In cyclic sports, such as swimming, running, or rowing, after the start of the race, the changes in metabolic rate are rather large and fast, forcing the cardiorespiratory system to respond promptly and precisely to prevent large variations of arterial blood gas and acid-base status (Burnley and Jones, 2007). The pulmonary oxygen uptake (V˙O2) kinetics is considered to be a useful, non-invasive measure of the integrated capacity of the organism to transport and utilize O₂ in order to support the increased rate of muscular energy turnover. It provides an important assessment of the physiological response of the athlete (Jones and Carter, 2000; Burnley and Jones, 2007). When constant-load exercise is performed at moderate intensity, i.e., below the first ventilatory threshold (VT), after the cardiodynamic phase, the V˙O2 rises mono-exponentially until a steady state is achieved. In heavy intensity domain, i.e., above the VT, the attainment of a steady state in V˙O2 is delayed by a supplemental rise in V˙O2 or “slow component” (Barstow and Molé, 1991; Borrani et al., 2001; Carter et al., 2002). The time constant of the V˙O2 response (τ_p; time for completion of 63% of the response) within the transition between two levels of energy requirement is typically around 20–35 s for young healthy subjects (Borrani et al., 2001; Carter et al., 2002; Poole and Jones, 2005).

Numerous studies have described the V˙O2 kinetics parameters within constant load cycling or running exercise (Whipp and Wasserman, 1972; Carter et al., 2000, 2002; Borrani et al., 2001; Murias et al., 2014), however, V˙O2 kinetics in swimming is considerably less studied (Rodriguez et al., 2003; Filho et al., 2012; Reis et al., 2012a,b; Sousa et al., 2015). In well-trained swimmers, the values reported for τ_p in heavy and severe intensities are similar to those reported in running and rowing (Reis et al., 2012a,b; Espada et al., 2014; Sousa et al., 2015). Similarly to other sports, faster V˙O2 kinetics in swimming have been associated with performance in middle distance events (Rodriguez et al., 2003; Reis et al., 2012b; Espada et al., 2014).

It has been shown that women have lower respiratory and cardiovascular capacities than their male counterpart. Namely, women have smaller stroke volumes, cardiac outputs, arterial oxygen content, hemoglobin and less red blood cells concentration than men in rest and in submaximal exercise both in absolute values or relative to body surface values (Wiebe et al., 1998; Wheatley et al., 2014). Furthermore, women also present smaller lung volumes, lower resting lung diffusion capacity and lower maximal expiratory flow rates (Harms, 2006). Therefore, it is not surprising that men present higher absolute V˙O2max and produce higher power outputs than women (Wiebe et al., 1998). Although the lower cardiac and respiratory capacities of women in rest and exercise could induce smaller O₂ delivery and utilization to the muscle, and consequently, slower V˙O2 kinetics than men, women also present an enhanced blood flow to the working muscle, namely they present higher blood flow and vascular conductance for the femoral blood flow in steady state exercise (Parker et al., 2007). Conversely, in the forearm exercise the vasodilatory responses do not differ between men and women (Limberg et al., 2010). Furthermore, it should be acknowledged that the microvascular O₂ delivery could be a determinant factor in the V˙O2 kinetics response, which is not expected to be influenced by sex (Murias et al., 2014). Thus, although the current physiological models seem to support the similarity the response between men and women, there are still some sex induced differences that can potentially influence the oxygen uptake kinetics response. Furthermore, since in swimming the training groups frequently include both men and women, it is of most importance to verify the possible differences in order to increase the specificity of the training bouts.

The characterization of V˙O2 kinetics in females has been considerably less studied than in males and the literature has provided conflicting results regarding the differences between sexes (Fawkner and Armstrong, 2003; Murias et al., 2010, 2011a). While for heavy exercise prepubertal boys present faster V˙O2 kinetics and smaller contribution of the slow component than maturational age matched girls, there are no differences in V˙O2 kinetics between male and female children and adults in moderate exercise (Fawkner and Armstrong, 2003). Also in moderate exercise Murias et al. (2010, 2011a) reported similar V˙O2 Kinetics for men and women of similar fitness levels. Conversely, recently Lai et al. (2016) reported that adolescent women had slower V˙O2 and heart rate kinetics in moderate and heavy intensity cycling and presented a higher slow component for the latter.

In swimming, the literature has mainly presented data for male swimmers: to the best of our knowledge, only one study has characterized separately the V˙O2 kinetics for female swimmers (Rodriguez et al., 2003). However, these researchers only studied 4 female swimmers, describing the V˙O2 kinetics for race-pace velocities in 100 and 400 m front crawl swimming.

Since the control for V˙O2 kinetics across exercise domains can be attributed to different physiological mechanisms (Carter et al., 2002; Poole and Jones, 2005), it is important to study the influence of sex on V˙O2 kinetics both below and above the ventilatory threshold. Additionally, to date, most studies in swimming described the oxygen uptake kinetics above VT, i.e., for heavy, severe and extreme exercise (Sousa et al., 2011; Filho et al., 2012; Reis et al., 2012a,b). To the best of our knowledge only two studies analyzed the V˙O2 kinetics response in moderate swimming and used only one repetition (Sousa et al., 2013) and/or reported the values obtained in an incremental protocol (de Jesus et al., 2015). The comparison between τ_p for moderate and heavy exercise, using multiple exercise transitions has yet to be done, which can be useful to describe the control of on-kinetics in such exercise modality. Furthermore, in swimming most of the water-training is performed at these intensities and the volume performed at such intensities is significantly correlated with performance (Mujika et al., 1996).

Therefore, the aim of the present study was to compare the V˙O2 kinetics response of male and female trained swimmers during moderate and heavy intensity exercise. We hypothesized that the V˙O2 dynamic response is faster in males than in females in moderate and heavy intensity swimming.

The swimmers responses obtained in the incremental test and in the square-wave transitions are given in Tables 1, 2, respectively. In the incremental test, male swimmers showed higher absolute and relative V˙O2max and vV˙O2max (p = 0.000). Conversely, male and female swimmers presented similar values of maximal heart rate and maximal lactate concentration, as well as VT expressed as a percentage of V˙O2max.

V˙O2 responses for representative subjects in square wave transitions to Moderate and Heavy swimming are presented in Figures 1, 2. There was a significant effect of increased intensity in A_p, EE V˙O2, EE HR and EE La, for both sexes (p = 0.000 for all parameters). For moderate exercise, the female swimmers presented lower relative values than men in A_p (p = 0.03) and EE V˙O2 (p = 0.04). The Gain A_p (p = 0.01) and EE gain for heavy exercise (p = 0.000) and EE gain for moderate exercise (p = 0.00) was significantly lower for females. However, τ_p and td_p were not influenced by either intensity (p = 0.804 and 0.326) nor sex (p = 0.527 and 0.908). Additionally, the A_sc' was not different between men and women (p = 0.476).

The main finding of this work is that the time constant for the primary component of the V˙O2 kinetics response is similar between female and male trained swimmers in square transitions for both moderate and heavy intensity swimming.

Up to now, only one study has reported the V˙O2 kinetics separately for female swimmers (Rodriguez et al., 2003). Even so, the sample size was just 4 swimmers, only one transition per intensity was performed and it was limited to transitions for maximal intensity in 100 and 400 m race pace efforts. Therefore, the novelty of this study relies on the description of the V˙O2 kinetics in moderate and heavy intensity in highly trained swimmers of both sexes.

When absolute values were considered, male swimmers presented higher Ap at moderate and heavy intensities. However, we observed that only, A_p at moderate intensity was smaller in women. This difference is not surprising, since it has been shown that women have a lower energy cost than men at low intensity (Pendergast et al., 1977), which the authors credited to the differences in body size and velocity, both inducing higher values of drag for men. Furthermore, we also found a decreased gain, both for the primary component (in heavy swimming) and end-exercise in females. Since the gain reflects the energy consumption corrected for the distance, this fact could also be associated with the higher swimming economy of women.

Contrary to our hypothesis, we did not find differences in the primary phase time constant between sexes in either moderate or heavy swimming. It has been reported that women have smaller hearts, smaller stroke volumes, cardiac outputs and hemoglobin concentration than men (Wiebe et al., 1998). Furthermore, they have smaller lung volumes and lower maximal expiratory flow rates (Harms, 2006). This author suggested that during work above 80% of VO₂max women are more susceptible to fatigue and exercise induced arterial hypoxemia. However, Murias et al. (2013) in a recent study reported that, in a maximal ramp test in the cycle ergometer, women presented higher O₂ extraction for the same relative intensity than men, presenting a less effective matching of the O₂ delivery and O₂ utilization, which seem to translate impairments in blood flow. However, said lower oxygen delivery to the muscle and the cardiac and respiratory characteristics of women does not seem to affect the oxygen uptake kinetics in moderate and heavy swimming. Nevertheless, since men present higher absolute amplitudes for the primary component with similar time constants, the gross rate of increase of oxygen uptake per second is higher in men, suggesting a quicker onset. This could be due to the higher maximal oxygen uptake and larger muscle mass presented by the male swimmers.

Whereas the comparison between male and females remains to be thoroughly addressed in the literature for other exercise modalities, our results are in accordance to what was reported for middle age subjects in cycle ergometer, for heavy and moderate exercise (DeLorey et al., 2005; Connor et al., 2012) and for children and adults in moderate exercise (Fawkner and Armstrong, 2003). One of the reasons that can explain the fact that τ_p for transitions both below and above VT are similar between women and men of similar training backgrounds is that the intensity was not high enough for the oxygen delivery to the active muscle to be sufficiently compromised. In addition, since all the swimmers were highly trained and of similar performance level, the time constant of the oxygen uptake kinetics was likely already minimized (Murias et al., 2014) and, therefore, differences between male and female swimmers within our relatively homogeneous group of study participants were not detected.

Our results confirm the existence of a slow component, only for swimming intensities above the VT, similarly to what previous literature has reported for other sports (Carter et al., 2000, 2002). There were no differences in the slow component between male and females, both relative to body weight and to the end-exercise V˙O2 kinetics.

The values observed for τ_p for heavy intensity swimming are similar or somewhat faster than those reported from previous studies from the literature (Filho et al., 2012; Reis et al., 2012a,b; Espada et al., 2014). Namely, the average of the time constant for all our subjects was 14.9 ± 4.2 s, whereas previous studies reported τ_p of 17.3 ± 5.4 s, 15.8 ± 4.8 s and 16.9 ± 3.9 s for the same exercise intensity (Bentley et al., 2005; Reis et al., 2012a,b). However, these studies only evaluated male swimmers, whom in our study presented a τ_p of 16.0 ± 4.5 s for heavy intensity swimming. For higher intensities, namely race pace or severe swimming, a wider range of τ_p has been reported: from 10.5 ± 2.5 s for a 200 m race pace swimming bout in elite athletes (Sousa et al., 2011) to 21 ± 3 s for the square-wave transition to vV˙O2max (Sousa et al., 2015). Due to the supine position and predominance of upper body use we could expect slower V˙O2 kinetics in swimming, however, surprisingly, most of the studies conducted in swimming, including the present, reported a kinetics similar to the upright exercise modalities (Sousa et al., 2011, 2015; Filho et al., 2012; Espada et al., 2014). One may speculate that the specific training adaptation surpass the possible impairments caused by the body position and muscle mass involved in swimming.

No differences were found in the τ_p between moderate and heavy intensity swimming in trained swimmers. Our work is in agreement with previous studies conducted in cycling and running that reported similar τ_p across exercise intensities (Barstow and Molé, 1991; Carter et al., 2002). According to Poole et al. (Poole and Jones, 2005) said invariant time constant across exercise intensities is generally attributed to the fact that the V˙O2 kinetics is restricted by metabolic inertia rather than oxygen delivery, since the oxygen delivery is compromised without similar decrease in the adaptation of the aerobic response. Therefore, since the time constant for heavy swimming is not slowed when the oxygen requirements of the exercise increases above the VT, one may suggest that in trained swimmers, with fast oxidative responses, the metabolic inertia, and not oxygen delivery, restricts V˙O2 kinetics at both moderate and heavy intensities. However, due to methodological constrains that restrict the measurements of muscle oxygenation and blood flow in swimming, the invariance of time constant cannot be categorically attributed to metabolic inertia, since for heavy intensity swimming the oxygen availability could not be sufficiently compromised to affect the immediate response of the aerobic system. Furthermore, the constrained breathing pattern imposed in front crawl swimming could potentially influence the O₂ delivery when comparing swimming with terrestrial activities. We also must acknowledge the recent work of Murias et al. (2014) that suggested that in subjects with fast V˙O2 kinetics, such as the swimmers participants in these study, the intracellular control mechanisms are mainly responsible for the rate of adjustment of oxidative phosphorylation, representing an improved matching of O₂ delivery (or distribution) to O₂ utilization, a parameter that is was not possible to determine in our study. Therefore, the invariant time constant between exercise domains could also be a consequence of an improved vascular responsiveness and vascularization of the muscle caused by years of swimming training in these intensities.

This study also shows an unexpected increase in both the gain for the primary component and end exercise gain, between moderate and heavy swimming. This relative increase in energy cost could be associated with the increase in drag, which, contrary to other exercise modalities, induces a cubic relationship between energy expenditure and velocity (Pendergast et al., 1977). Of importance is that, from moderate to heavy domain, the increase in velocity and then in drag was similar between men and women.

The exercise intensities used in our study are common in swimming training (Mujika et al., 1996). One of the practical applications of these findings for training is that coaches can prescribe interval-training similarly for both male and female swimmers of identical training background, since the most important V˙O2 kinetics parameters (i.e., time constant of the primary component and amplitude of the slow component) do not appear to be different for the same relative intensity. However, since there is a large interindividual variation in the V˙O2 response to swim interval training (Bentley et al., 2005), it is important for the coaches to have an individual evaluation of each swimmer, especially at the elite level.

Despite the encouraging results, some limitations are presented and should be considered: Although we acknowledge that the small number of subjects underpowered the present study, one may observe that the present sample size is within the usual range for similar studies with highly trained athletes.

We did not control the female swimmers for the menstrual cycle phase. However, Gurd et al. (2007) reported that there were no differences in either V˙O2 nor muscle deoxygenation kinetics between the follicular and luteal phases of the menstrual cycle in active women.

Future research should be conducted to refine this limitations.

This study was the first to compare the V˙O2 kinetics between male and female trained swimmers in moderate and heavy intensity swimming, which are the most frequent intensities prescribed during training by swimming coaches. In highly trained individuals, the time constant of the primary component was not significantly different between sexes in both intensity domains and was also independent from swimming intensity. However, due to the instrumental limitations imposed by swimming exercise, namely, the inability to use oximetry in the water, we could not verify the physiological mechanisms responsible for this results.

Conceived and designed the experiments: JR, GM, and FA. Performed experiments: JR. Analyzed data: JR, VV, and PB. Interpreted results of research: JR, GM, VV, and FA. Drafted manuscript and prepared tables/figures: JR and PB. Edited, critically revised paper and approved final version of manuscript: JR, GM, PB, VV, and FA.

The first author gratefully acknowledges the “Fundação para a Ciência e Tecnologia, Portugal” (“The Foundation for Science and Technology, Portugal”) for their post-doctoral fellowship award (reference number SFRH/BPD/84315/2012). The results of the present study do not constitute endorsement of the mentioned instruments by the authors or the journal.