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For almost 80 years, physiological studies have attempted to explain endurance performance and to develop ways of improving it by training. Performance for a runner can be represented by the relation of his/her personal power (velocity) to time to exhaustion (time limit).1
There are particular velocities that delineate intensity domains which are determined by oxygen uptake (V̇o2) and blood lactate response versus time.2,3 We are going to use them to define the slow phase of V̇o2 kinetics V̇o2 slow component) which only appears during intense exercise.
A high range of work can be identified at which there is a sustained increase in blood lactate and a decrease in arterial pH with time. These responses decline back towards a baseline value. Oxygen uptake increases in a monoexponential way and stabilises at about 80% in high level marathon runners for at least an hour and a half of continuous exercise. After that time, it is possible for oxygen consumption to increase because of thermoregulatory constraints, and this increase is called the “V̇o2 drift”. This intensity of exercise corresponds to the velocity that can be sustained during a marathon and is equal to about 80% of the velocity associated with V̇o2max determined in an incremental test—that is, vV̇o2max.4 During this type of exercise both lipids and carbohydrate are used as fuel.
At a higher intensity, the maximal lactate steady state occurs5 when the rate of appearance of blood lactate equals the rate of its disappearance. V̇o2 stabilises after three minutes at about 85% V̇o2max. This corresponds to the highest velocity that an athlete can sustain for an hour (85% vV̇o2max for a well trained endurance athlete); carbohydrate (and lactate even) is the main substrate for this exercise.
At a higher intensity, at about 90% vV̇o2max, the rate of appearance of blood lactate exceeds the rate of disappearance and therefore blood lactate increases. After the first monoexponential increase in V̇o2, there is a second increase after about three minutes which is defined as the V̇o2 slow component. V̇o2 reaches a delayed steady state which is higher than the V̇o2 requirement estimated from the relation between V̇o2 and moderate work rate. For instance, in this case the athlete can run at 90% vV̇o2max and reaches and stabilises at 95% V̇o2max at the sixth minute of exercise (time to exhaustion at this velocity being about 10–15 minutes). This corresponds to the so called “critical power” which is the vertical asymptote of the hyperbolic relation between power (velocity) and time.6 Time limit at the critical velocity is reduced to less than 30 minutes because of rapid glycogen depletion.7,8 The critical velocity is the highest velocity below its maximal level (V̇o2max) at which oxygen consumption can reach a steady state.
Above this critical velocity, during high intensity exercise, neither V̇o2 nor blood lactate can be stabilised, and both rise inexorably until fatigue ensues, at which point V̇o2 reaches its maximum value.9
The initial very small component (phase 1), resulting from a sudden change in the venous return in combination with a small change in the mixed venous gas tension, is not fitted into the following equation. In fact, the parameters for the oxygen uptake kinetics were obtained from a two component exponential model in which the first component accounted for the fast component (phase 2) and the second component accounted for the slow component (phase 3). The oxygen uptake kinetics are described as a function of time by the following equation10:
where A0 is the resting baseline value, A1 and A2 are the amplitudes for the two components, τ1 and τ 2 are the time constants for the two components, and TD1 and TD2 are the time delays from the onset of exercise for the two components.
Hence, the so called V̇o2 slow component is the second amplitude (A2) of the increase in V̇o2 that appears at TD2. This second amplitude represents about 10% of the first (A1) and depends on the absolute intensity of exercise because V̇o2 is regulated by the split of ATP and phosphocreatine.11 The value of the V̇o2 slow component can reach 500 ml/min and is generally considered to be significant when the value is above 200 ml/min. To avoid the use of this complicated equation which necessitates the use of software such as Sigma plot (SPSS), the V̇o2 slow component can be identified as described initially by Whipp and Wasserman12 by calculating the difference in V̇o2 measurement between the 6th and 3rd minute or, if the exercise is performed until exhaustion, between the third and last minute.13
The appearance of this slow V̇o2 component is mainly due to the recruitment of fast fibre type II fibres with fatigue.14 It has been shown that type II fibres have a phosphate to oxygen ratio that is 18% lower than in type I fibres, probably because of a greater reliance on the α-glycerophosphate shuttle than the malate-aspartate shuttle.15 Therefore more oxygen is required to produce the same level of ATP turnover and sustain a given power output. The other 15% is due to an increase in cardiac and ventilation work. Training decreases the V̇o2 slow component at the same absolute velocity, mainly because of an increase in the distribution of type I fibres and an increase in mitochondrial and capillary density.16,17 A decrease in the V̇o2 slow component can also appear for the same relative velocity (in % vV̇o2max) because of an increase in the maximal lactate steady state.18 However, during intense exercise, the amplitude of the V̇o2 slow component is not linked to endurance at all. Moreover, it has been reported that triathletes that had no V̇o2 slow component in running compared with cycling had the same endurance time in these two types of exercise (at 90% of the power or velocity associated with V̇o2max). These triathletes also had the same maximal lactate steady state at 82% of velocity or power associated with V̇o2max in running and cycling.
Endurance training decreases the V̇o2 slow component at the same velocity.19–22 Personal data on high intensity training have shown that the decrease in the V̇o2 slow component at the same absolute intensity (90% vV̇o2max) is not correlated with an improvement in performance (endurance time) at this velocity (+ 40% of time limit).
A more interesting fact about this V̇o2 slow component phenomenon is for training at V̇o2max as it creates a broad range of exercise intensities for which V̇o2max will occur, provided that the exercise is continued to the point of exhaustion.9
Hence, it may be possible to describe a new relation between time spent at V̇o2max (tlimV̇o2max) and velocity as a percentage of the velocity associated with V̇o2max determined in an incremental test (vV̇o2max). The relation between time to exhaustion at V̇o2max and velocity follows a function that has a peak around 100% vV̇o2max in well trained runners who have no, or only a low value for, the V̇o2 slow component (<200 ml/min). In less well trained subjects, the V̇o2 slow component means that they spend longer sustaining V̇o2max at 90% vV̇o2max than at 100% vV̇o2max.23,25,26 However, fit endurance athletes have to run at close to 100% of vV̇o2max to elicit V̇o2max because they have no V̇o2 slow component.23 24
Therefore, in training, if the aim is to elicit V̇o2max, it may be useful to determine the velocity for which time spent at V̇o2max is maximal.25 To determine at which velocity the longest time at V̇o2max is obtained during continuous exercise, the critical velocity at V̇o2max can be determined using the critical power model. Instead of total time limit run, only the time run at V̇o2max is plotted against the distance run at V̇o2max. The slope of this plot is the critical velocity at V̇o2max. This relation between tlimV̇o2max and velocity can be used to determine the velocity that elicits the longest time to exhaustion at V̇o2max.26,27 This velocity is not significantly different from vV̇o2max determined from an incremental protocol, but is significantly higher than the critical velocity classically determined using a two parameter critical power model and the total distance-time.26
The existence of this V̇o2 slow component phenomenon raises the question of how athletes can adapt their training to improve performance. In fit runners, who are not at a high level (vV̇o2max = 19 km/h), eight weeks of training at high intensity was shown to remove the V̇o2 slow component at the same absolute velocity (V Billat, A Demarle, J Slawinski and JP Koralsztein, unpublished work). This was because vV̇o2max increased, and at the same velocity was at a lower percentage of vV̇o2max than before training. The time limit at this previously high intensity training was doubled (20 v 10 minutes). At the same relative velocity to vV̇o2max, the V̇o2 slow component was comparable with that before training, which means that this high intensity training (twice a week) has to be calibrated at least every two months in this case.
In conclusion, the V̇o2 slow component phenomenon, which was first described by Margaria et al in the sixties28 and then by Whipp and Wasserman in the seventies,12 has been widely focused on in the nineties. In the light of this, it should be possible in the next five years to use the knowledge to diversify training and to explore endurance training effects and fitness.
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