Oxygen uptake kinetics during incremental- and decremental-ramp cycle ergometry

Abstract

The pulmonary oxygen uptake (VO2) response to incremental-ramp cycle ergometry typically demonstrates lagged-linear first-order kinetics with a slope of ~10-11 ml·min(-1)·W(-1), both above and below the lactate threshold (θL), i.e. there is no discernible VO2 slow component (or "excess" VO2) above θL. We were interested in determining whether a reverse ramp profile would yield the same response dynamics. Ten healthy males performed a maximum incremental -ramp (15-30 W·min(-1), depending on fitness). On another day, the work rate (WR) was increased abruptly to the incremental maximum and then decremented at the same rate of 15-30 W.min(-1) (step-decremental ramp). Five subjects also performed a sub-maximal ramp-decremental test from 90% of θL. VO2 was determined breath-by-breath from continuous monitoring of respired volumes (turbine) and gas concentrations (mass spectrometer). The incremental-ramp VO2-WR slope was 10.3 ± 0.7 ml·min(-1)·W(-1), whereas that of the descending limb of the decremental ramp was 14.2 ± 1.1 ml·min(-1)·W(-1) (p < 0.005). The sub-maximal decremental-ramp slope, however, was only 9. 8 ± 0.9 ml·min(-1)·W(-1): not significantly different from that of the incremental-ramp. This suggests that the VO2 response in the supra-θL domain of incremental-ramp exercise manifest not actual, but pseudo, first-order kinetics. Key pointsThe slope of the decremental-ramp response is appreciably greater than that of the incremental.The response dynamics in supra-θL domain of the incremental-ramp appear not to manifest actual first-order kinetics.The mechanisms underlying the different dynamic response behaviour for incremental and decremental ramps are presently unclear.

Keywords: Oxygen uptake-work rate gain; decremental-ramp exercise; incremental-ramp exercise; system linearity..

Figure 1.

Schematic representation of linear first-order O₂ uptake (VO₂) responses as a function of time (- - -) for (a) an incremental ramp exercise test, (b) a step-decremental ramp exercise test, (c) a step (or constant work-rate) exercise test and (d) a decremental ramp exercise test. The work rate profiles are shown as solid lines. See text for further detail.

Figure 2.

Breath-by-breath VO₂ responses as a function of time in a representative subject for (a) incremental-ramp exercise ( Ramp; 30 W.min^-1) and (b) maximal decremental-ramp exercise (Ramp; 30 W·min^-1). Asterisk represents θ_L. Solid circles represent steady-state responses, with line of best fit (solid line). Lines of best fit to the linear regions of the incremental and decremental ramp responses are shown as dashed lines.

Figure 3.

Individual subject VO₂-WR slope (∆VO₂/∆WR) responses for incremental-ramp exercise (solid circles) and maximal decremental-ramp exercise (open circles); solid and dashed lines, with error bars, represent the respective mean ± standard deviation (p 0.005).

Figure 4.

Breath-by-breath VO₂ responses as a function of work rate, displayed as mirror images, in a representative subject for: maximal decremental-ramp exercise (crosses) and sub- maximal decremental-ramp exercise (open circles). The dashed lines are the lines of best fit over the linear regions of the decremental VO₂ response.

Figure 5.

Individual subject VO₂-WR slope (∆VO₂/∆WR) responses for incremental-ramp exercise (solid circles) and sub-maximal decremental-ramp exercise (open triangles); solid and dashed lines, with error bars, represent the respective mean ± standard deviation.