Oxygen Uptake Kinetics During Exercise

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Abstract
The characteristics of oxygen uptake (V̇O2) kinetics differ with exercise intensity. When exercise is performed at a given work rate which is below lactate threshold (LT), V̇O2 increases exponentially to a steady-state level. Neither the slope of the increase in V̇O2 with respect to work rate nor the time constant of V̇O2 responses has been found to be a function of work rate within this domain, indicating a linear dynamic relationship between the V̇O2 and the work rate. However, some factors, such as physical training, age and pathological conditions can alter the V̇O2 kinetic responses at the onset of exercise. Regarding the control mechanism for exercise V̇O2 kinetics, 2 opposing hypotheses have been proposed. One of them suggests that the rate of the increase in V̇O2 at the onset of exercise is limited by the capacity of oxygen delivery to active muscle. The other suggests that the ability of the oxygen utilisation in exercising muscle acts as the rate-limiting step. This issue is still being debated. When exercise is performed at a work rate above LT, the V̇O2 kinetics become more complex. An additional component is developed after a few minutes of exercise. The slow component either delays the attainment of the steady-state V̇O2 or drives the V̇O2 to the maximum level, depending on exercise intensity. The magnitude of this slow component also depends on the duration of the exercise. The possible causes for the slow component of V̇O2 during heavy exercise include: (i) increases in blood lactate levels; (ii) increases in plasma epinephrine (adrenaline) levels; (iii) increased ventilatory work; (iv) elevation of body temperature; and (v) recruitment of type IIb fibres. Since 86% of the V̇O2 slow component is attributed to the exercising limbs, the major contributor is likely within the exercising muscle itself. During high intensity exercise an increase in the recruitment of low-efficiency type IIb fibres (the fibres involved in the slow component) can cause an increase in the oxygen cost of exercise. A change in the pattern of motor unit recruitment, and thus less activation of type IIb fibres, may also account for a large part of the reduction in the slow component of V̇O2 observed after physical training.