Maximal oxygen uptake (VO_2max), ventilatory anaerobic threshold (metabolic threshold), and respiratory compensation point (RCP) are important physiological measurements used to determine aerobic fitness, training adaptations, and training planning in soccer players (Cunha et al., 2016). Metabolic threshold (MT) based on gas exchange measurements is considered the point at which the contribution of anaerobic metabolism to overall metabolism begins to increase during incremental exercise (Wasserman et al., 2012). When MT is exceeded during incremental exercise testing, non-oxidative CO₂ production increases due to H⁺ buffering by blood bicarbonate (Wasserman et al., 2012). As exercise intensity increases further, the buffering system eventually becomes overwhelmed, causing a decrease in blood pH and further stimulation of hyperventilation (Binder et al., 2008; Oshima et al., 1998; Wasserman et al., 2012). This induces a second ventilatory threshold called the respiratory compensation point (RCP) (Meyer et al., 2004; Whipp et al., 1989). Physiologically, RCP marks the failure of the body’s buffering mechanism (Meyer et al., 2004). The period between MT and RCP is called the isocapnic buffering (IB) and represents the compensation phase for exercise-induced metabolic acidosis (Takano, 2000; Whipp et al., 1989). The period between the RCP and the exhaustion point of the exercise is called the hypocapnic hyperventilation (HHV) phase (Chicharro et al., 2000; Oshima et al., 1998).

The IB phase is assumed to be an indicator of cardiopulmonary function in athletes, healthy individuals, and patients with cardiac diseases (Yen et al., 2018). Although the IB phase is considered an indicator of endurance performance in healthy individuals and athletes (Oshima et al., 1998), it appears to be more associated with performance in activities that lead to a pronounced elevation in blood lactate and accompanying metabolic acidosis (Bentley et al., 2005). The range of the IB phase is related to factors such as buffering capacity, lactate kinetics, and sensitivity of the carotid bodies (Whipp et al., 1989). These factors are adaptive variables, and their response to physical training depends on exercise planning (e.g., intensity and volume) (Edge et al., 2006; Hasanli et al., 2015). Therefore, the IB phase can be a useful tool for coaches and physiologists to characterize metabolic adaptations induced by a specific training regimen (Hasanli et al., 2015). Although the IB phase decreases with age, it is longer in trained athletes independent of age (Lenti et al., 2011). While differences in respiratory response to maximal exercise have been documented between children and adults (Prado et al., 2010; Cooper et al., 1987; Weston et al., 2021), the IB phase has not received attention in children.

Ventilatory responses to exercise in the pediatric population appear to be maturity-dependent (Giardini et al., 2011; Rowland and Cunningham, 1997). Compared to teenagers, younger children have been reported to ventilate more during exercise to eliminate a given amount of CO₂ to maintain PaCO₂ at a lower level (Cooper et al., 1987; Gratas-Delamarche et al., 1993; Nagano et al., 1998). Smaller body size, smaller CO₂ storage capacity, and immaturity of the respiratory center are considered responsible for age-related differences (Ohuchi et al., 1999; Cooper et al., 1987; Weston et al., 2021). Compared to adults, children show a greater increase in VE relative to VCO₂ during exercise, reflecting lower ventilation efficiency (Takken et al., 2020; Giardini et al., 2011). Children’s respiratory centers have been suggested to be more sensitive to CO₂ during exercise than adults (Gratas-Delamarche et al., 1993; Nagano et al., 1998). However, ventilatory regulation related to the change in acid-base balance induced by lactic acidosis has been shown to be more efficient in boys than men during strenuous exercise (Ratel et al., 2002).

Although it has been shown that aging reduces the IB phase (Lenti et al., 2011), to our knowledge, the IB phase in children has not been studied previously. Children may exhibit different IB phases compared to adults due to children’s immature glycolytic capacity (Reybrouck et al., 1985; Fellmann and Coudert, 1994). On the other hand, children have a higher ventilatory response during exercise (Cooper et al., 1987; Gratas-Delamarche et al., 1993), which may enhance the IB phase by assisting in the respiratory compensation of pH. We hypothesized that instead of VO_2max, child soccer players might extend the IB phase by using the compensatory mechanisms mentioned above and increase their exercise tolerance compared to youth soccer players. This study aimed to examine the respiratory response and IB phase during incremental exercise in child and youth soccer players.

When the maturation status of child players was evaluated noninvasively, it was determined that they were in the pre-PHV period. VO_2max, maximal VE, and exercise duration were higher in youth players compared with child players (p < 0.05) (Table 1). MT and RCP occurred at a similar percentage of VO_2max in the two groups (p > 0.05). The absolute and mass-adjusted relative VO₂ and VCO₂ values at MT, RCP, and maximal were higher in youth players than in child players (p < 0.05). While relative VO₂ and VCO₂ at baseline were higher in child players than in youth players (p < 0.05), the absolute values did not differ between the two groups (p > 0.05). VO₂ (mL min⁻¹ kg^−0.75), expressed according to allometric scaling, was higher in youth players than in child players at MT, RCP, and maximal, while it was higher in child players than in youth players at baseline (p < 0.05). RER values were not different between child and youth players (p > 0.05). The HR values were higher at baseline in child players and at MT in youth players (p < 0.05) and were not different between the two groups at RCP and maximal (p > 0.05).

Relative VE was higher in child players at baseline (p < 0.05), but there was no difference between the two groups at 50%MT, MT, 50%∆MT-RCP, RCP, and maximal (p > 0.05) (Figure 2). Youth players had greater relative TV than child players at 50%MT, MT, 50%∆MT-RCP, and RCP (p < 0.05). Child players had higher respiratory frequency than youth players at baseline, MT, 50%∆MT-RCP, RCP, and maximal (p < 0.05). Relative O₂ pulse was higher in child players at baseline and higher in youth players at 50%MT, 50%∆MT-RCP, RCP, and maximal (p < 0.05).

Child players had lower PETCO₂ and higher PETO₂, VE/VCO₂, and VE/VO₂values compared to youth players during exercise (p < 0.05), with no difference between the two groups at baseline (p > 0.05) (Figure 3). There were no significant differences between child and youth players in RCP/MT VO₂, absolute and relative IBP, and HHP (p > 0.05) (Table 2). VE/VCO₂ slope and VE/VCO_2RCP slope were higher in child players than in youth players (p < 0.05).

The main findings of the present study are as follows: 1) child players showed an IB phase similar to youth players, 2) child players had lower aerobic capacity than youth players, and 3) child players had different ventilatory responses to incremental exercise compared to youth players. The results confirmed our hypothesis and indicated that child players exhibited similar IB phases despite having lower relative VO_2max compared to youth players. MT and RCP occurred at a similar VO_2max fraction in child and youth players, suggesting that both thresholds significantly impact the IB phase.

Respiratory frequency, VE/VCO₂ slope, ventilatory equivalent for CO₂ and O₂, and PETO₂ values of child players were higher, while TV and PETCO₂ values were lower than youth players. These findings may be interpreted as child players’ lower ventilation efficiency during exercise than youth players (Ohuchi et al., 1999; Takken et al., 2020). Similar to the findings of previous studies (Prado et al., 2010; Gratas-Delamarche et al., 1993), the VE/VCO₂ slope and the VE/VCO₂ were found to be higher in child players than in youth players. Even though VE relative to body mass was not different between the two groups, lower relative VO₂ and VCO₂ values of child players throughout exercise indicate increased ventilatory response during exercise. Consistent with our findings, lower TV in children compared with adults during exercise is compensated by a higher respiratory frequency (Takken et al., 2020; Rowland and Cunningham, 1997; Prado et al., 2010). The lower PETCO₂ observed in child players in this study compared to youth players during incremental exercise is consistent with previous studies (Prado et al., 2010; Ellis et al., 2017; Nagano et al., 1998; Cooper et al., 1987; Weston et al., 2021). Higher central ventilatory neural drive for children may lead to higher ventilatory rates for a given metabolic demand (Gratas-Delamarche et al., 1993).

The high VO_2max values of youth players indicate that they had a better aerobic capacity than child players. VO_2max may increase with biological maturation in regularly exercising individuals (Paterson et al., 1987; Baxter-Jones et al., 1993). Since child players have a shorter soccer training history than youth players, their lower VO_2max may be an expected finding. Two other important variables related to aerobic fitness, MT and RCP, occurred at similar VO_2max fractions in child and youth players. Consistent with our findings, studies have shown that relative RCP values in boys are similar to youth men (Cunha et al., 2008) or men (Klentrou et al., 2006), corresponding to approximately 80%–91% of VO_2max (or VO_2peak) (Weston et al., 2021; Cunha et al., 2008; Klentrou et al., 2006). In children, MT varies widely, with approximately 58%–73% of VO_2max (Cunha et al., 2008; Paterson et al., 1987; Reybrouck et al., 1985; Mahon et al., 1998; Mahon and Vaccaro, 1989). However, active boys have a higher ventilatory anaerobic threshold than less active boys (Reybrouck et al., 1985; Weymans and Reybrouck, 1989). Although some studies have reported that MT occurs at a higher percentage of VO_2max in children than in adults (Klentrou et al., 2006; Weston et al., 2021), other studies similar to our study found no difference in MT between groups (Mahon et al., 1998; Cunha et al., 2016). Individuals’ training status and training history from childhood appear to play an important role in MT values (Paterson et al., 1987). Although youth players had a twice higher weekly training volume than child players due to more training sessions, the relative intensity of each soccer training was similar. The metabolic pathways used during training sessions and matches were probably equivalent in terms of physiological strain. Since relative MT and RCP were similar between child and youth players, the physiological mechanisms that identify these thresholds may differ for these age groups, and exercise intensity may be the main determinant of these two variables.

During the IB phase, both aerobic and anaerobic systems simultaneously produce energy at varying rates (Hasanli et al., 2015). If the aerobic metabolic rate is relatively high after MT, the duration of the IB phase may be longer due to the slow increase in metabolic acidosis (Yen et al., 2018). The IB phase of youth soccer players is consistent with previous research on trained men (Lenti et al., 2011; Chicharro et al., 2000; Eryılmaz et al., 2018; Korkmaz Eryılmaz et al., 2018). We found that even though youth players had higher VO_2max, the absolute and relative IB range was similar to that of child players. In addition to that, the ratio of RCP VO₂ to MT VO₂ can be used to compare buffering capacity between groups of different ages (Kominami et al., 2022), and we did not find any significant difference in this ratio between youth and child players in our study. The present and previously published data (Hasanli et al., 2015; Korkmaz Eryılmaz et al., 2018; Röcker et al., 1994) indicate that the VO_2max value may not be enough to interpret IB and buffering capacity.

Physical training may directly affect the IB phase by changing MT and RC (Kominami et al., 2022; Lenti et al., 2011; Chicharro et al., 2000; Eryılmaz et al., 2018). Although there was no statistically significant difference in our study, child players tend to have a larger IB phase due to their lower relative MT than youth players. High-intensity training may enhance the IB phase by shifting RCP with minimal or no change in MT (Eryılmaz et al., 2018; Oshima et al., 1998). Several studies have shown higher lactate accumulation during the IB phase in anaerobic-trained athletes than in endurance-trained athletes (Hasanli et al., 2015; Röcker et al., 1994). Although we did not directly measure blood lactate, the maximal RER may reflect the lactate concentration (Stringier et al., 1995). Unlike adults, children have a muscle metabolic profile better equipped for the aerobic energy pathway rather than the glycolytic energy pathway (Fellmann and Coudert, 1994). Therefore, maximal RER values of children at the end of exercise can be expected to be lower than adults. RER values at the same relative exercise intensities did not differ significantly between child and youth players, and our data is consistent with other studies (Ohuchi et al., 1999; Giardini et al., 2011). Even though the training history of the child players was lower than that of the youth players, both groups were training specifically for soccer. It is known that regular physical training can increase the glycolytic capacity of skeletal muscles in boys (Eriksson, 1972; Paterson et al., 1987). Since youth soccer players have a more extended training history than child players, similar RER values indicate that training modalities are important for determining metabolic profiles.

Ratel et al. (2002) demonstrated a smaller decrease in blood pH relative to an increase in lactate in boys compared with men during repeated sprint exercise. This relatively small increase in blood [H⁺] in boys was explained by higher ventilatory regulation related to the change in acid-base balance compared to men during repeated sprints (Ratel et al., 2002). Children may be better able to tolerate lactic acidosis with a higher ventilatory response for a given CO₂ (Ratel et al., 2002). This may explain the mechanism of similar IB phases between child players with lower aerobic capacity and youth players. Future studies are necessary to explain this physiological phenomenon in child and youth athletes with matched aerobic capacities or specific training programs.

Child players had lower aerobic capacity and hyperventilatory response to exercise than youth players. MT and RCP occurred at similar VO_2max fractions in child and youth players, and both of these groups showed similar IB phases. Respiratory frequency, ventilatory equivalent for CO₂ and O₂, and PETO₂ values of child players were higher than youth players. Hyperventilation reduced PETCO₂ values, which may correspond to a reduction of PaCO₂. Therefore, a higher ventilatory response during exercise may help children buffer pH. Longitudinal studies are needed to confirm the responsiveness of the IB phase to physical training in children.