Cycling Assessment - Critical Power

In general, workouts prescribe both relative intensity and duration of the work to be performed. The duration is easy… this can be the total time of the workout and/or can be the work or rest portion of intervals, in hours, minutes, or seconds. Although there are various methodologies that can be used to articulate intensity, in general for cycling we’ll use a percentage of threshold power or percentage of threshold heart rate (HR). But what constitutes “threshold” power? Is it Functional Threshold Power (FTP), Critical Power (CP), Maximal Lactate Steady State (MLSS), Ventilatory Threshold 2 (VT2), Lactate Threshold 2 (LT2) or the power you can hold for an hour?

First, let’s chat about “threshold”. From a physiology standpoint (and associated research), there are four primary “intensity domains”, which are:

  1. Moderate Intensity: This domain involves exercising at a pace that feels comfortable and sustainable for a prolonged period. Physiologically, it primarily uses aerobic metabolism, where the body efficiently uses oxygen to produce energy. For most trained athletes, the body will oxidize/combust fat as the primary fuel source, and most of the work is done by slow-twitch (type I) muscle fibers. Most athletes can carry on a conversation at this intensity, without pausing or gasping for breath. Under many traditional 5 or more cycling training zone models, this would include both zone 1 (recovery) and zone 2 (endurance).

  2. Heavy Intensity: Moving from Moderate to Heavy, we transition across the first threshold (but not the transition that most refer to as “threshold” from a zone-setting standpoint). In this domain, exercise intensity increases, leading to a notable increase in heart rate and faster breathing. The body starts using a greater percentage of fast twitch (starting with IIa) muscle fibers, and relying on more glycolysis for fuel (breaking down glucose anaerobically). This means while the body starts to produce lactate (and blood lactate levels rise), the body is still clearing (and using all the lactate it is producing aerobically). Using more glycolysis, the body produces a greater proportion of carbon dioxide, resulting in a change in breathing (higher than proportional rise). Because of the physical changes that can be observed, this first “threshold”, its often called the Lactate Threshold 1 (LT1), Ventilatory Threshold 1 (VT1), or sometimes simplistically the aerobic threshold. From an athlete’s perspective, many can feel this transition with the change in breathing or RPE (rating of perceived effort). It takes more concentration to hold intensity in this range, and it would be difficult (or impossible for most) to be able to sing a song without gaps for breathing. Under many traditional cycling training zones, this would represent zone 3, tempo (and the subset of zone 3, Sweet Spot) training.

  3. Severe Intensity: This intensity crosses the lactate threshold. Here, lactate production exceeds the body's ability to clear it, leading to lactate accumulation in the muscles. This domain challenges the athlete's anaerobic metabolism and improves their ability to tolerate and buffer lactate. This intensity is often used in interval training to increase speed and power.

  4. Extreme Intensity: This is the highest intensity domain, characterized by maximal or near-maximal effort that can only be sustained for a short duration. It relies heavily on anaerobic metabolism. Training at this intensity improves an athlete's speed, power, and anaerobic capacity, essential for sprint finishes or overcoming steep inclines.



Intensity Domain

Three zone model, showing the Moderate, Heavy and Severe intensity domains. Note that as intensity goes up, the demarkation between Moderate and Heavy is threshold 1 (LT1, VT1) and the second threshold is between Heavy and Severe domains (LT2, VT2, MLSS). The Extreme Domain would be considered a subset of Z3 in this example.

A deep dive into the intensity domains, and how this model works across many species (besides humans), is included here.

Besides cycling, this concept is applicable to other exercise modalities, besides cycling, as discussed
here. Interestingly, researchers took the same concept for running, and used big data from Strava to use it to predict marathon times (called critical speed for running), as discussed here.

Some researchers argue that critical power should be “the” gold standard for defining zones, rather than other methods, as described here.