Doctor of Philosophy
John M. Kowalchuk
The pulmonary O2 uptake (V̇O2p) response to exercise has been characterized by exponential kinetics that remain constant regardless of the exercise protocol used to force the change in V̇O2p (kinetics are invariant). A system that responds in this way is classified as “dynamically linear”, implying that a first-order rate reaction controls V̇O2 at the muscle level (V̇O2m). However, slowed V̇O2p kinetics when initiating exercise from raised baseline intensities challenges this notion. The purpose of this thesis was to characterize the rate (τV̇O2p) and magnitude (gain) of adjustment of V̇O2p in response to step-transitions initiated from a wide range of exercise intensities to examine whether V̇O2 kinetics at the muscle level function as a dynamically linear system. In silico experiments were included to corroborate responses measured in vivo. Using breath-by-breath V̇O2p during step- and ramp-incremental exercise it was demonstrated that: 1) V̇O2p kinetics were invariant and fast (τV̇O2p ~20s) when transitions of varying ∆WR were initiated from a common WR (Chapter III); and 2) the V̇O2p response to ramp exercise was linearly related to WR and well described by a mono-exponential (Chapter IV) – consistent with dynamically linear control. However, it was also demonstrated in the same groups of participants that τV̇O2p and gain increased as a function of baseline intensity (Chapters III and IV) – refuting this notion. Modelling the summed influence of muscle compartments based on in vivo measurements in Chapter III revealed that τV̇O2p could appear fast (20s) despite being derived from τV̇O2m values ranging 15-40s and τQ̇m ranging 20-45s. Additionally, it was demonstrated that the V̇O2p response to ramp exercise in Chapter IV could also be characterized by an exponential function with τV̇O2p and gain parameters that vary as a function of WR. Collectively, these data suggest that V̇O2p kinetics are slowed dependent on WR and may be strongly influenced by muscle metabolic and circulatory heterogeneity. Therefore, it is proposed that at the muscle level V̇O2 kinetics operate as a linear system and that non-linear V̇O2p responses to exercise may reflect a “heterogeneity of linear responses” within the range of muscle fibres recruited to address the exercise challenge.
Keir, Daniel A., "Examination of the Non-linear V̇O2p Response to Exercise: Non-invasive Evidence of Linear Systems Control Using V̇O2p Kinetic Analyses" (2015). Electronic Thesis and Dissertation Repository. 3181.