Oxygen uptake kinetics during exercise

The characteristics of oxygen uptake ((V) over dot O-2) kinetics differ with exercise intensity. When exercise is performed at a given work rate which is below lactate threshold (LT), (V) over dot O-2 increases exponentially to a steady-state level. Neither the slope of the increase in (V) over dot O-2 with respect to work rate nor the time constant of (V) over dot O-2 responses has been found to be a function of work rate within this domain, indicating a linear dynamic relationship between the (V) over dot O-2 and the work rate. However, some factors, such as physical training, age and pathological conditions can alter the (V) over dot O-2 kinetic responses at the onset of exercise. Regarding the control mechanism for exercise (V) over dot O-2 kinetics, 2 opposing hypotheses have been proposed. One of them suggests that the rate of the increase in (V) over dot O-2 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 (V) over dot O-2 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 (V) over dot O-2 or drives the (V) over dot O-2 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 (V) over dot O-2 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 (V) over dot O-2 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 (V) over dot O-2 observed after physical training.