Perhaps the best example in sport of where breathing frequency is naturally reduced is during front crawl swimming. During such exercise breathing (and specifically inspiration) must be coordinated with stroke mechanics and as a result tidal volume ([V.sub.T]) is increased to compensate for the reduced breathing frequency (RBF) (Dicker et al., 1980). Such a restricted breathing pattern during front crawl swimming limits minute ventilation ([V.sub.E]) and can lead to hypercapnia i.e. carbon dioxide retention (Cordain and Stager, 1988). The breathing pattern that swimmers adopt during front crawl largely depends on the swimming distance. For example, it is usual practice for swimmers to breathe every second stroke cycle during events of 200 metres or more, but to adopt a more restricted pattern during shorter front crawl distances (Maglischo, 2003). Consequently, it is not surprising that coaches have included RBF sets i.e. taking a breath every fourth, fifth, sixth or eighth stroke cycle during training: a practise which has been used in swimming since the 1970's. Several studies have examined the influence of RBF on different physiological parameters in swimming since then. RBF during swimming reduced [V.sub.E], increased the alveolar C[O.sub.2] concentration (Cordain and Stager, 1988; Holmer and Gullstrand, 1980; Peyrebrune et al., 2003; Town and VanNess, 1990) and induced higher partial pressures of carbon dioxide in capillary blood (Kapus et al., 2002; 2003). Due to the technical limitations of measuring respiratory and blood parameters during swimming, the impact of RBF on physiological parameters has been investigated during cycle ergometry test (Kapus et al., 2009; 2010a; 2010b; Sharp et al., 1991; Yamamoto et al., 1987; 1988) and treadmill running (Matheson and McKenzie, 1988): here breathing frequency can be modified and respiratory and blood parameters measured with greater ease. These studies confirmed the presence of marked hypercapnia as a result of RBF during exercise. In addition, these studies also examined hypoxemia by measuring capillary blood [Po.sub.2] and oxygen saturation during exercise with RBF (Kapus et al., 2009; 2010a; 2010b; Matheson and McKenzie, 1988; Sharp et al., 1991; Yamamoto et al., 1987).
All of the above studies investigated the acute effects of RBF. To our knowledge only one study has examined the influence of RBF training on exercise performance and did so in swimming. After four weeks of RBF training (breathing every fourth stroke cycle during front crawl) swimmers reduced their breathing frequency from 32 [+ or -] 5 breaths per minute to 25 [+ or -] 7 breaths per minute during a maximal 200 meter front crawl swim (Kapus et al., 2005). Although the mechanisms responsible for this training adaptation are not clear, it is possible that RBF training increased tidal volume and did so sufficiently to the extent that [V.sub.E] was increased. Given that RBF creates a hypercapnic training stimulus (Dicker et al., 1980; Peyrebrune et al., 2003) it is possible that the adaptation to hypercapnia could be the result of RBF training. Consequently the purpose of this study was twofold: firstly, to investigate the influence of RBF training on [V.sub.T] during incremental exercise where breathing frequency is restricted. Secondly, to investigate the effect of RBF training on the ventilatory response during exercise when breathing a 3% C[O.sub.2] mixture, which is thought to be an indicator of ventilatory sensitivity to hypercapnia (Florio et al., 1979). Considering suggestions from previous studies, we hypothesise that RBF training will increase [V.sub.T] during incremental exercise where breathing frequency is restricted and decrease ventilatory sensitivity during exercise when breathing a 3% C[O.sub.2] mixture.
Twelve healthy male participants volunteered to participate in this study. As students of the Faculty of Sport, they were active but none were currently participating in a regular training programme. During the study period, all participants stopped their usual physical activity (recreational) and only exercised during the training sessions as part of the experiment. Descriptive measures of participants and training groups are presented in Table 1.
None of the participants were smokers and all were free from respiratory diseases. They were fully informed of the purpose and possible risks of the study before giving their written consent to participate. The study was approved by the National Medical Ethics Committee of Slovenia.
All testing and training was performed on an electromagnetically braked cycle ergometer (Ergometrics 900, Ergoline, Windhagen, Germany) with a pedal cadence of 60 rpm. Participants completed the following exercise tests in the same order pre- and post-training: 1) an incremental test to obtain peak power output with spontaneous breathing (SB) ([PPO.sub.SB]) and peak oxygen uptake (V[O.sub.2] peak); 2) an incremental test to obtain peak power output with RBF ([PPO.sub.RBF]); and 3) a constant load test with SB to determine the ventilatory response during exercise when breathing a 3% C[O.sub.2] mixture (ventilatory sensitivity). At pre-training testing, two additional tests were performed: 1) a constant load test with SB at [PPO.sub.SB]; 2) a constant load test with RBF at [PPO.sub.RBF]. The data obtained from these tests were used for determining the training interval sets only. Each exercise test was performed on a different day. All testing took place under controlled environmental laboratory conditions (21[degrees]C, 40-60% RH, 970-980 mbar) and at the same time of day. Participants were asked to maintain their usual eating habits and to avoid consuming food 2 hours before testing. Post-training testing began 2 days after the last training session.
RBF was defined as 10 breaths per minute and was regulated by a breathing metronome. The breathing metronome was composed of a gas service solenoid valve (24 VDC, Jaksa, Ljubljana, Slovenia) and a semaphore with red and green lights. Both were controlled by micro-automation (Logo DC 12/24V, Siemens, Munich, Germany). The participants were instructed to exhale and inhale during a two second period of open solenoid valve (the green semaphore light was switched on) and to hold their breath, using almost all lung capacity (holding breath near total lung capacity), for four seconds when the solenoid valve was closed (the red semaphore light was switched on). Prior to testing and training, the participants were familiarized with cycling on the cycle ergometer and breathing in time with the metronome.
Incremental exercise test: The participants initially performed an incremental exercise test with SB to obtain V[O.sub.2] peak and [PPO.sub.SB]. The test began at an intensity of 30 W and increased by 30 W every two minutes until volitional exhaustion. V[O.sub.2] peak was defined as the highest oxygen uptake averaged over a 60 second interval. On the next testing day, the participants performed an incremental exercise test with RBF to obtain [PPO.sub.RBF]. With the exception of breathing pattern, the exercise protocols of the two tests were identical. In both incremental exercise tests peak power output (thus [PPO.sub.SB] and [PPO.sub.RBF]) was defined as the highest work stage completed (the last work stage that was actually sustained for two minutes), and hence power output obtained.
Constant load test with SB to determine ventilatory sensitivity: [V.sub.E]ntilatory sensitivity was calculated from data obtained during 30 min cycle ergometery exercise at 50 W with a pedal cadence of 60 rpm. Participants started this test by breathing room air (initial 15 minutes) and then switched to breathing a humidified gas mixture containing 3% carbon dioxide, 21% oxygen and 76% nitrogen for a further 15 minutes (Kelley et al., 1982). The breathing mixture was directed to the inspiratory side of a Hans Rudolph respiratory valve (Hans Rudolph, Kansas City, Mo.). [V.sub.E]ntilatory sensitivity was calculated using the following equation (Kelley et al., 1982):
ventilatory sensitivity = ([V.sub.E]2-[V.sub.E]1)/([P.sub.ET][co.sub.2]2-[P.sub.ET]c[o.sub.2]1) (1)
where [V.sub.E]2 and [P.sub.ET][co.sub.2]2 are the average minute ventilation and the end tidal partial pressure of...