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 …