Original episode & show notes | Raw transcript
This guide synthesizes and expands upon the advanced concepts discussed in the “Empirical Cycling” podcast, offering a detailed exploration of Functional Threshold Power (FTP), its physiological underpinnings, testing methodologies, and its relationship with other models like Critical Power.
The central argument of the podcast is that FTP should not be viewed as the result of a single test protocol (e.g., 95% of a 20-minute effort). Instead, it’s a physiological concept.
The most crucial definition of FTP is the inflection point on your power-duration curve. Imagine plotting every maximal effort you could do, from a few seconds to several hours.
Below FTP: The curve is relatively flat. A small decrease in power leads to a large increase in how long you can sustain the effort. You fatigue slowly.
Above FTP: The curve becomes very steep. Even a small increase in power leads to a dramatic decrease in how long you can sustain it. You fatigue very quickly.
FTP is the tipping point between a slow and a rapid state of fatigue. As the podcast host explains with a concrete example:
An athlete with a 200-watt FTP can hold it for 40 minutes.
At 190 watts (5% below), they can last 60 minutes (a 50% increase in duration).
At 210 watts (5% above), they can only last 21 minutes (a 50% decrease in duration).
This inflection point is the “threshold” we are trying to identify. It is a physiological reality that can be felt by an experienced athlete.
The podcast correctly states that FTP was originally intended by its creator, Dr. Andy Coggan, to be a practical, field-based proxy for Maximal Lactate Steady State (MLSS). MLSS is the highest exercise intensity at which the rate of lactate production in the muscles equals the rate of lactate clearance from the blood.
Below MLSS: Your body can clear lactate as fast as it’s produced. You can maintain a physiological steady state for a long time.
Above MLSS: Lactate production outpaces clearance, leading to a rapid accumulation of metabolic byproducts (like H+ ions, which cause the “burn”) that accelerate fatigue.
Therefore, the “inflection point” on the power curve is the real-world manifestation of crossing your MLSS.
The podcast critically examines popular testing methods, highlighting their strengths and, more importantly, their weaknesses.
How it works: Perform an all-out 20-minute effort. Your FTP is estimated as 95% of your average power for that effort.
The Problem: This 95% figure is just a population average. An athlete’s ability to sustain a percentage of their 20-minute power depends on their unique physiology, particularly their anaerobic capacity.
An anaerobically strong athlete (a “puncher” or sprinter) might only be able to sustain 90-92% of their 20-minute power for an hour.
An aerobically dominant athlete (a “diesel” time trialist) might be able to sustain 96-98% or even more.
Conclusion: It works for some, but for a “large chunk of the population,” it produces an inaccurate FTP, leading to poorly set training zones.
How it works: The trainer increases resistance in set increments (e.g., 20 watts every minute) until exhaustion. FTP is calculated as a percentage (often ~75%) of the final minute’s power.
The Problem: As the podcast emphasizes, the result is heavily influenced by anaerobic capacity. An athlete can “hang on” for the last few steps using anaerobic energy reserves, which inflates the final number. This leads to a significant overestimation of the actual aerobic threshold (MLSS).
The Consequence: The athlete who got an inflated FTP from a ramp test finds their prescribed training zones (like over-unders or sweet spot) to be “basically impossible,” because they are working well above their true threshold.
The host’s preferred method is a more dynamic, perception-based approach that teaches the athlete to identify the inflection point by feel.
Start Low: Begin the effort well below your suspected FTP (e.g., 20-30 watts lower).
Slowly Ramp: Over 10-15 minutes, gradually increase the power. Avoid sudden jumps.
Find the Edge: The goal is to find the precise wattage that feels like the “edge of a knife.” You feel that if you go any harder, you’ll blow up, but at this exact intensity, you feel you can hold it for a very long time.
Hold It: Once you find that edge, you hold it for as long as you can (typically aiming for 30-60 minutes).
This method’s primary advantage is that it trains the most important skill: pacing and internal perception of the threshold state, which is more valuable than what any single number can provide.
The podcast dives into the often-confused relationship between FTP and Critical Power (CP).
The CP model is a mathematical formula that describes the relationship between power and time for high-intensity efforts. It has two components:
Critical Power (CP): A theoretical power output that you can maintain indefinitely without drawing on your finite energy reserves. This is the asymptote of the power-duration hyperbola.
W’ (pronounced “W prime”): A fixed amount of work (measured in kilojoules) that you can perform above your CP.
The model is very good at predicting performance for short, high-intensity efforts (e.g., 2 to 20 minutes).
Physiological Basis: FTP is a proxy for a specific metabolic state (MLSS). CP is a mathematical parameter derived from a curve fit; its direct physiological equivalent is debated.
The “Indefinite” Problem: A core assumption of the CP model is that CP is sustainable forever. This is physiologically untrue due to factors like glycogen depletion, muscle damage, and central fatigue.
Practical Discrepancy: For most trained athletes, CP is significantly higher than FTP/MLSS. The two are not interchangeable. Using your CP value to set your threshold training zones would lead to excessively hard workouts.
The podcast mentions a recent paper identifying a “gray zone” between FTP/MLSS and Critical Power. This is an intensity range where you are clearly above your sustainable steady state (you will eventually fatigue), but you are still below the intensity that the CP model would consider its threshold. This highlights that these are two distinct physiological boundaries.
The latter half of the podcast discusses the practical nuances of applying these concepts in a year-round training plan.
This is a fascinating insight. If an athlete does a lot of training at the same FTP wattage, their brain and body can become “stuck” on that level of perceived exertion (RPE). When they re-test, even if their fitness has improved by 15-20 watts, they may subconsciously settle at the old, familiar RPE and produce the same test result. The solution is to break this pattern with novel stimuli:
External Motivation: A long Strava KOM attempt, a hard group ride, or a local time trial.
Zwift Racing: Competing in a category where you are slightly outmatched, forcing you to push harder than you think you can just to stay with the group.
Instead of relying on single FTP tests, a modern approach is to use software like WKO5 that models the entire power-duration curve. By “feeding the model” with periodic maximal efforts across different durations (e.g., a 15-second sprint, a 1-minute effort, a 5-minute effort, and a 20+ minute effort), the software can accurately estimate FTP without the need for a formal, stressful test day.
Precision: FTP is not a razor-sharp number. It exists within a range of about ±5 watts. Don’t get lost in over-precision. When in doubt, it’s always better to set your training zones from a slightly lower FTP.
Fluctuation: Your ability to express your FTP can change day-to-day due to fatigue, nutrition, stress, and hydration. However, your actual physiological FTP does not change by 20 watts week-to-week. If your performance drops significantly, you need rest, not a new FTP.
When to Test: Every 4-8 weeks is a standard guideline for new athletes. For highly trained athletes, where a 10-watt gain in a year is a huge success, testing is far less frequent. Never simply add 5 watts to your FTP every month without validation.
The podcast concludes with a meta-discussion on why these topics are so heavily debated.
The Definition Problem: The scientific community often defines FTP by its most simplified (and often incorrect) marketing definition—95% of 20-minute power. This creates a strawman that is easy to criticize, harming the credibility of the underlying physiological concept (MLSS).
Bad Nomenclature: Names like “Functional Threshold Power” and “Critical Power” are not descriptive and can be misleading. “Functional” can imply it’s just a proxy, not the real thing. “Critical” is an ambiguous term. This poor naming creates barriers to understanding.
Different Worldviews: The debate often stems from people approaching the same problem from different backgrounds. A biochemist thinks about metabolic pathways, an exercise physiologist thinks about VO2 kinetics and established testing protocols, and a coach thinks about real-world performance and feel. They are all looking at the same coin from different sides, using different language, which leads to confusion and disagreement.
The ultimate goal, as the podcast concludes, is to find the same thing: that crucial point where fatigue rapidly accelerates. The specific name or testing protocol used is secondary to understanding the underlying principle.