In some sports, the environment is inappropriate for direct measurement of respiratory and blood gas parameters during exercise. Moreover, the attachment of cumbersome measurement equipment may influence the motion technique and consequently increase energy cost. To overcome this problem, respiratory and blood gas parameters have been measured at the end of exercise to estimate physiological responses during the exercise. Backward extrapolation of the [O.sub.2] recovery curve has been used to calculate the peak oxygen uptake during swimming (Rodriguez et al., 2002) and synchronized swimming (Bante et al., 2007). This method requires that measurements are made as soon as possible after the end of exercise. Data collection lasts a few minutes, and the recovery curve is extrapolated back to time zero, i.e., to the end of exercise.
Extrapolation from post-exercise measurements has also been used to estimate changes in partial pressures of blood gases induced by reduced breathing frequency (RBF), as observed in competitive swimming. However, Strumbelj et al. (2006) found that measurements of respiratory and blood gas parameters taken after the end of a maximal front crawl swimming test did not reflect the conditions which appeared during the swimming test. RBF during swimming increased the alveolar C[O.sub.2] concentration (Dicker et al., 1980; Peyrebrune et al., 2002; Town and Vanness, 1990; West et al., 2005) and induced higher partial pressure of carbon dioxide in capillary blood after it (Kapus et al., 2002; Kapus et al., 2003). Nonetheless, these studies failed to demonstrate a reduction in oxygen saturation due to RBF either by analysing expired air during swimming (Holmer et al., 1980) or by sampling capillary blood after swimming (Kapus et al., 2002; 2003). On the contrary, RBF during cycle ergometry has been shown to cause a reduction in oxygen saturation and lower partial pressure of oxygen, measured in arterial (Yamamoto et al., 1987) and capillary (Kapus et al., 2007; Sharp et al., 1991) blood. Considering that hypoxia has been detected during cycling but not after swimming, the timing of measurement may be the reason for the apparent difference in response to RBF. The timing of measurement may be especially important when spontaneous breathing follows the exercise with RBF. The present study was designed to elucidate this problem. To date, no data of oxygen saturation and respiratory parameters during recovery after exercise with RBF have been presented. Therefore, the aim of the present study was to ascertain whether measurements of oxygen saturation and blood gases measured immediately after exercise could estimate their values during exercise with RBF.
Eight healthy male subjects (age 25 [+ or -] 1 years, height 1.81 [+ or -] 0.03 m, weight 80 [+ or -] 7 kg, peak oxygen uptake (V[O.sub.2]peak) 44.26 [+ or -] 2.93 ml x [kg.sup.-1] x [min.sup.-1], forced vital capacity of 5.98 [+ or -] 0.58 l and forced expiratory volume of 4.76 [+ or -] 0.59 l in 1 s) volunteered to participate in this study. None of the subjects were smokers and were free of respiratory disease at the time of the study. The subjects 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 University's Research Ethics Committee.
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 a micro automation Logo DC 12/24V (Siemens, Munich, Germany). The subjects were instructed to expire and inspire during a 2 s period of open solenoid valve (the green semaphore light was switched on) and to hold their breath, using almost all lung capacity (breath holding near total lung capacity), for 4 s when the solenoid valve was closed (the red semaphore light was switched on). Prior to the exercise testing, the subjects were familiarized with breathing through the breathing metronome. After familiarization, each subject performed 4 exercise tests on an electromagnetically braked cycle ergometer Ergometrics 900 (Ergoline, Windhagen, Germany) with pedal cadence at ~60 revolutions per minute (rpm). Tests were performed in a prescribed order, each of them on a different day.
Preliminary tests: The subjects initially performed an incremental exercise test (IT) to obtain V[O.sub.2]peak. The test began at 30 W and increased by 30 W every 2 min until volitional exhaustion. V[O.sub.2]peak was defined as the highest [O.sub.2] uptake averaged over 60-s interval. The subjects then performed an incremental exercise test with RBF (ITB10) to obtain peak power output. Except breathing, the protocol of this test was identical to the protocol of IT. The peak power output was defined as the highest work stage that each subject completed. From these results, the work rate for the constant load test with RBF was chosen for each subject.
Experimental protocol: After preliminary testing, a constant load test with RBF (B10) was performed to exhaustion at the peak power output obtained during ITB10. This test started with a 5 min warm-up at 50 W. After that, the resistance was increased to match the subject's peak power output and the subject continued to exhaustion. The constant load test was completed by 10 min of active recovery at 20 W with spontaneous breathing. Finally, the subjects repeated the constant load test, however, with spontaneous breathing (SB). The protocol (intensity and duration) of this test was otherwise identical to the protocol of B10.
During the constant load tests (warm-up, exercise and 10 min recovery; SB and B10), the subjects breathed through a mouthpiece attached to a pneumotachograph. The subject's expired gas was sampled continuously by a VMAX29 (SensorMedics Corporation, Yorba Linda, USA) metabolic cart for a breath-by-breath determination of respiratory parameters (pulmonary ventilation...