Kinetics of V̇O₂ with impulse and step exercise in humans

The constancy of the time course (i.e., dynamic linearity) of the O₂ uptake (V̇O₂) response to exercise was examined by testing the law of superposition on data from impulse and step work rate forcings. Two impulses (10 s at a 235-W increase above a 25-W base line, I-235; and 5 s at 475-W increase above a 25-W base line, I-475), four steps (ST) (25-65 W, ST1; 65-105 W, ST2; 25-105 W, ST3; and 25-145 W, ST4), and the corresponding off-transient responses were performed six to eight times by each of five subjects. The integrated area (G) of the V̇O₂ response for I-235 was similar to that of ST1 and ST2 (P > 0.05); the I-475 G was significantly greater (P < 0.05). The time constant of V̇O₂ during the step function on-transient response for the second exponential component was significantly faster for ST1 and significantly slower for I-235 and I-475 than for ST2, ST3, and ST4 (P < 0.05). However, I-235 and I-475 time constants for V̇O₂ were not different from the ST off-transient values. Attempts to superimpose the integral of the impulse on the ST data showed that the early rapid increase in V̇O₂ in the ST was underpredicted by the impulse and that the impulse response lagged behind the ST at all points before steady state. It can be concluded that V̇O₂ kinetics failed the test of superposition and are therefore described by a nonlinear dynamic system.