Introduction

Exercise during hypoventilation may occur either by exercising at high altitude or during exercise at sea level. The control of the breath frequency during exercise is a training method used by many coaches to increase the athletes’ performance. Physical training with reduced breath frequency (RBF) focuses on the adaptation to the situation of hypercapnia, where the athlete keeps his breath for a longer duration during a swimming effort. This method is suggested for anaerobic adaptations [1] in aquatics sports such as free diving, synchronised swimming and finswimming.

Exercise with RBF causes hypoventilation and leads to reduced oxygen saturation and increases the heart rate (HR) and the cardiac output [2]. Some of the reported changes during exercise with RBF (i.e. hypoventilation) are similar to those that have already been reported after adequate duration and submaximal intensity of intermittent hypoxic training (IHT) [3]. In fact, a 4-week training period with RBF showed a reduced level of acidosis in the blood at an exercise intensity of 90% of the maximum heart rate [4]. Breath holdings and apneas or the level of exhalation are controlled during the training in the aquatic environment. Such a control requires specific equipment for continuous evaluation [5]. In this case, the RBF practice may be an alternative of hypoxic training, although hypercapnia may also occur under these exercise conditions [6].

The purpose of our study was to investigate the effect of sub-maximal freestyle leg kick of different breath techniques on the inspiratory muscles, heart rate and arterial oxygen saturation. We hypothesized that the different breath techniques [breath holding (BH) and intermittent breath holding (IBH)] could affect the finswimming performance.

Materials and methods

Participants

Ten young finswimmers that were selected from the local swimming club participated in this study (Table 1). The participants had a main aerobic training time of 8 h/week and their training age was 7.8 ± 1.8 years. The study was conducted according to the Helsinki Declaration for use in Human subjects (No. of Ethical Committee; 2-5/2.2.2011). All the participants’ parents submitted a written consent.

Table 1 Athlete’s characteristics
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Measures

The study design included repeated measures in a group. Independent variables were the breathing techniques: control treatment—normal breath frequency and/or self-selected breath frequency (NB), breath holding (BH: voluntary breath-holding as its breaking point, violent expiration, violent inspiration and breath-holding again the cycle of BH.) and intermittent breath holding (IBH: voluntary breath-holding as its breaking point, slow and deep expiration, violent inspiration and again the cycle of IBH). The dependent variables were the control parameters, maximum inspiratory pressure (PImax), heart rate (HR) and arterial oxygen saturation (SpO2)].

Procedures

Anthropometric characteristics body height, body mass (Seca 700), body mass index, body surface area [7] and percentage of body fat (7 skinfold points measurement, Harpenden) [8] and stages of biological maturation [9] were measured prior to the test procedures. The estimated strength of inspiratory muscles (PImax) was recorded by a MicroRPM portable device (Care Fusion, California, USA). The measurements were made in accordance with the ATS/ERS recommendation [10, 11]. SpO2 and the HR were measured by a pulse oxymetry (Nonin Onyx II 9550, Plymouth, MI, USA) at the end of all trials. A maximum single trial of 25 m of freestyle leg kick trials was first performed to calculate the finswimmers intensity at the 80% of their personal best. The time performance (time/s) was recorded with a digital handheld chronometer (Cei-Ultrak 499, Cardena, CA, USA).

The trials were performed with a difference of 24, 48 and 72 h between them. The swimming trials were performed at a submaximal intensity of 80% of personal best. The duration of each 25 m was approximately 25 s and the rest of the intervals were 25 s. All the sessions were performed in a 25 m indoor swimming pool with water temperature at 26 ± 1 °C and environmental temperature at 23 ± 1 °C. The evaluation was made between 14:30 and 16:30 p.m. The test parameters (PImax, HR and SpO2) were recorded immediately after the end of the tests. The tests were performed in random sequence on the breathing method (NB, BH and IBH) and athletes to avoid learning effects. The athletes did not warm-up before the 8 × 25 m tests.

Statistical analysis

The Kolmogorov–Smirnov test was used for the normality of the distribution. Analysis of variance for repeated measurements on one factor (1 group × 3 measurements × 3 parameters) with independent variable the breathing method (NB, BH and IBH) and dependent variables the control parameters (SpO2, HR and PImax). A Tukey post hoc test was used to locate any differences between means. The level of significance was set to p < 0.05 and the data are presented as mean value and standard deviation (Mean ± SD). The SPSS 15 statistical package (SPSS inc., Chicago, IL, USA) was used for the statistical analyses.

Results

Heart rate

Results showed differences between the three breathing techniques in HR immediately after the completion of the test. The IBH observed higher values in heart rate compare to BH and NB [F (2,40) = 1456, p < 0.001; IBH: 177 ± 4.2 bpm−1; BH: 165.7 ± 7.9 bpm−1; NB: 158.3 ± 2.2 bpm−1; Fig. 1].

Fig. 1
figure 1

Heart rate responses after the three breathing techniques (# p < 0.001)

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Arterial oxygen saturation

The results also showed differences between the three breathing techniques in SpO2 immediately after testing [F (2,21) = 153, p < 0.001, IBH: 88.2 ± 0.3%; BH: 93 ± 0.2%; NB: 97.1 ± 0.3%; Fig. 2]. The IBH observed higher desaturation compare to BH at 5% and NB at 5.3% and BH showed higher desaturation compare to NB at 10.3%.

Fig. 2
figure 2

Arterial oxygen saturation responses after the three breathing techniques (# p < 0.001)

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Maximal inspiratory pressure (PImax)

Additionally, PImax showed differences between the three breathing techniques [F (2,21) = 153, p < 0.001, IBH: 168.3 ± 5.3 cmH2O; BH: 166 ± 11 cmH2O; NB: 161.7 ± 11.4 cmH2O; Fig. 3]. The IBH observed higher values compared to BH at 2.3 cmH2O and NB at 6.7 cmH2O and BH showed higher values compared to NB at 4.3 cmH2O.

Fig. 3
figure 3

Maximum inspiratory pressure responses after the three breathing techniques (# p < 0.001, † p < 0.05)

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Time (s)

The time of trials was not different between the three breathing techniques (NB: 22.9 ± 1.5 s; BH: 23.1 ± 1.4 s and IBH: 22.7 ± 1.3 s, p > 0.05).

Discussion

Our study reveals that the heart rate, the arterial oxygen saturation and the maximal inspiratory pressure are affected differently following the three different swimming breathing techniques. The results of the present study indicate that the IBH applied to these young athletes positively affected the respiratory parameters associated with the power of inspiration and caused increased tolerance to hypoxia.

Previous studies have suggested improvements in the endurance capacity during short duration high intensity time trial performance in healthy individuals after specific respiratory muscle training [12]. In our study, the IBH method showed higher PImax, HR and SpO2 values compared to NB and BH methods. The differences in PImax, HR and SpO2 values are explained by the different intrathoracic pressure during the swimming trails. Progressive loading also ensures that an end-point is achieved in a timely manner without the need to control a breathing pattern. However, an important factor in testing the inspiratory muscles by using this method is the requirement of a learning period. A previous study has suggested that the threshold loading is associated with systematic changes in breathing patterns that act to optimize the muscle strength and increase the endurance. Task failure occurred when these compensatory mechanisms were maximal. Inspiratory muscles appeared relatively resistant to fatigue, which was late but persistent [13]. Following this learning period, the task is reproducible, thereby allowing comparisons from occasion to occasion in a particular group.

The significant improvement in the inspiratory muscles’ strength might have been due to the strength being constrained by an initial high baseline value in the respiratory muscle training group [14]. An alternative explanation is that the concurrent force (pressure-threshold loading) and resistance (voluntary isocapnic hyperpnoea) caused by the respiratory muscle training performed by individuals might have inhibited the strength development [15]. The PImax with IBH method increased 9% from the baseline. IBH exercises probably enhanced the inspiratory muscles and increased the PImax (Fig. 3).

After the probationary period, the IBH method showed an increased HR. That is likely to mean a higher intensity effort and this may justify the lower price of SpO2. In addition, the mechanisms which contribute to the breath holding by the IBH procedure affect the functions of the brain [16]. In other words, the controlled frequency breathing exercise activates the parasympathetic system, particularly the vagus nerve innervating the lungs [16]. The signs that are transferred through the vague nerve when the athlete develops volitional hypoventilation, cause desaturation and stimulate the central chemoreceptors of the retrotrapezoid nucleus in the brainstem. Increased PaCO2 decreases pH and shifts the oxygen–hemoglobin dissociation curve right and as a result also increases the frequency and the breadth (range) of respiration in order to return to normal through the procedure of hyperventilation (pH 7.4; SpO2 > 96%). The increase of the breathing frequency is a result of the need to provide O2 in tissues and excrete CO2.

Woorons et al. [17] found that IBH exercise causes activation of the anaerobic metabolism that leads to increased lactate production. The mechanism of action for the neutralization of lactic made from NaHCO3 plasma results in the production of the lactate solution and the release of CO2 which in turn incorporates the CO2 transported in the blood. The mechanism of hypercapnia (↓SpO2) increases the pulmonary ventilation over those levels necessary for adequate blood oxygenation. Probably the athletes of our study were influenced from the same mechanism which was activated during exercise with IBH and BH.

The arterial oxygen saturation following NB was within normal limits while lower levels occurred after BH and IBH (Fig. 2). Woorons et al. [17] reported that voluntary hypoventilation can cause severe hypoxemia due to the prolonged effort of hypoventilation at submaximal intensity. This happens because of the changes in the frequency and the depth of the breathing affecting the CO2 and O2 levels in the arterial blood and performance [18]. Athletes who train with IBH maintain longer hypoxic ability to exercise, displaying a tolerance to hypoxia [19] than athletes who do not use it enough [20]. Woorons et al. [17] observed that the exercise with infrequently breathing displays similar adjustments to those exhibited by athletes who train at altitude, such as increased activity of respiratory muscles and improve aerobic and anaerobic capacity.

Limitations of the study

In our study there were some limitations. There was no control group so the participants were not related to another group. Additionally, the participants were at the age of 15 years and possibly the different stage of puberty could affect their physical growth, biological maturation and behavioural development [21]. Moreover, the low number of participants might be a statistical bias in our conclusions.

Conclusion

To conclude, data from the present study support that indeed breath holding (BH) and even more so intermittent breath holding (IBH) training acutely affect the inspiratory muscle performance. That is an important training tool which can be used to improve the inspiratory muscles in subjects which need to improve their inspiratory capacity and increase their performance. However, the training with BH and IBH has benefits in PImax, but needs further investigation.