Book Description

He pulmonary O2 uptake (VȮ2p) response to exercise has been characterized by exponential kinetics that remain constant regardless of the exercise protocol used to force the change in VȮ2p (kinetics are invariant). A system that responds in this way is classified as dynamically linear, implying that a first-order rate reaction controls VȮ2 at the muscle level (VȮ2m). However, slowed VȮ2p kinetics when initiating exercise from raised baseline intensities challenges this notion. The purpose of this thesis was to characterize the rate (VȮ2p) and magnitude (gain) of adjustment of VȮ2p in response to step-transitions initiated from a wide range of exercise intensities to examine whether VȮ2 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 VȮ2p during step- and ramp-incremental exercise it was demonstrated that: 1) VȮ2p kinetics were invariant and fast (VȮ2p 2̃0s) when transitions of varying ∆WR were initiated from a common WR (Chapter III); and 2) the VȮ2p 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 VȮ2p 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 VȮ2p could appear fast (20s) despite being derived from VȮ2m values ranging 15-40s and Qṁ ranging 20-45s. Additionally, it was demonstrated that the VȮ2p response to ramp exercise in Chapter IV could also be characterized by an exponential function with VȮ2p and gain parameters that vary as a function of WR. Collectively, these data suggest that VȮ2p 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 VȮ2 kinetics operate as a linear system and that non-linear VȮ2p responses to exercise may reflect a heterogeneity of linear responses within the range of muscle fibres recruited to address the exercise challenge.