LINEAR AND NONLINEAR CHARACTERISTICS OF OXYGEN-UPTAKE KINETICS DURING HEAVY EXERCISE

We assessed the linearity of oxygen uptake (VO2) kinetics for several work intensities in four trained cyclists. VO2 was measured breath by breath during transitions from 33 W (baseline) to work rates requiring 38, 54, 85, and 100% of maximal aerobic capacity (VO2max). Each subject repeated each work rate four times over 8 test days. In every case, three phases (phases 1,2, and 3) of the VO2 response could be identified. VO2 during phase 2 was fit by one of two models: model 1, a double exponential where both terms begin together close to the start of phase 2, and model 2, a double expontential where each of the exponential terms begins independently with separate time delays. VO2 rose linearly for the two lower work rates (slope 11 ml.min-1 W-1) but increased to a greater asymptote for the two heavier work rates. In all four subjects, for the two lighter work rates the double-exponential regression reduced to a single value for the time constant (average across subjects 16.1 +/- 7.7 s), indicating a truly monoexponential response. In addition, one of the responses to the heaviest work rate was monoexponential. For the remaining seven biexponential responses to the two heaviest work rates, model 2 produced a significantly better fit to the responses (P < 0.05), with a mean time delay for the slow component of 105 +/- 46 s. The amplitude of the primary component (A1) increased linearly across all four work rates (slope 9.1 ml.min-1 W-1, r = 0.957). In contrast, the time constant for the fast component (tau-1) exhibited no significantly change as work rate increased. Thus, although the overall oxygen cost increases nonlinearly with work intensity, the primary component of the VO2 response exhibits approximately linear behavior: linear increase in gain (A1) and invariant time constant (tau-1).