Accumulative excessive physical load increases central arterial stiffness and smooth muscular tone in the peripheral vascular beds of endurance athletes, likely due to enhanced sympathetic nervous activity and oxidative stress (Iellamo et al., 2002; Knez et al., 2006; Barnes et al., 2010; Vlachopoulos et al., 2010). We previously reported that, in well-trained male collegiate endurance runners, aortic systolic and pulse pressures, along with central arterial stiffness, increased after a 7-day intense endurance training camp, with these changes correlating to the total running distance (Tomoto et al., 2015; 2018). Therefore, the vascular condition—characterized by central arterial stiffness and peripheral vascular smooth muscle tone—may reflect exercise-induced physiological fatigue and recovery.

It should be noted that postexercise hemodynamic responses to acute endurance exercise bouts differ between men and women (Senitko et al., 2002). However, our studies subjected male-only groups, and thus, the hemodynamic response to repeated, intense endurance exercise bouts in women is not entirely known.

A photoplethysmogram (PTG) measures changes in the absorbance of hemoglobin based on the Lambert–Bernstein law, indicating regional blood flow (Jespersen and Pedersen, 1986). The waveform, recorded at the finger, has two main components: an early systolic wave caused by left ventricular ejection, and a reflected wave from the periphery (Westerhof et al., 1972a; Hashimoto et al., 2002). These components could be characterized by the five inflection points on PTG waves using second derivatives (e.g., SDPTG). This technique provides a composite measure of vascular aging (Takazawa et al., 1998), and the detailed wave analysis offers surrogate markers for both central arterial stiffness and peripheral vascular smooth muscle tone (Westerhof et al., 1972b; Hashimoto et al., 2002). Thus, it is a valuable tool for assessing the vascular condition of athletes.

Athletes competing at higher levels require intense, high-volume training, which often results in prolonged imbalances between fatigue and recovery. This can reduce trainability, impair performance, and increase the risk of overtraining syndrome (Lee et al., 2017). Ideally, the relationship between training load and physiological responses should be monitored individually. However, much of the current evidence is based on cross-sectional studies, which fail to capture intra-individual changes over time (Lee et al., 2017). Additionally, conventional correlation analysis applied to repeated measures can produce biased results due to non-independent observations (Bakdash and Marusich, 2017).

In this field study, we sought to identify sensitive vascular biomarkers that reflect physical fatigue and recovery in athletes. We aimed to elucidate the impact of a brief series of matches on vascular conditions in young female soccer players. Additionally, we determined the relationship between intra-individual responses of vascular condition, hemodynamics, and fatigue to the repetitive matches using the repeated measures correlation analysis (Bakdash and Marusich, 2017). Given that the required running speed and distance in soccer vary depending on a player’s position and the strength of the opposing team, this approach allowed us to efficiently clarify the individual relationship between physical fatigue and vascular responses. We hypothesized that a brief series of soccer matches would increase central arterial stiffness and arterial wave reflection from the periphery in young female football players, correlating with fatigue, and that this vascular condition would be associated with hemodynamic markers such as blood pressure and heart rate.

During the experimental period, there were a total of two instances of missing data for both B/A and D/A ratios. Additionally, there were four instances of missing data for RPE and seven instances for KES. Therefore, estimated marginal means and 95% confidence intervals were reported using LMM. As shown in Table 1, body weight was lower (time effect: P< 0.0001), and RPE was higher (time effect: p = 0.001) post-match compared with pre-match. KES tended to be lower post-match than pre-match (time effect: p = 0.084).

Figure 3 depicts comparisons of the estimated marginal means (via linear mix model) of hemodynamic and SDPTG indices between pre- and post-match. Heart rate was higher (time effect: p = 0.019). Systolic and diastolic BP, and D/A ratio were lower than the pre-match value (time effects: p = 0.021, p = 0.016, and p = 0.023, respectively). The B/A ratio did not show a significant change over time (p = 0.653). However, the D/A ratio exhibited a significant effect related to the match sequence (p = 0.009). The D/A ratio at the third and fourth matches was lower than that at the first (Bonferroni test: p = 0.027 and 0.031). Rmcorr revealed that the D/A ratio exhibited significant intra-individual correlations with KES (r_rm = 0.200, p = 0.049) and RPE (r_rm = −0.229, p = 0.022) (Figure 4). Furthermore, the D/A ratio demonstrated significant intra-individual correlations with diastolic BP (r_rm = 0.259, p = 0.008) and heart rate (r_rm = −0.381, P< 0.001) (Figure 5). However, no significant correlation was found with systolic BP (r_rm = 0.155, p = 0.118).

The main findings of the present study are as follows: first, during the daily repetition of soccer matches, the post-match values of the D/A ratio and BP were lower than the pre-match values, while the B/A ratio showed no significant changes. Second, the D/A ratio exhibited significant intra-individual correlations with diastolic BP and heart rate. These results suggest that, in young female football players, a brief series of matches did not increase central arterial stiffness but instead led to a repeated, temporary attenuation of arterial wave reflection, which was associated with a reduction in diastolic BP and a compensatory rise in heart rate.

In young female football players who engaged in a brief series of matches, the D/A ratio was lower 4 h after the soccer matches compared with that the night before the matches, suggesting that the attenuated arterial wave reflection lasted more than 4 h, followed by gradual recovery. Rmcorr indicated that the D/A ratio had significant intra-individual relationships with the indices of fatigue (i.e., KES and RPE). While the level of fatigue likely varied individually depending on each player’s position and the strength of the opposing team, the series of matches consistently induced repeated, temporary attenuation of arterial wave reflection, which was associated with physical exertion in each individual.

We found mild hypotension in our subjects after the matches, as depicted in Figure 3. Specifically, the D/A ratio exhibited a significant intra-individual correlation with diastolic BP but not with systolic BP. While systolic BP is influenced by several hemodynamic factors, including arterial stiffness, stroke volume, and left ventricular ejection fraction, the primary hemodynamic determinants of diastolic BP include total peripheral resistance, heart rate, arterial stiffness, and systolic BP (Tanaka et al., 2016). It is important to note that postexercise hemodynamic responses to acute endurance exercise bouts differ between men and women. Endurance-trained women typically experience postexercise hypotension due to a persistent reduction in systemic vascular resistance, whereas endurance-trained men showed a substantial drop in cardiac output with unchanged systemic vascular resistance (Senitko et al., 2002). Therefore, the prolonged reduction in peripheral vascular tone may explain the postexercise hypotension in our subjects.

Exaggerated hypotension may be unfavorable for normotensives. Orthostatic hypotension, a hemodynamic malregulation, is often observed following endurance exercise (Mundel et al., 2015) and in women (Bryarly et al., 2019), mainly due to peripheral blood pooling and insufficient offset by increases in cardiac output. It may precipitate lightheadedness, dizziness, fatigability, and presyncope (Ma et al., 2024). Fortunately, no one complained of unfavorable hypotension-related symptoms. We could speculate on the contribution of tachycardia as an underlying mechanism. The increase in heart rate lasted 4 h after the match, as did the decrease in the D/A ratio. In addition, the individual response of the D/A ratio is inversely correlated with that of heart rate, as shown in Figure 5. Thus, the increase in heart rate may partly compensate for the prolonged reduction in systemic vasodilation after the soccer match, not to drop BP substantially.

In contrast to the D/A ratio, the B/A ratio did not change significantly during the observation period. The B/A ratio seems to be associated with central arterial stiffness (Westerhof et al., 1972a; Hashimoto et al., 2002), which typically reduces 30–60 min after moderate-intensity endurance exercise (Kingwell et al., 1997). Since we did not assess SDTPG indices within a couple of hours after the soccer match, it remains uncertain whether the B/A ratio might decrease tentatively and return to the baseline level by 4 h post-match. Our previous research on well-trained male collegiate endurance runners demonstrated increased arterial stiffness and elevated aortic systolic and pulse pressures after a 7-day intense endurance training camp (Tomoto et al., 2015; 2018), reflecting heightened vascular tone after short-term, intense endurance training. However, the current study suggests a gradual decrease in peripheral vascular smooth muscle tone without significant changes in central arterial stiffness after repeated soccer matches. Taken together, these findings imply that peripheral vascular tone is more sensitive to physical exertion than central arterial stiffness in endurance-trained females.

A brief series of matches in young female football players induced the repeated, temporary attenuation of arterial wave reflection, which was associated with reduced diastolic BP and a compensatory increase in heart rate.