“Linear” Versus “Nonlinear” O2 Responses to Exercise: Reshaping Traditional Beliefs

A number of basic tenets in traditional exercise physiology have been formulated on the assumption that pulmonary oxygen uptake (O2) adapts to changes in metabolic rate with linear, first-order response kinetics. However, questions regarding this premise have been raised for over half a century and clear contradictions have been reported. Specifically, Boltzmann's principle of superposition that defines linearity is violated for exercise transitions of different magnitudes, and the symmetry between on- and off-responses that first-order kinetics implies is not always present. Furthermore, a single exponential model does not adequately describe the O2 response to high-intensity exercise because a supplementary response compartment of delayed onset is manifest. Collectively, these findings reflect a range of nonlinear behaviors that indicate greater complexity of the O2 response, and it is imperative that these deviations be universally recognized, both to reshape our interpretation of the acute metabolic adaptation to exercise and also to provide clues regarding cellular mechanisms of respiratory control.