The performance of young athletes training in competitive swimming depends on their circulatory and respiratory abilities, skeletal muscle energetics, and somatic traits, which are developed simultaneously with maturation and training. Therefore, testing and then proper training with appropriate intensity, corresponding to physiological changes, supporting the current development of the body and specific motor skills, constitute a priority. In addition, acting in a harmonious manner ensures long-term progress focused on the development of an adult elite swimmer (Pichardo et al., 2018).

In young swimmers, in the period of building up the foundations of endurance, aerobic capacity assessed as maximal oxygen uptake ( V ˙ O 2 max ) is the key to successful conditioning (Olbrecht, 2015). In the swimmer’s energy profile, the ability of the body to produce energy from aerobic vs. anaerobic metabolism in 100-m races, considered sprints in swimming, can be more than half (Hellard et al., 2018). Maximal aerobic power assessed in different V ˙ O 2 protocols is among the best predictors of swimming aptitude and concerns the most common Olympic 200-m races. Based on the latest reports, it can be concluded that the protocols for assessing aerobic capacity include a graded test procedure, with the measurement of the anaerobic threshold and then reaching V ˙ O 2 max (Sokołowski et al., 2021). Other tests, most often performed in 400-m all-out swimming, assessed fast ability and also the fast component of reaching V ˙ O 2 max during heavy or severe intensities (Costill et al., 1985; Costill et al., 1992; Lätt et al., 2009; Zacca et al., 2019). In swimming competitions, the aerobic energy system participates in the overall energy production from the first part of the all-out swimming effort of 400-m (Rodríguez et al., 2003), 200-m (Figueiredo et al., 2011; Sousa et al., 2011), and even 100-m races (Ribeiro et al., 2015; Hellard et al., 2018) or 60 s (Sokołowski et al., 2022a). To control V ˙ O 2 toward its peak, close to its maximum at 60 s (Gastin and Lawson, 1994), such extreme efforts are used for swimmers’ conditioning evaluation.

In competitive swimming, developing the fast component of V ˙ O 2 through intensive aerobic training is sustainably accompanied by the conditioning of muscle strength needed for a swimmer to generate propulsion in appropriate stroking, which is at last manifested by a correspondingly longer stroke length (SL) and stroke index (SI) (Hellard et al., 2018). The ability of 13-year-old swimmers to develop fast 60-s V ˙ O 2 kinetics can be useful in races longer than 100 m, i.e. 200-m (Sokołowski et al., 2022a) or 400-m races, enabling them to open the race with more ease by reducing the initial oxygen deficit and simultaneously reducing excessive fatigue.

During exercise, the fast phase of V ˙ O 2 response to transition from rest to exercise may be shorter in trained than in untrained persons; the shorter time constant (τ) for the fast phase of V ˙ O 2 was linked with better exercise performance (Arend et al., 2021). A significant positive correlation between the fast component and V ˙ O 2 max was observed (Ingham et al., 2007; Arend et al., 2021), but previous studies also reported no relationship between the fast component and V ˙ O 2 max (Pringle et al., 2003; Reis et al., 2012). The differences in fast V ˙ O 2 , as evidenced in peer swimmers competing together in the same race, can be seen as one of the deterministic factors useful for practitioners. These differences may be noted not only in elite adult swimmers; it is equally interesting to see how they are exhibited within a given age group, as growth, development, and maturation may be masked or may be more intensely expressed than those caused by training (Baxter-Jones et al., 1993). Perhaps it is very difficult to completely separate the effects of maturation and training on the development of swimming skills. Nevertheless, one can assess the relationship between the biological age (BA) and the level of features determining swimming ability, which can be developed with appropriate intensity at the age of puberty and later (Strzała et al., 2019; Zacca et al., 2019; Fiori et al., 2022).

Nevertheless, to gather useful results to evaluate young swimmers’ strength potential and physiological endurance, tethered swimming can be used as a valid and reliable tool to measure exerted forces in water, where simultaneously physiological variables are not significantly different from free swimming effort of similar duration (Morouço et al., 2014; Ruiz-Navarro et al., 2020).

This study aimed to assess differences in BA of 13-year-old swimmers and show their ability, as biologically younger or older, to develop fast 60-s V ˙ O 2 kinetics and tethered swimming strength. In addition, the interplay between swimming strength and V ˙ O 2 was examined. Furthermore, the impact of these indices on performance in a 400-m front crawl race was determined, including stroke kinematics. It can be hypothesized that BA is a factor making a difference in fast V ˙ O 2 kinetics and strength development, with further influence on swimming performance.

In this study, the division of swimmers into early-mature (older BA) and late-mature (younger BA) groups by presenting the most important variables ( V ˙ O 2 and F _ave), which explained their impact on swimming results, was justified. The groups did not differ significantly in terms of date of birth (t = −1.27, p = 0.213, Cohen d = 0.44); however, for these groups (early mature, n = 13 and late mature, n = 23), BA was significantly different (t = −9.82, p < 0.001, Cohen d = 3.41). In the second analysis, the assumption of homogeneity of variances was not met (F = 7.12, p = 0.012), and applying the robust version of the t-test did not change the results.

The older—early-mature group had higher V ˙ O 2 values than the late-mature group (Figure 3) (F = 6.75, p = 0.02, eta² = 0.17). The two-factor (6 × 2) ANOVA results revealed that V ˙ O 2 in the 10-s periods was significantly higher in the early-mature group of swimmers from the third ( V ˙ O 2 _21-30) to the sixth V ˙ O 2 _51-60) period. The differences (0.23 and 0.61 L min^–1) between the groups were not significant for V ˙ O 2 _1-10 (F = 1.02, p = 0.32, eta² = 0.03) and V ˙ O 2 _11-20 (F = 3.07, p = 0.89, eta² = 0.08), i.e., the first and second periods of the 1-min tethered test.

When analyzing the groups separately, in the late-mature group, the ANOVA results showed no difference from the first to the third V ˙ O 2 10-s periods (1 vs. 2: F = 0.97, p = 0.33, eta² = 0.03; 1 vs. 3: F = 2.01, p = 0.16, eta² = 0.06; 2 vs. 3: F = 2.42, p = 0.13, eta² = 0.07), whereas in the early-mature group, all the 10-s periods were statistically different.

F _ave (Figure 4) in the 10-s periods of the 1-min tethered test was different (two-factor ANOVA, 6 × 2) between the BA groups and across the subsequent 10-s periods within the late-mature and the early-mature swimmers.

Considering V ˙ O 2 during the 1-min tethered test, expressed in ml∙min^–1∙kg^–1∙N⁻¹, (Figure 5), the variance analysis showed a general difference between the late-mature and early-mature groups (F = 4.82, p = 0.011, Cohen d = 0.12). Furthermore, the ANOVA results of the particular 10-s periods (two-factor ANOVA, 6 × 2) between the late-mature and early-mature swimmers presented no significant differences in the average V ˙ O 2 values expressed in ml∙min^–1∙kg^–1∙N⁻¹ for period 2: 0.09 (ANOVA F = 1.10, p = 0.30, Cohen d = 0.03), period 3: 0.01 (ANOVA F = 0.02, p = 0.90, Cohen d = 0.01), and period 4: 0.07 (ANOVA F = 0.51, p = 0.48, Cohen d = 0.01). The differences in average V ˙ O 2 values in both the late-mature and the early-mature groups were significant from the first to the sixth 10-s period.

An increased, ranging from low to very strong, correlation was noted between V ˙ O 2 and F _ave in the subsequent 10-s periods in the entire group of swimmers (n = 36) (Table 1).

V ˙ O 2 _1-10 was not significantly correlated with BA, the force indices of the 1-min tethered test, and the 400-m front crawl speed or kinematics (Table 2). The subsequent 10-s V ˙ O 2 periods had a higher relationship with force indices and SL. V ˙ O 2 _21-30 was highly correlated with the swimming speed (V _total).

The subsequent 10-s periods of F _ave showed a significant moderate relationship with the swimming race results of V _surf, V _total, and SL and a strong or very strong relationship with BA (Table 3).

As depicted in Table 4, there was a significant moderate correlation between the 400-m swimming speed (V _surf and V _total) and SL kinematic and V ˙ O 2 : V ˙ O 2 _1-60 (2.51 ± 0.97 L min^–1), V ˙ O 2 _31-60 (3.15 ± 1.11 L min^–1), and V ˙ O 2 _41-60 (3.47 ± 1.05 L min^–1). The correlations were strongest with the tethered force (F _ave1-60). SR kinematics presented a relationship with V _surf close to significance (p = 0.09).

Additionally, V ˙ O 2 kinetic variables V ˙ O 2 _1-60, V ˙ O 2 _31-60, and V ˙ O 2 _41-60 were correlated with abilities corresponding with the beginning of a 400-m race, i.e., the first 100-m lap (1.35 ± 0.09 m s^–1). These coefficients were significant (0.38, p = 0.02; 0.36, p = 0.03; 0.35, p = 0.04, respectively). BA showed a strong or very strong relationship with the consecutive variables, but its correlation with SR kinematics was negative.

This study aimed to evaluate the oxygen kinetics during a 1-min tethered test in young swimmers with different BAs. It was hypothesized that oxygen kinetics might be different in participants with different BAs. Calendar age-homogeneous 13-year-old swimmers were significantly differentiated in terms of biological development. Dividing them in accordance with BA into early-mature and late-mature groups revealed their differences in V ˙ O 2 kinetics (Figure 2). In the 1-min tethered test after the first two 10-s periods of V ˙ O 2 , the next four 10-s periods were significantly higher in the early-mature group than the late-mature group. The early-mature group revealed a stepwise V ˙ O 2 increase, with each subsequent 10-s V ˙ O 2 being significantly higher. The late-mature group exhibited a not-significant increase in the first three 10-s periods of V ˙ O 2 . In the examined 1-min tethered test, the production of a specific swimming pulling force (F _ave) decreased significantly in each 10-s period in both BA groups and was higher in the early-mature group. The decrease in F _ave in both groups was almost parallel, although it could be expected in the late-mature group; the lower general level of V ˙ O 2 kinetics, especially in the third period, would enhance this decrease in F _ave. These results suggest, however, that late-mature swimmers, owing to their well-trained aerobic power and higher gain in V ˙ O 2 , were able to bring a slight decrease in F _ave, similar to the early-mature peers, presenting a higher O 2 relative usage (ml∙min^–1∙kg^–1∙N⁻¹). It should be added that the lack of correlation between V ˙ O 2 and force, swimming speed, or kinematics and the lack of difference between both groups found in the first 10-s are most probably due to the high initial use of the anaerobic pathway in accordance with the almost instantaneous cardiac output increase, which is initiated by vagal withdrawal and the mechanical pumping action of the contracting muscles in the absence of altered arterial or venous O 2 contents compared to the level before beginning the test (Poole et al., 2012).

In this study, V ˙ O 2 in short term,1-min extreme intensity swimming between late-mature and early-mature swimmers of the same calendar age was compared, which showed that late-mature swimmers could be less aerobically economical, requiring a greater percentage of their peak V ˙ O 2 (per unit of body mass per unit of force). Being aerobically less economical, they were less efficient in the production of pulling force (F _ave), having a less developed or less mature metabolism to produce maximal anaerobic power (Van Praagh and Doré, 2002). The late-mature swimmers cannot be as efficient as their older counterparts because of their smaller “metabolic reserve,” as suggested by Bar-Or (1982). This remains in accordance with this study because of similar or higher relative O 2 usage (ml∙min^–1∙kg^–1∙N⁻¹) in an extreme swimming effort. The statement in this paragraph may be taken with caution, as early maturing swimmers were heavier and developed much greater thrust.

It can be hypothesized that, generally, a slower increase in V ˙ O 2 among late-mature swimmers during the short-term intense effort (i.e., in the first 30 s of 60 s) was most likely due to a lower percentage of employing or utilizing type II oxidative muscle motor units. This could be ascribed to their recruitment immaturity and to a higher type I muscle fiber contribution to prepubertal to postpubertal boys (Dotan et al., 2012). When the exercise intensity increases, human muscles exhibit organized recruitment adequately to force requirements, which demonstrates that higher-order fibers (i.e., type II) contribute more to force production at higher contraction frequencies (Beelen and Sargeant, 1993).

As explained by Poole et al. (2012), the available evidence supports a greater bias toward oxidative vs. glycolytic metabolism in children compared with adults (in the present study, late-mature vs. early-mature swimmers). This helps explain a greater percentage of O 2 usage vs. anaerobic sources in children despite their general lower potential for muscle O 2 delivery (resulting from a lower cardiac output) at a given work rate and V ˙ O 2 . Speeding the primary V ˙ O 2 response is essentially limited to exercise engaging the trained musculature and is generally associated with elevated oxidative enzyme activity within those muscles (Poole et al., 2012). The observed differences in fast oxygen kinetics could be caused by differences in muscle mass (resulting from different BAs) and muscle blood perfusion. Greater muscle mass recruitment could potentially impair muscle perfusion, especially during heavy exercise. If muscle perfusion were a limiting factor in V ˙ O 2 kinetics, this would result in a prolonged fast component when recruiting greater muscle mass (Arend et al., 2021).

In this study, the influence of the V ˙ O 2 p e a k on 400-m front crawl performance was examined, and a significant moderate relationship between the V ˙ O 2 _1-60, V ˙ O 2 _31-60, and V ˙ O 2 _41-60 indices and the swimming speed (V _surf or V _total) was revealed. Studies on competitive swimming have demonstrated that both aerobic and anaerobic metabolisms are important to swimming performance (Zamparo et al., 2000; Figueiredo et al., 2011; Kalva-Filho et al., 2015; Campos et al., 2017). In their meta-analysis of the gathered data, Birat and Ratel (2020) suggested a greater contribution of aerobic energy turnover in 50–400-m swimming races, e.g., 65%–75% and 80%–90% of the total energy turnover is metabolized during 200-m and 400-m races, respectively. In shorter swimming efforts, swimmers require high aerobic power even below 1 min, reaching almost V ˙ O 2 max at the finish wall of the race (Costill et al., 1992; Rodríguez et al., 2003; Lätt et al., 2009). On the other hand, Campos et al. (2017) found that 200- and 400-m swimmers’ performance involved higher total values of anaerobic contributions, considering 50-, 100-, and 800-m performance also; both distances (200 and 400 m) can be used to estimate the anaerobic capacity of swimmers and assess changes arising from specific training (Almeida et al., 2021). In addition, Kalva-Filho et al. (2015) demonstrated that total anaerobic capacity was highly associated with 400-m performance and, on the other hand, might be inversely related to the aerobic conditioning level but associated with V ˙ O 2 slow-component contribution and blood lactate concentration, which should be high (Massini et al., 2023).

It could be stated that short-duration extreme performance is dependent on fast V ˙ O 2 kinetics (Hellard et al., 2018; Sokołowski et al., 2022a), but this need was also discernible within the moderate exercise intensity domain, requiring 60% and 80% of the gas exchange threshold to produce higher power output in endurance-trained cyclists (Koppo et al., 2004). It was suggested that V ˙ O 2 kinetics may be attributed to other factors besides O 2 availability, such as the recruitment of higher threshold motor units. This is in agreement with former studies by Zhang et al. (1991), where V ˙ O 2 and V ˙ C O 2 kinetics were faster at each fraction of the work capacity in fitter subjects.

Pulling force results of the all 36 swimmers showed moderate (I _ave1-60) and high (F _ave1-60) correlations with 400-m front crawl performance and a high relationship with the main contributor of swimming speed, i.e., SL kinematics, similar to the case of V ˙ O 2 . Those 400-m main determinants were highly interrelated with the swimmers’ BA. This generally remains in agreement with another study, which presents a significant relationship between tethered swimming strength and middle distance front crawl performance (Morouço et al., 2012; Santos et al., 2016; Sokołowski et al., 2022a) and a simultaneous high relationship with V ˙ O 2 and BA (Sokołowski et al., 2022b) and can be related to body composition (Massini et al., 2021; Sokołowski et al., 2023). In the current study, SR kinematics, as a basic variable of swimming speed (correlation with V _surf close to significance, p = 0.09), was negatively correlated with BA and also with the most prominent determinants of 400-m performance, i.e., F _ave and V ˙ O 2 in the subsequent 10-s periods. This can suggest that less mature, late-mature swimmers may compensate for lower SL, F _ave, and V ˙ O 2 with a higher rate of stroking, which could be introduced in individualized training. It should also be emphasized that late-mature competitors can eliminate the advantage of early-mature competitors not only within the surface front crawl swimming zone analyzed in this study but also through techniques such as well-trained dolphin kicks in underwater swimming (Wadrzyk et al., 2021).

In future studies, skeletal muscle volume relating to performance could be calculated to assess the muscle’s effectiveness in producing force during tethered swimming. Considering similar tests closer to the natural conditions, e.g., in a flume, could be seen as an alternative to the static tests carried out in this work, which was its limitation (Ruiz-Navarro et al., 2020).

In conclusion, the 1-min tethered swimming test showed significant differences in the homogeneous calendar age/heterogeneous BA groups of swimmers. These were distinguished by a higher level of V ˙ O 2 kinetics and pulling force indices in early-mature swimmers. Lower aerobic power and a less efficient aerobic system in late-mature peers were also noted when considering relative O 2 usage during extreme intensity tethered swimming, with a higher relative O 2 expenditure per unit of body mass per unit of force (ml∙min^–1∙kg^–1∙N⁻¹). These differences, such as higher V ˙ O 2 kinetics and pulling force, were further translated into 400-m front crawl performance, including SL kinematics. Swimming training should support the development of these aptitudes in accordance with the BA level of swimmers.