Oxygen uptake kinetics during exercise

The characteristics of oxygen uptake ((V) over dot O-2) kinetics differ wit h exercise intensity. When exercise is performed at a given work rate which is below lactate thre shold (LT), (V) over dot O-2 increases exponentially to a steady-state leve l. Neither the slope of the increase in (V) over dot O-2 with respect to wo rk 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 dynam ic relationship between the (V) over dot O-2 and the work rate. However, so me factors, such as physical training, age and pathological conditions can alter the (V) over dot O-2 kinetic responses at the onset of exercise. Rega rding the control mechanism for exercise (V) over dot O-2 kinetics, 2 oppos ing hypotheses have been proposed. One of them suggests that the rate of th e increase in (V) over dot O-2 at the onset of exercise is limited by the c apacity of oxygen delivery to active muscle. The other suggests that the ab ility of the oxygen utilisation in exercising muscle acts as the rate-limit ing 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 slo w 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 exerci se intensity. The magnitude of this slow component also depends on the dura tion of the exercise. The possible causes for the slow component of (V) ove r dot O-2 during heavy exercise include: (i) increases in blood lactate lev els; (ii) increases in plasma epinephrine (adrenaline) levels; (iii) increa sed ventilatory work; (iv) elevation of body temperature; and (v) recruitme nt 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 t he slow component of (V) over dot O-2 observed after physical training.