Exercise Transients

Abstract

The steady state of light constant-power submaximal exercise is attained after at least 3 min since the exercise start. Although the application of mechanical power is practically immediate, the rate of oxygen uptake _ V O 2 À Á increases at a remarkably slower rate: a metabolic transient phase occurs, during which _ V O 2 , despite increasing, is insufficient to provide all the metabolic power that is necessary to sustain ATP resynthesis, so that an oxygen deficit is incurred. The oxygen deficit consists of (i) an obligatory component, covered by phosphocreatine breakdown, reflecting the kinetics of glycolysis activation in the contracting muscles, and characterized by an invariant time constant independent of the exercise intensity and (ii) a facultative component, covered by anaerobic lactic metabolism (early lactate accumulation), due to the dissociation between cardiopulmonary response and muscular response at elevated powers, and characterized by a time constant depending on the exercise intensity. The kinetics of _ V O 2 upon exercise onset is described with exponential equations. The implications of single-and doubleexponential models are discussed. The former model carries along the assumption that the kinetics of _ V O 2 determined at the mouth reflects the kinetics of _ V O 2 in the contracting muscles. This correspondence, however, does not hold anymore when early lactate appears. The latter model, in particular, implies a rapid phase (phase I), which has been related to the fast cardiovascular responses at exercise start. A singleexponential model assumes also that the exercising muscles behave as dynamic linear first-order systems that, as such, admit only one transfer function. On this basis, ramp exercise and sinusoidal exercise, typically two unsteady-state conditions, are discussed. More recently, at powers higher than the critical power (see Chap. 5), the so-called slow component, or third phase of the _ V O 2 kinetics, has been also characterized. The energetic meaning of the slow component is also discussed. Finally, some methodological aspects of single-breath analysis of pulmonary gas exchange are discussed.