Investigation of VO2 kinetics in humans with pseudorandom binary sequence work rate change

Abstract

The dynamic response of oxygen uptake (VO2) was investigated with two different cycle ergometer tests in which the work rate changed as a pseudorandom binary sequence (PRBS). One sequence had 15 units, each of 30-s duration for a total of 450 s (PRBS1). The second had 63 units, each of 5-s duration for a total of 315 s (PRBS2). The useful range of frequencies available for investigation of the dynamic characteristics of the VO2 response as described by their bandwidth were 0.002-0.013 Hz for PRBS1 and 0.003-0.089 Hz for PRBS2. Eight subjects each completed both PRBS tests. Data from four or five consecutive sequences were ensemble averaged to reduce the biological noise. A Fourier analysis was then conducted, with the range of frequencies investigated spanning those of the bandwidth for PRBS2. This was up to the 28th harmonic. For PRBS1, the VO2 response could be adequately reconstructed by including Fourier coefficients only up to the 5th harmonic. In contrast, for PRBS2, there was still a clear pattern in the residuals at the 5th harmonic. The data were not adequately reconstructed until higher-frequency components up to the 28th harmonic were included. Evidence for this came from analysis of the mean square error. The mean square error at the 28th harmonic was reduced to 83 +/- 8% of the mean square error at the 5th harmonic for PRBS1 and to 31 +/- 3% for PRBS2 (P less than 0.0001). These data obtained by Fourier analysis and reconstructed for comparison with the original VO2 response indicate the presence of a high-frequency component that was not apparent when a test with a smaller bandwidth was used as the work rate forcing.