This article provides a good starting point for the basic physiological reasons behind the use of different levels of training intensity.

Aerobic Threshold (AeT) – the base from which the first up-tick in lactate occurs; From an athletes perspective, the sensation you feel as you pass beyond AeT is an increase in your breathing rate. For most, this is the transition from nose- to mouth-breathing. This point marks the bottom of the Steady Zone as well as where your slow twitch fibers are materially recruited. Beyond this point your fast oxidizing glycolytic (FOG) fibers are recruited and, therefore, lactate increases in your bloodstream.

Ventilatory Threshold #1 (VT1) or Lactate Threshold (LT) – the top of the moderately-hard zone, estimated at 1mmol above baseline; the sensation at this point is audible rhythmic breathing, driven by increasing CO2 in the bloodstream. At this point, your fast oxidizing glycolytic (FOG) fibers are materially recruited.

NOTE: The term “lactate threshold” is often used interchangeably with Functional Threshold. In this definition we are following the generally accepted terminology used in cycling and exercise physiology.

Functional Threshold (FT) or Ventilatory Threshold #2 (VT2)– threshold intensity, estimated as best average pace/power for a best effort 60-minute time trial; the breathing sensation at this point is panting. At this point, your fast twitch fibers are heavily recruited. This effort is characterized by an increase in power/pace increases lactate such that a reduction in power/pace is required to clear the lactate (hence the “functional” nature of this threshold).

Maximal Aerobic Function (VO2max) – this power/pace that results in maximal oxygen uptake. At this point all muscle fiber types are close to maximally recruited. This effort is generally best effort power/pace for a six minute time trial.

For endurance training adaptations, athletes are best served by training around these four physiological markers. The recovery cost increases (in some cases greatly increases) with training above these markers, therefore athletes are advised to train slightly under these points. Briefly the purpose of training at each marker is outlined below:

Steady Training (~AeT) – Improve your ability to generate energy from fat, enhance mitochondrial proliferation and capillary density (these points are important for aerobic energy production). Training in this zone, and lower, creates biomechanical adaptations that allow training at more intense levels. Training above this level places increasing risks for overuse injuries, especially in the novice athlete.

Moderately-Hard Training (~VT1/LT) – Improve the ability of your FOG fibers to produce energy aerobically as well as being intense enough to stimulate many of the Threshold adaptations without the high level of recovery cost.

Threshold Training (~FT) – Develop the ability of your fast twitch fibers to produce energy as well as enhancing cardiac stroke volume and VO2max.

VO2max Training (~VO2)– Maximally develop the ability of your cardiovascular system to deliver oxygen to your working muscles.