What follows is an explanation of the terminology around different energy systems and the physiological adaptation targeted for athlete progression.
Aerobic Threshold (AeT)
The base from which the first up-tick in lactate occurs. The sensation of passing beyond AeT is marked by an increase in the depth of breathing. For most athletes, this is the transition from nose- to mouth-breathing. This point marks the bottom of what I refer to as the Steady Zone, in which your slow twitch fibers are materially recruited. Beyond this point the fast oxidizing glycolytic (FOG) fibers are recruited and, consequently, lactate increases in the bloodstream.
Ventilatory Threshold #1 (VT1) or Lactate Threshold (LT)
This is the where the moderately-hard zone occurs, estimated at 1 mmol above baseline blood lactate level; the sensation is an audible, rhythmic breathing, driven by increasing CO2 in the bloodstream. At this point, the athlete’s fast oxidizing glycolytic (FOG) fibers are materially recruited.
Note: Lactate Threshold is often used interchangeably with Functional Threshold. This definition follows the generally accepted terminology used in cycling and exercise physiology.
Functional Threshold (FT) or Ventilatory Threshold #2 (VT2)
This is threshold intensity, estimated as the average pace or power for a best effort 60-minute time trial. The sensation at this point is labored breathing or panting. At this point, your fast-twitch fibers are heavily recruited. This effort is characterized by an increase in power/pace, which 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 is the power/pace that results in maximal oxygen uptake. At this point all muscle fiber types are close to maximally recruited. This effort is generally reflected in an athlete’s best effort power/pace for a 6-minute time trial.
Physiological markers to evolve endurance
Athletes are best served by training around these physiological markers for endurance adaptations. The recovery cost increases (in some cases, it greatly increases) with training above these markers. This is why athletes are advised to train slightly under these points. The purpose of training at each marker is briefly outlined below:
Easy Training (<AeT)
Provides a stimulus for improving the number of mitochondria within slow twitch fibers, while not further significantly depleting the athlete’s glycogen stores, and thus facilitating recovery between the harder workouts. As the athlete’s metabolic fitness grows, easy training can grow and take the place of pure rest between the harder workouts.
Steady Training (~AeT)
Improves the athlete’s maximal ability to generate energy from fat. It also enhances production of mitochondria and capillary density within those lower threshold FOG fibers—both of which are important for favoring aerobic energy production in these fibers. 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)
Improves the ability of FOG fibers to produce energy aerobically while also being intense enough to touch the lower threshold FG fibers and stimulate many of the threshold adaptations without the high level of recovery cost.
Threshold Training (~FT)
Develops the ability of the fast-twitch fibers to produce energy at their aerobic limit, i.e., without significant acidosis, as well as improving cardiac stroke volume and VO2max.
VO2max Training (~VO2)
Maximally develops the ability of the cardiovascular system to deliver oxygen to the working muscles & develops the ability of the athlete to tolerate increasing acidosis at these higher levels of intensity.