements were not possible during team training, we used calendar-defined phases. As a result, between-phase comparisons are interpreted as phase-associated, not hormone-confirmed, effects.

2.4. Variables Assessed

The independent variable was MC phase (FP vs. LP). The dependent variables included: Sweat rate (L·h⁻¹), estimated sweat sodium concentration (mg·L⁻¹), heart rate (beats·min⁻¹), body mass changes (kg), fluid intake during training (mL), urine specific gravity, and rating of perceived exertion (RPE). Control variables included ambient temperature and relative humidity, training duration, and training time of day. Nutritional and hydration status prior to training were recorded as contextual variables. Participants were instructed to maintain their habitual hydration patterns and to avoid alcohol and strenuous exercise during the 24 h preceding each session.

2.5. Instruments and Measurements

2.5.1. Sweat Rate and Sweat Sodium Concentration

Sweat rate and estimated sweat sodium concentration were assessed using the Gatorade Gx Sweat Patch and the accompanying mobile application (iOS) (Gatorade Sports Science Institute, Barrington, IL, USA). This single-use, microfluidic patch was placed on the participant’s forearm approximately 5 cm below the elbow crease. The patch collects sweat through microchannels that visually indicate sweat volume and chloride concentration via colorimetric reactions. The application then combines the patch outputs with individual (e.g., sex, body mass), environmental (temperature, humidity), and exercise information to generate algorithm-based estimates of whole-body sweating rate (L·h⁻¹) and sweat sodium concentration/losses (mg·L⁻¹). This Gx platform has been described and validated against established laboratory reference methods, including mass-balance approaches for whole-body sweating rate and whole-body washdown for sweat electrolyte concentration, supporting its use as a practical field tool when laboratory collection is not feasible [26].

Nevertheless, because the Gx outputs are estimates derived from a single anatomical site and embedded algorithms, their accuracy may vary across individuals and field conditions. Independent field evaluations have also reported that, while the system can provide useful ballpark information for sweat electrolyte concentration, sweat-rate estimates may be less consistent and occasional patch failures can occur; therefore, absolute values should be interpreted cautiously. To strengthen measurement validity, sweat rate was also calculated from pre- to post-exercise body mass change (accounting for fluid intake) as a standard field benchmark. Patch-derived estimates were therefore interpreted alongside this gravimetric method to contextualize absolute values under applied training conditions.

2.5.2. Body Mass and Fluid Intake

Body mass was measured before and after each training session using a calibrated digital scale (OMRON HBF-514, Kyoto, Japan). Measurements were taken with participants wearing light sports clothing and without footwear. Sweat rate was additionally calculated using pre- to post-training body mass differences, accounting for fluid intake. Fluid intake during training was recorded by weighing individual water bottles before and after the session. Participants were provided with up to 2 L of water and were allowed to drink ad libitum.

2.5.3. Heart Rate Monitoring

Heart rate was continuously monitored throughout training using a validated chest-strap heart rate monitor (Myzone, Nottingham, UK). Average heart rate values were extracted for each session to assess cardiovascular response.

2.5.4. Hydration Status

Hydration status was assessed using urine specific gravity (USG). Urine samples were collected immediately after exercise to assess post-exercise hydration (Mettler-Toledo, Columbus, OH, USA). No strict hydration threshold (e.g., USG < 1.020) was imposed prior to participation, as the aim of the study was to reflect habitual field conditions during regular training sessions.

2.5.5. Perceived Exertion

Perceived exertion was assessed immediately after each training session using the Borg CR-10 scale, a validated tool for estimating internal training load in athletic populations.

2.5.6. Environmental Conditions

Ambient temperature (°C) and relative humidity (%) were measured using a portable digital thermo-hygrometer (ThermoPro TP55, Hong Kong, China). Training sessions were conducted between 13:00 and 15:00 to minimize circadian variability.

2.6. Procedure

Each participant completed two monitored training sessions: one during the LP and one during the FP. The order of sessions was not randomized, as measurements were scheduled according to each participant’s natural menstrual cycle phase. Therefore, the interval between sessions corresponded to the individual cycle duration (approximately 2–3 weeks). Upon arrival, menstrual phase was confirmed, and a brief interview was conducted to record food and fluid intake during the previous hour. Pre-training body mass was measured, and participants were asked to void their bladder. Individual height and age were also registered in the first session.

The sweat patch and heart rate monitor were then applied, and environmental conditions were recorded. No statistically significant differences in ambient temperature were observed between the training sessions. Training sessions followed the team’s regular training plan and lasted approximately 70 min. After training, body mass, fluid intake, perceived exertion, sweat patch data, heart rate, and urine specific gravity were collected following standardized procedures.

2.7. Statistical Analysis

Data are reported as mean ± SD. Normality of within-player differences was checked using a Shapiro–Wilk test; when questionable, Wilcoxon signed-rank results are shown as sensitivity. Between-phase comparisons were analyzed as within-subject (paired), to determine differences in both MC phases. For each outcome the paired-samples t-test (df = n − 1), the mean difference (LP–FP) with 95% CI, and within-subject effect sizes (Cohen’s dz and Hedges’ gav) are reported (bias-corrected standardized mean change using the average SD across phases). For completeness, a one-factor repeated-measures ANOVA (phase: FP vs. LP) was also run and reported as F(1, df_error) with partial η²; with two levels, F = t² and partial η² = F/(F + df_error). Paired sample sizes vary by outcome due to one missing patch reading (n = 7 for patch-based variables instead of n = 8). Statistical significance was set at p-value ≤ 0.05. Jamovi package 2.6.26 (2025)) was used to run statistical analysis. Environmental variables (ambient temperature and relative humidity) were recorded during each session but were not included as covariates in the statistical models due to the exploratory nature of the study and the relatively small sample size.

3. Results

3.1. Participant Characteristics and Training Conditions

Eight university female football players from a first-division team completed all testing sessions. Participants had a mean age of 20.0 ± 1.51 years, mean height of 1.63 ± 0.07 m, and mean body mass of 66.25 ± 11.22 kg (see Table 1). Each participant completed two training sessions, one during the estimated-FP and one during the estimated-LP of the MC, with a standardized training duration of 1.16 h per session. Environmental conditions were similar but not identical between sessions. Mean ambient temperature was 20 °C during FP sessions and 22 °C during LP sessions, while relative humidity averaged 51.2% and 48.0% during FP and LP, respectively. Because sweating rate is a function of metabolic intensity and environmental heat load, small differences in ambient temperature may contribute to variability in sweat loss in field settings [27]. Nevertheless, sessions were performed under similar, temperate conditions and standardized training constraints, allowing meaningful within-player comparisons while acknowledging potential environmental influence

Table 1. Descriptive statistics of the sample.

	N	Mean	Median	SD	Minimum	Maximum
Age (years)	8	20.50	20.50	1.51	19	23
Height (m)	8	1.63	1.63	0.0693	1.52	1.73
Weight (kg)	8	66.25	64.55	11.2176	53.80	80.70

Notes: SD (standard deviation), m (meters), kg (kilograms).

3.2. Physiological and Perceptual Responses During the Luteal Phase

During the LP of the MC, the mean sweat rate calculated using body mass changes was 0.83 ± 0.20 L·h⁻¹, with individual values ranging from 0.43 to 1.03 L·h⁻¹. Sweat Patch-estimated sweat rate was slightly lower, with a mean value of 0.71 ± 0.14 L·h⁻¹ (range: 0.47–0.86 L·h⁻¹).

Estimated sweat sodium concentration during LP averaged 695 ± 305.50 mg·L⁻¹, showing substantial interindividual variability, with values ranging from 409 to 1186 mg·L⁻¹. Post-exercise urine specific gravity was 1.019 ± 0.01, with some participants exceeding the 1.020 threshold, indicating mild dehydration in certain cases.

Perceived exertion, assessed using the Borg CR-10 scale, averaged 4.63 ± 1.41, with individual scores ranging from 2 to 6, reflecting a moderate subjective training intensity. Mean heart rate during training in LP was 135 ± 10.2 beats·min⁻¹. Fluid intake during LP sessions averaged 0.99 ± 0.19 L, and net body mass change showed minimal variation (−0.025 ± 0.36 kg), suggesting partial compensation for fluid losses through ad libitum drinking.

3.3. Physiological and Perceptual Responses During the Follicular Phase

During the FP of the MC, sweat rate measured via body mass changes was lower than in LP, with a mean value of 0.55 ± 0.25 L·h⁻¹ (range: 0.35–0.95 L·h⁻¹). Sweat Patch-estimated sweat rate was also lower than when using body mass changes and averaged 0.49 ± 0.12 L·h⁻¹, with values ranging from 0.32 to 0.74 L·h⁻¹.

Estimated sweat sodium concentration during FP was 403.63 ± 159.44 mg·L⁻¹, with a minimum of 104 mg·L⁻¹ and a maximum of 557 mg·L⁻¹. Post-exercise urine specific gravity averaged 1.015 ± 0.0068, remaining within values generally considered indicative of adequate hydration.

Perceived exertion during FP was lower than during LP, with a mean Borg CR-10 score of 3.13 ± 0.83 (range: 2–4), indicating a light to moderate perceived training intensity. Mean heart rate during FP sessions was 130.3 ± 7.92 beats·min⁻¹. Fluid intake during FP averaged 0.49 ± 0.25 L, and participants experienced a small net body mass loss (0.15 ± 0.19 kg) following training.

3.4. Comparison Between Menstrual Cycle Phases

All descriptive analysis comparing both phases’ results are summarized in Table 2.

Table 2. Descriptive statistics for both menstrual cycle phases.

	Menstrual Cycle Phase	N	Mean	Median	SD	Minimum	Maximum
Exercise time (hours)	Luteal	8	1.160	1.160	0	1.160	1.160
	Follicular	8	1.160	1.160	0	1.160	1.160
Sweat rate (L·h−1)	Luteal	8	0.830	0.862	0.201	0.431	1.034
	Follicular	8	0.550	0.431	0.248	0.345	0.948
Sweat Patch-estimated sweat rate (L·h−1)	Luteal	8	0.750	0.737	0.113	0.468	0.856
	Follicular	8	0.491	0.467	0.124	0.316	0.741
Sodium concentration Sweat Patch (mg·L−1)	Luteal	8	715.000	557.000	324.502	409	1186
	Follicular	8	403.625	414.500	172.444	104	557
Urine specific gravity	Luteal	8	1.019	1.022	0.011	1	1.030
	Follicular	8	1.015	1.013	0.007	1.010	1.030
Rating of perceived exertion (Borg 1–10)	Luteal	8	4.625	4.500	1.408	2	6
	Follicular	8	3.125	3.000	0.835	2	4
Mean heart rate (bpm)	Luteal	8	135.500	133.500	10.240	123	154
	Follicular	8	130.250	127.500	7.924	121	144
Fluid intake (L)	Luteal	8	0.988	1	0.189	0.600	1.300
	Follicular	8	0.487	0.450	0.247	0.200	0.900
Total weight loss (Kg)	Luteal	8	−0.025	0	0.362	−0.500	0.600
	Follicular	8	0.150	0.150	0.193	−0.100	0.500

Notes: SD (standard deviation), L·h⁻¹ (liters per hour), bpm (beats per minute), L (liters), Kg (kilograms).

Compared with FP, the LP showed a higher body mass-derived estimated sweat rate (Δ = 0.252 L·h⁻¹, 95% CI 0.068 to 0.437; t(6) = 3.35, p = 0.015; d_z= 1.27; g_av= 1.77; F(1,6) = 11.25, η²p = 0.65), and a higher estimated sweat sodium concentration (Δ = 312.9 mg·L⁻¹, 95% CI 74.0 to 551.7; t(6) = 3.21, p = 0.018; d_z= 1.21; g_av= 1.05; F(1,6) = 10.27, η²p = 0.63). Fluid intake was also greater in LP (Δ = 0.500 L, 95% CI 0.277 to 0.723; t(7) = 5.29, p = 0.001; d_z= 1.87; g_av= 2.02; F(1,7) = 28.00, η²p = 0.80), alongside a higher perceived exertion (Δ = 1.50 a.u., 95% CI 0.159 to 2.841; t(7) = 2.65, p = 0.033; d_z= 0.94; g_av= 1.15; F(1,7) = 7.00, η²p = 0.50). No clear between phase differences were observed for urine specific gravity, mean heart rate, or total body mass change (p ≥ 0.05; effects small to moderate). Sensitivity analyses using Wilcoxon yielded consistent inferences (significant for patch estimated sweat rate, sweat sodium, RPE, and fluid intake, non-significant for the remainder) (see Table 3).

Table 3. Inferential statistics for both menstrual cycle phases.

Variable	t (df)	95% CI	p-Value	Cohen’s d	Hedges’ gav	F(1,df)	η2
Body mass sweat rate (L·h−1)	2.30	−0.007/0.568	0.045 *	0.81	1.10	5.30	0.43
Patch estimated sweat rate (L·h−1)	3.35	0.068/0.437	0.015 *	1.27	1.77	11.25	0.65
Patch estimated Sodium Concentration (mg·L−1)	3.21	74.0/551.7	0.018 *	1.21	1.05	10.27	0.65
Urine Specific Gravity	0.93	−0.005/0.012	0.40	0.33	0.33	0.86	0.11
RPE (Borg 1–10)	2.65	0.159/2.841	0.033 *	0.94	1.15	7.00	0.50
Mean Heart rate (bpm)	1.26	−4.568/15.068	0.247	0.45	0.45	1.60	0.19
Fluid intake (L)	5.29	0.277/0.723	0.001 *	1.87	1.87	28.00	0.80
Total body mass change (Kg)	−1.47	−0.457/0.107	0.185	−0.52	−0.54	2.16	0.24

Notes: CI (confidence interval), F (F value), η² (eta square), L·h⁻¹ (liters per hour), bpm (beats per minute), L (liters), Kg (kilograms). * Significant difference between groups (p < 0.05).

4. Discussion

4.1. Influence of the Menstrual Cycle on Sweat Rate and Sodium Loss

The present study shows phase-associated differences within individuals in sweat-related responses during the estimated-phases of the MC in university female football players. The higher sweat rates and sodium concentrations observed in the estimated LP within individuals may reflect distinct thermoregulatory and electrolyte responses, potentially related to differences in physiological adaptation or fluid management strategies [28].

These results suggest that hormonal fluctuations across the MC (higher levels of progesterone during the LP) may be associated with changes in sympathetic activity, thermoregulatory and fluid–electrolyte responses during exercise [29].

The higher sweat rate observed during the estimated LP is consistent with the known physiological effects of progesterone [1], which predominates during this phase and has been associated with an elevation in basal and exercise-induced core temperature [11]. An increase of 0.3–0.5 °C in body temperature during LP has been previously reported [15], which may contribute to an earlier onset of sweating and higher sweat output in order to dissipate excess heat [16]. This mechanism aligns with findings by Cheung et al. [15], who described altered thermoregulatory efficiency during LP due to progesterone-mediated changes in heat production and dissipation.

Although the present data showed higher estimated sweat sodium concentration during LP compared with FP, the existing literature is not fully consistent. Some studies have reported no statistically meaningful differences in sweat sodium concentration across MC phases [17,29,30], suggesting that the magnitude and direction of phase-related changes may depend on contextual factors and the substantial interindividual variability typically observed in sweat electrolyte losses. In the current study, environmental conditions were broadly comparable across sessions; however, ambient temperature was approximately 2 °C higher during LP testing than FP (22 °C vs. 20 °C). Even small thermal differences may increase sweat output and, consequently, total electrolyte loss, potentially contributing to the higher sweat rate and sodium losses observed during the estimated-LP. However, environmental temperature was not included as a covariate in the statistical analysis, and its potential influence on sweating responses cannot be fully excluded [31].

In parallel, estimated sweat sodium concentration was substantially higher during LP than FP. This suggests that LP may be associated not only with greater fluid loss, but also with increased electrolyte loss. Baker and colleagues [18] have highlighted the large interindividual variability in sweat sodium concentration, which was also evident in the present study, and emphasized that hormonal status may contribute to this variability. Elevated progesterone levels may influence aldosterone sensitivity and sodium reabsorption at the sweat gland level, potentially increasing sodium loss during LP [10,30]. These interpretations should be considered in light of the measurement approach used, as sweat sodium concentration was estimated using a single-site, algorithm-based wearable device.

4.2. Hydration Status and Fluid Intake Across Estimated Menstrual Cycle Phases

Despite higher sweat rate and sodium loss during LP, urine specific gravity values remained, on average, within ranges indicative of adequate hydration in both phases. However, individual values exceeding the 1.020 threshold were observed during LP, suggesting a greater risk of mild dehydration in some athletes during this phase.

Fluid intake during training was nearly twice as high during LP compared with FP, indicating a greater perceived need for hydration. This observation is consistent with previous literature showing that hormonal fluctuations during LP may alter fluid balance regulation, thirst perception, and fluid retention [30,32,33]. Baker [18] reported that dehydration-related increases in perceived exertion and discomfort may persist even when fluid is consumed ad libitum, reinforcing the importance of proactive hydration strategies rather than reliance on thirst alone. The higher fluid intake observed during LP, together with very large effect sizes (which should be interpreted cautiously given the small sample size), highlights the potential importance of individualized hydration strategies. Interestingly, despite differences in sweat rate and fluid intake, no between-conditions differences were observed in heart rate or total body weight loss. This suggests that cardiovascular strain and overall dehydration remained within comparable ranges.

It should also be noted that pre-exercise hydration status was not strictly standardized, as the aim was to reflect habitual field conditions. While this approach enhances ecological validity, it may have introduced additional variability in hydration-related responses. In addition, dietary sodium intake was not controlled, which may have contributed to inter-individual variability in sweat sodium concentration.

4.3. Perceived Exertion and Physiological Responses

Perceived exertion and mean heart rate were both higher during the estimated LP compared with FP, despite comparable session duration and broadly similar environmental conditions. These findings may suggest that LP is associated with a greater internal load for the same external workload [14]

Higher perceived exertion during LP has been reported in previous studies and may be linked to increased core temperature, which can shift thermos-effector thresholds [34], altered substrate metabolism [35], and heightened sympathetic nervous system activity [36]. In controlled endurance exercise at ~22 °C, women tested in the mid-luteal phase showed higher rectal temperature and, importantly, higher heart rate (≈10 beats·min⁻¹) and higher RPE later in the bout, despite similar oxygen uptake and sweat loss, suggesting that phase-related increases in core temperature can translate into greater cardiovascular strain and higher perceived effort [37]. In field settings such as ours, a higher thermal drive may also increase skin blood flow demands, thereby elevating cardiac output requirements. Under these conditions, progressive cardiovascular adjustments (including cardiovascular drift) can increase heart rate disproportionately relative to external workload, particularly when hyperthermia and fluid loss are present [38,39].

In addition, de Poli et al. [35] and Pardo et al. [34] have suggested that progesterone-related changes may increase neuromuscular fatigue and cardiovascular strain, thereby elevating perceived effort during exercise. The slightly higher mean heart rate observed during the estimated LP in the present study supports this interpretation and indicates increased cardiovascular demand during this phase.

Autonomic modulation across the MC provides an additional mechanistic explanation. Direct measurements have shown that resting muscle sympathetic nerve activity and plasma norepinephrine may be higher in the mid luteal phase than in the early follicular phase, indicating greater sympathetic outflow during LP [40]. Given that thermoregulation and cutaneous blood flow are strongly influenced by autonomic control, and that reproductive hormones can modulate thermoregulatory pathways, this shift may contribute to higher heart rate and perceived effort during exercise [9]. Taken together, a higher thermoregulatory set point, increased cardiovascular demands, and greater sympathetic activation provide a plausible explanation for the higher RPE and mean heart rate observed during LP in this study. However, because menstrual phase classification was based on calendar methods and hormonal concentrations were not directly measured, these mechanisms should be interpreted as physiologically plausible rather than directly confirmed.

4.4. Evaluation of Field-Based Monitoring Technologies

The comparison between sweat rate calculated via body mass changes and estimates obtained using the Gx Sweat Patch showed consistent trends across estimated MC phases, supporting the practical applicability of this wearable technology in field settings. Although small differences in absolute values were observed between methods, these discrepancies are expected, given differences in measurement principles, patch placement, and individual sweat distribution patterns. In the context of female football, where non-invasive, time-efficient, and field-based tools are particularly valuable, the use of wearable sweat sensors may facilitate individualized hydration strategies that account for MC-related variability. Given these considerations, it is important to interpret these findings alongside the methodological characteristics of the wearable device used.

In the present field-based setting, the Gx Sweat Patch offered a practical, non-invasive way to estimate sweat loss and electrolyte losses during regular training; however, its output should be interpreted in light of how the system works and its known limitations. The patch provides local measures of sweat volume and sweat chloride via colorimetric microfluidic channels, and the app then applies image-processing and prediction algorithms (including athlete and environmental inputs) to estimate whole-body sweating rate and sweat sodium losses. Although the Gx platform has been validated against standard reference approaches (e.g., mass-balance methods for whole-body sweating rate and whole-body washdown for sweat electrolytes), any single-site, algorithm-based estimate can vary across individuals, sessions, and field conditions. Some independent studies have also reported variability in agreement between wearable sweat sensors and reference methods, highlighting potential limitations in accuracy depending on measurement conditions [41]. Also, occasional patch failures have been reported in real-world use. For this reason, we triangulated sweat rate using an established field benchmark (pre–post body mass change corrected for fluid intake) and examined agreement between gravimetric and patch-derived estimates to contextualize measurement uncertainty. Accordingly, we place greater emphasis on within-player, between-phase comparisons (where each athlete serves as her own control) than on the absolute magnitude of patch-derived sodium concentration values, and we frame practical implications as preliminary and contingent on confirmation with larger samples and, where feasible, laboratory-grade sweat electrolyte testing.

4.5. Limitations

Several limitations of this study should be acknowledged. First, the sample size was relatively small (n = 8), which limits the generalizability of the findings and increases the likelihood of variability in the observed responses. Second, menstrual cycle phase classification was based on calendar methods and was not confirmed by hormonal measurements, which may have led to potential misclassification. This is particularly relevant given the still limited body of research in female football players, especially in relation to physiological responses across the menstrual cycle [42].

Third, environmental variables such as ambient temperature were not included as covariates in the statistical analysis, and therefore their potential influence on sweating and thermoregulatory responses cannot be fully excluded. In addition, pre-exercise hydration status and dietary sodium intake were not strictly standardized or controlled, which may have contributed to inter-individual variability in hydration and electrolyte responses.

Finally, although wearable technology (Gx Sweat Patch) provided a practical field-based approach, its estimates are based on single-site measurements and algorithm-derived predictions, which may introduce measurement variability and should be interpreted with caution.

Therefore, the findings should be considered preliminary, and future studies with larger samples, hormonal verification, and tighter control of environmental and nutritional variables are warranted.

4.6. Practical Implications for Training and Hydration Strategies

The findings of this study have practical implications for the planning of training and hydration strategies in female football players. The LP may be characterized by higher sweat rate, greater sodium loss, and increased RPE, suggesting increased physiological stress during this phase. Coaches, sports scientists, and nutrition professionals may consider adjusting hydration strategies during LP by emphasizing adequate fluid and electrolyte intake, particularly during high-intensity or warm-environment training sessions. Additionally, monitoring RPE and cardiovascular responses across the MC may help inform individualized load management and recovery strategies.

5. Conclusions

Overall, the findings suggest the presence of phase-related differences in thermoregulation, hydration, and perceived effort. LP may be associated with higher sweat rate and greater estimated sweat sodium concentration, alongside higher mean heart rate and greater perceived exertion.

Incorporating accessible monitoring technologies during the different phases of the MC may represent a meaningful step toward more personalized and inclusive approaches to performance optimization in women’s football.

This study contributes to the growing body of evidence highlighting the importance of considering the MC in female athlete monitoring. Given the exploratory nature of the study, the lack of hormone status confirmation and the small sample size, future research should incorporate larger cohorts, hormonal confirmation of MC phases, and tighter control of environmental and pre-exercise factors to clarify the magnitude of phase-related differences and their performance implications.