Acute ventilatory responses to swimming at increasing intensities

Introduction

Breathing is a natural and fundamental human behavior that allows the exchange of respiratory gases between the lungs and the atmosphere. When we are under stress, as physical exercise, minute ventilation ( $\dot{\mathrm{V}}$E) increases (Pelarigo et al., 2016; Tipton et al., 2017) due to the respiratory frequency (f_R) and tidal volume (V_T) rise. During an incremental exercise, f_R increases non-linearly and V_T tends to present a plateau, with the $\dot{\mathrm{V}}$E rise at lower intensities depending on both f_R and V_T increases. The further growth in $\dot{\mathrm{V}}$E at higher exercise intensities seems to be explained by the increase in f_R, a phenomenon known as the tachypneic breathing pattern (Sheel & Romer, 2012; Nicolò, Marcora & Sacchetti, 2020). Despite the well-established knowledge on the f_R and V_T contributions for $\dot{\mathrm{V}}$E increase during an incremental exercise, further research focusing on the different regulatory mechanisms that drive these contributions is welcome (Figueiredo et al., 2013; Tipton et al., 2017).

Central command, muscle afferent feedback and metabolic inputs are the major $\dot{\mathrm{V}}$E behavior determinants despite acting with different timings when exercise intensity changes (Forster, Haouzi & Dempsey, 2012; Duffin, 2014; Tipton et al., 2017). The regulation of f_R and V_T is less studied but it was previously suggested that the inputs driving $\dot{\mathrm{V}}$E act separately on these variables, with central command and muscle afferent feedback preferentially regulating f_R (Amann et al., 2010; Nicolò et al., 2017), while metabolic responses are responsible for the V_T regulation (Nicolò et al., 2017). Considering the great importance of the central command on f_R control and the close association between breathing patterns, exercise modes and limbs movement (Sheel & Romer, 2012; Forster, Haouzi & Dempsey, 2012), it is of great importance to understand how the ventilatory response adapts to different exercise related constraints.

In swimming, the aquatic environment imposes significant restrictions on the human body, such as the increase of the hydrostatic pressure around the chest, resulting in an augmented work of the inspiratory muscles (Lomax & McConnell, 2003; Leahy et al., 2019). In addition, the swimming typical horizontal position leads to face immersion and, consequently, to restricted ventilation (Holmér et al., 1974; McCabe, Sanders & Psycharakis, 2015). These restrictions oblige swimmers to synchronize active inspiratory and expiratory phases with movements of upper and lower limbs, resulting in specific swimming breathing patterns (Leahy et al., 2019). Front crawl is the most common (in training and competition conditions) from the four swimming conventional techniques, with swimmers more generally inspiring on every two or three upper limbs actions, i.e., using unilateral and bilateral breathing patterns (Seifert, Chollet & Allard, 2005; Figueiredo et al., 2013). Despite the existing variability of the V˙E responses along the different intensity domains, particularly when using the front crawl technique (Ribeiro et al., 2015; Monteiro et al., 2022), the f_R and V_T behaviors are still scarcely studied when swimming at increasing paces.

Since further research on the f_R and V_T responses is necessary to improve the overall understanding of breathing physiology and ventilatory control, we have aimed to assess the acute ventilatory responses when swimming from low to extreme intensity domains. For achieving that purpose, swimmers were required to wear a breathing snorkel attached to a gas analyzer along a standard incremental front crawl protocol. Complementarily, we aimed to characterize the swimmers breathing patterns along the exercise intensity rise to understand if the synchronization with the upper and lower limbs motion is maintained even when using the respiratory snorkel, i.e., without constraining the inspiratory and expiratory phases. We have hypothesized that: (i) despite the respiratory constraints, gas exchange variables increase concomitantly with the swimming velocity rise, with f_R and V_T presenting a nonlinear increase and a stabilization (respectively); and (ii) swimmers keep the breathing patterns used in free swimming when breathing into a snorkel (due to the breathing synchronization with stroke rate).

Materials and Methods

Data analysis

The pulmonary gas exchange and ventilation data were examined to exclude occasional errant breaths (eventually caused by swallowing, coughing or signal interruptions). For the analysis, only the values of oxygen uptake ( $\dot{\mathrm{V}}$O₂) between ±3 SD were considered (Monteiro et al., 2020), which were then smoothed using a three breaths moving average and 10 s time average (Fernandes et al., 2012). The mean values from the last 30 s of exercise per step were selected and conventional physiological criteria were applied to stablish the maximal oxygen uptake $\dot{\mathrm{V}}$O₂max; (Howley, Bassett & Welch, 1995; Zacca et al., 2020). The lactate-velocity curve modelling method, through the determination of the interception point of the best fit of a combined linear and exponential pair of regressions, was used to determine the individual anaerobic threshold (Carvalho et al., 2020; Monteiro et al., 2022). Using the $\dot{\mathrm{V}}$O₂max and the anaerobic threshold as physiological indicators, the following intensity domains were identified (Fig. 1): (i) the low and moderate domains, corresponding to two steps below and the step at the anaerobic threshold; (ii) the heavy and severe domains, matching the step below and the step where $\dot{\mathrm{V}}$O₂max was elicited; and (iii) the extreme domain, allocated to the maximum 100 m at the end of the incremental protocol (Fernandes et al., 2012; de Jesus et al., 2015; Ribeiro et al., 2017). Swimmers breathing patterns were determined by calculating the ratio between f_R and SR.

Results

Depending on the swimmer, the establishment of the low and moderate intensity domains corresponded to the swimming velocity between the first-third and third-fifth steps (respectively), while the heavy, severe and extreme intensity domains corresponded to the sixth, seventh and maximal last protocol steps (in this order). The low, moderate, heavy, severe and extreme efforts were performed at 1.04 ± 0.11, 1.13 ± 0.11, 1.22 ± 0.10, 1.26 ± 0.10 and 1.39 ± 0.11 m∙s⁻¹ (respectively) and all the physiological and performance variables are presented in Fig. 2. With the swimming intensity rise (Z_4.68 = 305.79, ${\eta }_{\mathrm{p}}^2$ = 0.95, p < 0.001), f_R, [La⁻] and SR increased ( ${\eta }_{\mathrm{p}}^2$ = 0.81, ${\eta }_{\mathrm{p}}^2$ = 0.95 and ${\eta }_{\mathrm{p}}^2$ = 0.88, respectively; p < 0.001) and SL decreased ( ${\eta }_{\mathrm{p}}^2$ = 0.59, p < 0.001). $\dot{\mathrm{V}}$O₂, $\dot{\mathrm{V}}$E, $\dot{\mathrm{V}}$CO₂ and HR increased from low to severe intensities ( ${\eta }_{\mathrm{p}}^2$ = 0.91, ${\eta }_{\mathrm{p}}^2$ = 0.88, ${\eta }_{\mathrm{p}}^2$ = 0.89 and ${\eta }_{\mathrm{p}}^2$ = 0.83, respectively; p < 0.001), but all stabilized at extreme exertion and V_T presented similar values from low to severe exertions and decreased at the extreme intensity (p = 0.006). The f_R/SR ratio increased from low to moderate (p = 0.01) and from moderate to heavy domains (p = 0.02) and lower values were observed at extreme compared to severe intensity (p = 0.02; Fig. 3).

Discussion

The main purpose of the current study was to assess swimmers acute ventilatory responses when performing front crawl from low to extreme intensities. As hypothesized, the values of the selected gas exchange variables increased along with the swimming intensity rise (until the severe intensity domain) and f_R and V_T presented a nonlinear increase and a plateau (respectively) in the incremental protocol. Concurrently, we aimed to analyze swimmers breathing pattern behavior as swimming pace was rising, being observed an f_R/SR ratio increase until reaching the severe intensity domain and a decrease at extreme exertions. This does not confirm our initial hypothesis that the front crawl breathing pattern was going to remain stable when swimming with a respiratory snorkel.

It is well established that the 7 × 200 m front crawl intermittent incremental protocol allows collecting capillary blood for [La⁻] analysis (Fernandes et al., 2005; Pelarigo et al., 2016; Carvalho et al., 2020) and, together with the gas exchange assessment, ensures a complete physiological characterization of the low, moderate, heavy and severe intensity domains (de Jesus et al., 2015; Zacca et al., 2019; Monteiro et al., 2022). The current results are similar to those previously presented for the low-severe swimming intensity domains when using the same methodological approach, particularly regarding V˙O₂ (Fernandes et al., 2012; de Jesus et al., 2015), $\dot{\mathrm{V}}$E and f_R (Pelarigo et al., 2016; Monteiro et al., 2022), and [La⁻] values (Štrumbelj et al., 2007; Sousa, Vilas-Boas & Fernandes, 2014; Monteiro et al., 2022).

However, most official swimming events (such as the 50, 100 and 200 m distances) occur at the extreme intensity domain, and is why a complete swimming ventilatory characterization should also include extreme efforts. This is a fundamental training zone for excelling competitive swimmers performances where the exertions are so intense that fatigue occurs and exercise ends before $\dot{\mathrm{V}}$O₂max can be reached (Hill, Poole & Smith, 2002; Ribeiro et al., 2017). Studies focusing on the anaerobic capacity development are very scarce, with swimmers acute ventilatory responses remaining almost unexplored, justifying the inclusion of a 100 m maximal bout at the end of the front crawl incremental protocol. This maximal intensity short duration effort, when swimming up at the standard 7 × 200 m step protocol, allows swimmers to have their physiological profile fully characterized.

The current ventilatory results at the maximal 100 m front crawl evidenced lower $\dot{\mathrm{V}}$O₂ and V˙E values compared to those reported for the same intensity domain (Sousa, Vilas-Boas & Fernandes, 2014), probably due to the sample characteristics (higher level and male swimmers only) and the higher swimming velocity achieved (the current 100 m bout was part of an incremental protocol instead of an isolated rectangular test). Additionally, higher f_R values were observed at the current extreme intensity domain compared to a maximal 200 m front crawl bout performed at a lower swimming velocity (Štrumbelj et al., 2007). This demonstrates that the extreme exertions (only inferiorly delimited by the $\dot{\mathrm{V}}$O₂max) can include a wider range of swimming velocities, being important to consider them when comparing the results obtained at this intensity domain.

Swimming faster implied a $\dot{\mathrm{V}}$O₂, V˙E, f_R, $\dot{\mathrm{V}}$CO₂, [La^-] and SR increase and a SL decrease from low to severe exertions, as previously described (Figueiredo et al., 2013; de Jesus et al., 2015; Monteiro et al., 2022), while $\dot{\mathrm{V}}$O₂, V˙E, $\dot{\mathrm{V}}$CO₂ and HR stabilized from severe to extreme intensities (Sousa, Vilas-Boas & Fernandes, 2014). The attainment of $\dot{\mathrm{V}}$O₂max at severe intensity paces, and the fact that these abovementioned variables are highly related, explain the maintenance of similar values despite the swimming intensity rise (Sousa, Vilas-Boas & Fernandes, 2014; Nicolò et al., 2018; Monteiro et al., 2022). HR at $\dot{\mathrm{V}}$O₂max corresponded to 90.4% ± 3.1% of its maximum, in accordance with the secondary criteria used to confirm $\dot{\mathrm{V}}$O₂max (Howley, Bassett & Welch, 1995; Zacca et al., 2020). Maximal [La⁻] values were observed at extreme exertions, where the energy production is highly dependent on anaerobic metabolism, with a higher production of lactate and, consequently, its progressive accumulation in the bloodstream (Hargreaves & Spriet, 2020). Contrarily to what was described (Sousa, Vilas-Boas & Fernandes, 2014), our [La⁻] values increase from severe to extreme intensity domains is explained by a bigger velocity rise (~10% instead of 5%), corroborating the existence of a wide range of swimming velocities at this domain.

The observed V˙E, f_R and V_T behaviors along the low to extreme swimming intensity domains spectrum corroborates what is described in the literature, independently of the exercise modality performed (Amann et al., 2010; Nicolò et al., 2017). This seems to indicate that, regardless of the swimming movements and the different constraints imposed by the aquatic environment, the central command, muscle afferent feedback and metabolic inputs have the same influence on their regulation along the intensity domains spectrum (Štrumbelj et al., 2007; Sheel & Romer, 2012; Forster, Haouzi & Dempsey, 2012). However, the swimming intensity rise resulted in the selection of different breathing patterns at each intensity domain. Diversely to what was initially expected, the f_R/SR ratio tended to increase until the heavy intensity domain, indicating that swimmers took advantage of free breathing while using the respiratory snorkel (Štrumbelj et al., 2007). The f_R/SR ratio decrease from severe to extreme intensity domains can be justified by both the maximal intensity and the short time duration effort of the 100 m exertion, where SR increased more than f_R (16% vs. 8%, respectively). In addition, the lower increase in f_R compared to SR seems to indicate that this extreme effort is characterized by moments of apnea.

Conclusions

The f_R and, consequently, the f_R/SR ratio values were influenced by the use of the respiratory snorkel and its interpretation may be different compared to free swimming. However, this is the only methodology that provides a real time and breath-by-breath assessment of the swimmers ventilatory responses. In conclusion, by proposing the addition of a maximal effort at the end of the front crawl intermittent incremental swimming protocol, the current study provides a novel framework of the acute ventilatory responses to the large spectrum of swimming intensity domains, particularly at the extreme exertion, used both in training and competition contexts. V˙O₂, V˙E and V˙CO₂ stabilized, and V_T decreased, from severe to extreme intensity domains, differently to what happened from low to severe exertions, while f_R and SR increased along the swimming intensities spectrum. The breathing pattern varied along the incremental protocol and its synchronization with stroke rate was not verified when using the respiratory snorkel.

Supplemental Information