In this article about the physiology of intensity monitoring, we define terms and explain the underlying metabolic processes that determine the ventilatory markers of intensity.

Aerobic Threshold (AeT), aka Ventilatory Threshold (VT1)

When ventilation is slow and controlled to the point where the athlete can carry on a conversation without needing to catch his or her breath or can maintain breathing through the nose, then fat metabolism is providing half or more of ATP for locomotion. This ventilation threshold is often referred to as the Aerobic Threshold (AeT) or the First Ventilatory Threshold (VT1). Its upper limit corresponds to the point where the blood lactate levels have risen by 1mMol/L above their baseline reading or have hit the 2mMol/L level of blood lactate. Up to that point, exercise can be carried out essentially indefinitely and will ultimately be limited in duration by fuel stores.

Once the Aerobic Threshold has been passed, glycolytic metabolism begins to dominate and makes a steadily increasing contribution to ATP production. At moderate intensities, lactate, which is a by-product of glycolysis, is easily remetabolized by the aerobic processes in the slow twitch fibers in the working muscles. This rate of lactate uptake directly determines endurance capacity. The faster this by-product lactate can be utilized by the working muscles as a fuel source and not accumulated as a waste product, the higher the endurance capacity of those muscles.

As the power output (speed) increases, so does the percent of glycolytic metabolism’s contribution to the overall energy demands. With this higher intensity comes a higher lactate production rate. At some point the lactate removal rate is surpassed by the lactate production rate. At that point the Second Ventilatory Threshold (VT2), or Anaerobic Threshold (AnT), is reached. Beyond this point the athlete is operating on borrowed time and will be forced to slow the pace.

VT2: Anaerobic Threshold (AnT)

CO2 is one of the products of the metabolic processes at the cellular level. As the ATP production rate increases, so does the production of CO2. This triggers the ventilation center of the brain to breathe deeper and faster in an effort to blow off the excess CO2 through exhalations.

When the VT2/AnT point is reached, another ventilation marker becomes obvious. This intensity corresponds very closely to the point where breathing becomes noticeably deeper and more rapid. It has been referred to as the “Breakaway Breathing Point.” Above this, conversation becomes difficult and only a couple of words can be spoken before one needs another breath. Above this intensity, exercise will not be sustainable for more than a few minutes.

The duration that the exercise can be sustained is inversely related to the intensity above VT2. The higher the intensity, the shorter the time it can be maintained.

How AeT Affects an Athlete’s Best Performance

The main determinant of the VT2 or MLSS (maximum lactate steady state) pace is the lowly AeT pace. That is because the faster the slow twitch muscle fibers can remove the lactate being produced by the faster twitch, high-power fibers, the greater the intensity that can be sustained for longer. Therefore having a high AeT pace will mean that you will have a higher MLSS/VT2 pace. This very important point is often misunderstood by coaches and athletes alike.

Many studies have shown that VT2 or MLSS or AnT is the best predictor of endurance performance in events lasting from around 2 minutes to about 2 hours. Increasing the AnT is clearly one of the best ways known to improve endurance performance. However, increasing the pace at AeT is the first and most important step in increasing the pace at AnT.

Vince Anderson climbing on the normal route (NW Ridge) of Makalu, the world’s fifth highest peak, at approximately 23,000 feet. Credit: Steve House
Vince Anderson climbing on the normal route (northwest ridge) of Makalu, the world’s fifth-highest peak, at approximately 23,000 feet. Photo by Steve House


Let’s summarize:

  1. Ventilation is a direct measure of the intensity of exercise.
  2. Ventilation offers a personalized, instantaneous metric for each athlete to control training intensity.
  3. Ideally you are using a sports watch that is recording heart rate and other relevant data such as pace, distance, and duration.
  4. Gauging where your AeT heart rate is plus your breathing rate in real time during training allows the athlete and coach to gauge recovery and readiness for the next high-intensity session (or climb), as it will vary based on fitness and recovery state.

This is Part 3 in a series of three articles. You can read Parts 1 and 2 here:

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