Oxygen uptake in the frequency domain as a test for cardiorespiratory fitness.

Oxygen uptake kinetics describe the characteristics of the rate of change of VO[2] in response to the onset of exercise or a change in work rate. There is a lack of knowledge concerning the use of VO[2] kinetics in the frequency domain as a test for cardiorespiratory fitness. The PRBS exercise test has been developed to study the dynamic responses of the cardiorespiratory system to random changes in submaximal work rate. This exercise test technique provides a multi-frequent assessment of VO[2] kinetics that can be expressed in terms of amplitude (ml-min[-1]W[-1]) or phase shift (degrees) over a frequency range of 0.0022 to 0.0089 Hz. The VO[2] kinetics of young women were investigated using this submaximal test during which the work rate was alternated between two levels. The upper work rate level was chosen to be below the ventilatory threshold. In the first experiment, the variability of replicate tests was investigated in a cohort of eight moderately active women (age = 22.6 +/- 0.8 years). Although there were wide limits of agreement between the two tests there was no significant difference between test 1 and test 2.In a second experiment to test the discriminant ability, oxygen uptake kinetics were compared to VO[2peak] in twenty-eight sedentary or moderately active young women (age = 22.9 +/-3.1 years). The PRBS exercise test technique was able to discriminate between a group of subjects with lower VO[2peak] (VO[2peak] = 32.3 +/- 3.3 ml-kg-1min-1) and a group of subjects with higher VO[2peak] (VO[2peak] = 41.1 +/- 3.2 ml-kg-1min-1). Differences in VO[2] kinetics occurred at frequencies of 0.0022 Hz for amplitude, and at frequencies of 0.0022 Hz to 0.0067 Hz for phase shift. Significant relationships were found to exist between VO[2peak] and VO[2] kinetics at frequencies of 0.0022Hz, 0.0044 Hz and 0.0067 Hz. The following model explained the highest proportion of the variation between VO[2peak] and VO[2] kinetics (r = - 0.72, P0.001): VO[2peak] (in ml-kg[-1]min[-1]) = 0.503(phase shift at 0.0067 Hz) (in degrees) + 72.24In a third experiment to test the sensitivity to detect change, both VO[2] kinetics and VO[2peak] were measured before, during and after an eight week endurance-type training programme completed by fifteen young women (age = 21.6 +/- 1.9 years). Thirteen young women (age = 24.3 +/-3.5 years) acted as a non-training control group. Faster VO[2] kinetics were measured at a frequency of 0.0044 Hz for amplitude and at frequencies of 0.0022Hz to 0.0067 Hz for phase shift following the training programme. Increases in VO[2peak] also occurred as a result of the exercise regimen. No changes in either VO[2] kinetics or VO[2peak] were observed in the non-training group. This study showed that the PRBS exercise test technique was sensitive to short-term endurance-type training adaptations. In conclusion, the parameters measured during the PRBS exercise test provide valuable information that can not be gained from a standard assessment of VO[2] kinetics in the time domain. It is proposed that this exercise test technique has potential as a means of assessing cardiorespiratory fitness within the area of sports science and within the clinical environment.