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.
ASJC Scopus subject areas
- Physiology (medical)