Study strategies used and the study limitations
The study strategy was set at aiming to provide evidence and to elucidate the swimming specific issues, which can affect the clinical testing quality in highly fit competitive swimmers.
This study had two main goals. The first goal was to elucidate the effects of water immersion on pulmonary function in highly fit swimmers and our second goal was to demonstrate evidence of the ventilation levels required to provoke a reaction in the airways of highly trained swimmers. With these goals this study´s design strategy was to answer these goals by generating swimming-specific evidence and did not aim to imitate clinical testing.
A limitation of this study was that only healthy subjects were studied. Testing swimmers with asthma would have demanded further resources as more specialized personnel including physician specialized in sports and exercise medicine to be present in the testing. We thought to extend our study in the future testing swimmers with respiratory symptoms depending on the findings of this study. Another limitation was that, during swimming the water resistance during exhalation could not be achieved, as the testing tubes did not cause resistance.
Effects of a water environment on the pulmonary function in competitive swimmers
The transition from land and a vertical sitting body position into a prone swimming body position in water caused a 1–8 % decrease in spirometry variables, which may look minor. However, even a small effect of water immersion in water on pulmonary function may be essential, in swimmers with allergic sensitivity, pulmonary disease and a lowered the ratio of forced expiratory volume in one second (FEV1) and forced vital capacity (FVC); FEV1/FVC. That may be essential also especially with the extreme demands of competition speed swimming put upon ventilation function.
Water immersion showed gender differences in forced vital capacity FVC and the ratio of forced expiratory volume in one second (FEV1) and forced vital capacity (FVC); FEV1/FVC. The decrease in FVC was significantly greater in males by a mean of 6 % in comparison to females with a mean of 1 % (Table 2). Body composition with better buoyancy in female swimmers and smaller lungs may be the causes for the result .
A higher reporting of respiratory symptoms in asthmatic female swimmers in previous study  was presumably due to their smaller lungs compared to asthmatic males and FEV1/FVC change, which in turn, decreases more in females in transition from land into water. Furthermore, the opposite situation may occur. Larger lungs in male asthmatic swimmers with a minor decrease in FEV1/FVC in connection to water immersion connects to no reporting of respiratory symptoms during swimming .
The additional effect of a water environment may be a trigger for respiratory symptoms. For example, a previous study on swimmers’ baseline spirometry findings on land showed that a 4 % decrease in FEV1/FVC associated significantly with the reporting of respiratory symptoms during swimming . A similar association in endurance athletes was reported by Dickinson et al. .
The measured effect of swimming on pulmonary function depends on the performed measurement protocol; on land or in water
The outcome of the effects of swimming on pulmonary function are different if measuring it on land or in water. Swimming caused an approximately 4 % increase in forced expiratory volume in one second (FEV1) in water suggesting swimming-induced bronco-dilation. However, the findings measured on land had no difference in FEV1, which does not suggest any bronco-dilatation occurred (Fig. 2). This finding may be due to water immersion-induced parasympathetic activation, which may affect pulmonary function. Pöyhönen and Avela  demonstrated water immersion-induced parasympathetic neural activation. This may explain why the swimming-induced bronco-dilatation in the water is not observable, when measurements are performed after swimming on land.
It is notable that even though the mean swimming-induced change in FEV1 differed approximately 4 % between land and water measurements, the SD`s overlapped (Fig. 2). Thus, even though the differences between measurements on land and in water look minor at 1–8 %, the difference of the spirometry results on land and in water was clear. Therefore, the results suggest a separate interpretation.
Pulmonary ventilation during swimming at competition-speed intensity
During swimming when the intensity increases, the ventilation increases mainly by the VT, because the timing of breathing is dependent on the upper limb cycle. This is different from the exercise on land, where pulmonary ventilation increases in relation to the intensity of physical exercise on land during free breathing. During exercise the ventilation is increased first by increasing tidal volume (VT) and then by increasing the breathing frequency (fb) .
There were no sex differences in breathing frequency, but the VT was about one liter smaller in females. In both, in females and males, the VT was about half of the measured FVC.
The swimming-specific breathing pattern, which is restricted according to the upper limb cycle, causes swimmers to inhale their lungs fuller in comparison to breathing during physical exercise on land, and this may influence lung compliance and the work of breathing. However, the resistance of water during exhalation and hydrostatic pressure may compensate for that.
Special observations for clinical testing in competitive swimmers
In clinical testing, a certain level of achieved pulmonary ventilation is used for triggering airway reactivity and respiratory symptoms. The target ventilation during clinical exercise challenge testing is 60 % of the calculated maximal voluntary ventilation (cMVV) . The questionnaire survey of 412 competitive swimmers showed that the competition speed intensity swimming triggered the most respiratory symptoms. The measurements in this study showed that the ventilation levels during that same intensity, were 80 and 76 % of (cMVV). Thus, at the same intensity, the minute ventilation was approximately 15–20 % higher than is typically used as a target ventilation in clinical exercise challenge tests on land. This finding may be crucial, when respiratory symptoms in elite competitive swimmers are examined in clinical trials. Correspondingly, if highly fit competitive swimmer performs a challenge test on level of 60 % of cMVV then that would be a ventilation level of zone II intensity swimming which is moderate intensity and triggers significantly less respiratory symptoms as shown in the Fig. 1.
The exercise challenge test is typically performed on a treadmill or bicycle ergometer for 6–8 min with a target heart rate of 80–90 % of predicted maximum of 220 minus the age, and ventilation should reach 40–60 % of the predicted cMVV (FEV1 × 35). It is preferable to maintain the target ventilation for at least 4 min during the challenge test . However, the findings by Rundell et al. (2001)  and this study suggest that the higher exercise intensity near maximal effort may be required when elite athletes are tested.
In eucapnic voluntary hyperventilation (EVH) testing, the target ventilation is 85 % of cMVV , which is closer to the ventilations observed during swimming at the competition speed.
Findings in a study by Pedersen et al.  are consistent with those found in this study. They studied different challenges such as EVH, a field-based exercise test by a swimming competition race, a laboratory-based exercise test and a methacholine test for competitive swimmers. They found that both EVH testing and a field-based exercise, a competition race challenge, showed post-challenge FEV1 fall the most positive for airway hyperresponsiveness .
Thus, the level of exercise intensity and ventilation during an exercise challenge may play an essential role. Carlsen et al.  demonstrated the importance of exercise load during an exercise challenge. In their study, 40 % of the studied children had a diagnostic FEV1 fall of greater than 10 %, when the exercise intensity was 85 % of calculated maximum exercise load, but all 100 % of the studied subjects had a positive finding when the exercise load was 95 % of the calculated maxim . That load would be considered close to the intensity swimming at competition speed.
Theoretically, a ventilation level of greater than 75–80 % of cMVV, which was obtained at very high intensity competition speed swimming, would be more appropriate for highly fit swimmers. That is consistent with the results for elite athletes in previous studies [24, 27].
Furthermore, Schwartz et al.  suggested that during exercise the ventilation rate and total ventilation strongly relate to the broncho-responsiveness in elite athletes.
The time duration, in their opinion, seemed unimportant .
In previous study during increased pulmonary ventilation, cooling and drying of airways, during increased pulmonary ventilation at high intensity physical exercise, are reported to be the stimuli for respiratory symptoms in previous studies [31,32,33] demonstrated an exercise-induced asthma (EIA) reaction, that is directly proportional to the thermal load on the airways and that the reaction is quantifiable in term of respiratory heat exchange (Deal et al. 1979). However, during swimming, the thermal loss is minimal, because the inhaled air is warm and humid. Therefore, swimming is considered beneficial for breathing.
Swimming as a low astmogenic sport induces fewer symptoms than running or cycling . Therefore, especially testing during swimming in high fitness competitive swimmers, a higher intensity and more severe provocation, than typically used, may be required to trigger airway reactivity and symptoms.
Finally, the majority of the previous studies reports that exercise-challenge testing was performed without measuring the ventilation. However, the ventilation level may be crucial for the quality and to successfully perform the test. The findings of this study suggest, that observing the ventilation function could help to determine the desired exercise intensity for exercise-challenge tests especially for highly fit swimmers with developed pulmonary function [3, 35].