Endurance training produces a physiological adjustment response in the cardiovascular, musculoskeletal and haematological systems. However, the structural and functional properties of the lungs do not change significantly in response to training.1,2

Exercise in the terrestrial environment does not provide sufficient stimulus for the remodelling and growth of the lungs.3 The biological adaptation of the human body to exercise has not found a way within the laws of physics to increase lung functionality,3 and the evidence is clear that the lungs do not well adapt to increased fitness.4 Consequently, the lungs could be a limiting factor for performance within the extraordinary physiology developed by highly trained athletes.

The diffusing capacity of the lungs for carbon monoxide (DLCO) provides an integrated representation of the mechanisms involved in the transfer of O2 from atmospheric air to the pulmonary capillaries,5 and provides a universally accepted measure of gas diffusion in the lungs.6 During high-intensity exercise, pulmonary diffusion can be limited, leading to an increase in the alveolo-capillary oxygen difference and exercise-induced arterial hypoxaemia,7 justifying the study of DLCO in high-performance sport.

The scientific literature has not extensively described the changes in DLCO in elite water-based athletes. While land-based sports do not alter DLCO,8 water-based sports have been associated with increased lung capacity and diffusion, including swimming,9 artistic swimming10 and free diving.11

Swimming has been described as one of the healthiest forms of physical activity, providing complete development of the body.12 Unlike other sports, and despite the lack of mechanical impact, swimming involves both locomotor and respiratory muscles in a demanding way. Moreover, certain conditions associated with swimming have an impact on the lungs and airways that could favour a physiological or pathophysiological adjustment in the pulmonary system (Fig. 1). These conditions may be present with greater or lesser intensity, depending on the particular exposure to swimming and the structural conditions of each individual.

Figure 1.

Factors inherent to the practice of swimming related to the stress on respiratory function.

(0.21MB).

It has been reported that a horizontal body position and water immersion stimulate an increase in lung capacity and diffusion. During exercise in the aquatic environment, swimmers are also exposed to the hydrostatic forces produced by the water, which necessitate stronger inspirations that improve the strength of the inspiratory muscles13 and longer breathing cycles, mimicking intermittent hypoxic training in which hypercapnia and hypoxia occur.14

This divergence between land and water sports may also have implications for the treatment of respiratory diseases and has been studied in the treatment of lung diseases such as asthma15 and chronic obstructive pulmonary disease (COPD),16 with positive lung function outcomes demonstrated in patients who practiced aquatic exercise.17 These results are important considering that DLCO is an important predictor of mortality in the general population.18

The clinical management of lung diseases can also be improved based on physiological assessments of elite athletes, who demonstrate physiological optimization at the highest level.19 As a result, the aim of this study was to evaluate the alveolo-capillary diffusion and lung volume of different elite aquatic athletes: swimmers, artistic swimmers and water polo players.

This study shows that aquatic athletes have higher lung capacity and gas diffusion than the reference population for their same height and age. By sport, male swimmers have higher lung capacity and diffusion than water polo players, while female artistic swimmers have higher lung capacity than water polo players.

The nature of the three water-based sports presented is different, but they all share, to a greater or lesser extent, movement in the water through swimming. It has previously been reported that swimmers have larger lungs and better diffusion capacity than land-based athletes or untrained subjects,9,22,23 but to the best of our knowledge, this is the first study analysing aquatic athletes of different modalities with a considerable number of subjects (n=64). An augmented lung capacity increases the area available for gas exchange and improves flotation in the water, decreasing resistance to progression24 and promoting performance in this sport.

Immersion of the body in water provokes a well-known physiological response named the “immersion response”.25 The immersion response (diving response) induces changes mediated by a vagal parasympathetic response in the cardiovascular and respiratory systems. On the one hand, there is a redistribution of peripheral blood flow towards the central circulation, and, on the other hand, peripheral vasoconstriction, increased blood pressure and decreased cardiac output.26 All this together with the forces caused by the hydrostatic pressure of the water against the chest wall and the high resistance of the airways leads to an increase in the work of breathing during swimming compared to other disciplines such as cycling.27

The mechanics of breathing in swimming are complex, combining a forced-inspiration phase with the head out of the water with a prolonged exhalation phase with the head in the water. Therefore, swimmers must establish a controlled breathing pattern, which must be coordinated with the stroke mechanics and the swimming stroke.9 Swimmers, and to a lesser extent water polo players, alternate short periods of apnoea with almost maximum inspirations while performing a long-duration exercise with high metabolic involvement. Artistic swimmers, in contrast, have long periods of apnoea for a shorter period of time, according to their artistic routine. Both set of conditions cause a low respiratory rate and high pulmonary vascular pressure that can stress the respiratory system through hyper-expansion of the chest wall, hypercapnia, hypoxia and mechanical loading.28,29

The exact mechanism by which lung volumes and diffusion capacity improve in swimmers is not known. However, all the aquatic athletes in this study had extremely well-developed lung capacities (female – VA: 6.00±9.38L; 135% and male – VA: 7.88±1.48L; 124%), and an increased diffusion capacity (female – DLCO: 135% and male – DLCO: 148%). It remains to be elucidated whether these parameters are the result of genetic endorsement or swimming training, although the second explanation seems to be more plausible. It has been reported that intensive swimming training in pre-pubescent children causes isotropic lung growth and better development of the airways and alveolar space.30 Some metabolic pathways related to genes activated by hypoxia and mechanical strain in the alveolar capillaries seem to be related to lung growth. Both the lung expansion to TLC24 and the exposure to repeated apnoea in the water3 that occur during swimming could be the reason why swimmers,9 free divers11 and female swimmers10 have greater lung capacity and diffusion than land-based athletes.

However, Armour et al.22 demonstrated that the diffusion capacity is higher in swimmers due to their larger ribcage and a larger number of alveoli compared to runners and control subjects. However, the aquatic athletes in the current study had a higher KCO than reference values in both females (115±12%) and males (119±13%), suggesting that elite aquatic athletes also have a higher gas transfer coefficient in the lungs for a given alveolar surface.

During swimming, the requirements of rapid sub-maximum inspirations from the functional residual capacity (FRC) to TLC, for short periods of time, as well as the ventilatory restriction in each respiratory cycle during swimming, constitute a conditional stimulus of intermittent hypoxia and mechanical loading, already described as the main stimuli for the improvement of breathing.3,31 In this study, the pulmonary parameters of swimmers and artistic swimmers exceeded those of water polo players. This coincides with the fact that both swimmers and artistic swimmers are exposed to a greater extent to the factors that promote improved lung function, such as the abovementioned mechanical strain and intermittent apnoea, suggesting that, in addition to the aquatic environment itself, the type of activity that takes place in the water is also important for improving lung function.

In this context, it has been reported that there is no additional benefit from respiratory muscle training as long as the swimmers train on an elite level basis.9 Our interpretation of this phenomenon is that swimming training in itself is already a sufficient stimulus for the development of the respiratory muscles’ strength, thus improving the pulmonary function of the practitioners. Therefore, it remains possible that other athletes or even patients with respiratory insufficiency could benefit from this characteristic of swimming by improving their functional lung capacity as well as their overall fitness. This is a factor to consider when performing rehabilitation of respiratory diseases in the aquatic environment.

In addition to the aquatic environment, the stroke mechanics combined with a low respiratory rate and intermittent apnoea could improve the prognosis of some lung diseases such as asthma15 and COPD,16 although those studies did not use swimming as an exercise method but instead used a combination of strength and mobility exercises in the aquatic environment (aqua-gym). It has also been shown that the distance walked in the shuttle endurance test improved more after an exercise programme in the aquatic environment than on land.17

Evidence is currently limited regarding whether training in the aquatic environment offers more benefits than training in the terrestrial environment for improving aerobic capacity. Future studies should include swimming in these aquatic exercise protocols, due to the beneficial effects of mechanical stress and intermittent hypoxia, in addition to strength and mobility exercises, to increase the beneficial effects of aquatic therapy on lung function.

One of the main challenges of pulmonary physiology is to understand the mechanisms underlying lung plasticity, and to find ways to improve structural and functional capabilities.32 Accordingly, the study of swimming exercise as a conditional model could be useful.

The elite aquatic athletes studied had larger lungs and better lung diffusion capacity than their reference population for the same age and height. Therefore, the practice of sports in the aquatic environment could help improve lung function. Specifically, male swimmers had better lung capacity and diffusion than water polo players, and female artistic swimmers had better lung capacity than female water polo players. Future research should include a longitudinal analysis to understand the impact of the practice of different water-based and land-based sports on lung function. The results of such studies would be relevant to the therapeutic exercises applied in the general population and to the rehabilitation of lung function in people with pathologies.

The authors declare that they don’t have any conflict of interests.