Original episode & show notes | Raw transcript
In the world of endurance sports, two terms are frequently discussed, often interchangeably and with some confusion: VO2 Max and Functional Threshold Power (FTP). While both are critical measures of aerobic fitness, they represent distinct physiological concepts. Understanding their individual roles and, more importantly, their interplay is fundamental to grasping the true nature of endurance performance.
This guide will delve into these concepts, using the framework of a landmark 1988 study, “Determinants of Endurance in Well-Trained Cyclists,” to illustrate why the relationship between these two metrics—not just their absolute values—is the key to unlocking superior endurance.
Before we explore their relationship, we must first establish a clear understanding of each component.
VO2 Max, or maximal oxygen uptake, is the maximum rate at which your body can take in, transport, and utilize oxygen during intense exercise. It is a measure of the absolute upper limit of your aerobic (oxygen-dependent) energy system.
The Science: It’s typically measured in a lab in milliliters of oxygen per kilogram of body weight per minute (mL/kg/min) or as an absolute value in liters of oxygen per minute (L/min). It represents the highest rate at which your mitochondria, the powerhouses of your cells, can produce ATP (the body’s energy currency) through aerobic oxidation.
The Analogy: Think of VO2 Max as the maximum potential horsepower of a car’s engine. A bigger engine has a higher potential top speed.
Common Misconception: A once-popular idea was that VO2 Max is a genetically fixed ceiling that you are “born with.” While genetics play a role, it is highly trainable and can be significantly improved. However, it is fundamentally a ceiling for your aerobic performance, not a direct predictor of your performance in a 2-hour race.
Functional Threshold Power (FTP) represents the highest power output an athlete can maintain in a quasi-steady-state for a prolonged duration (typically around 30-70 minutes) without a rapid onset of fatigue.
The Science: In scientific literature, the concept most analogous to FTP is the Maximal Lactate Steady State (MLSS). This is the highest exercise intensity where the rate of lactate production in the muscles equals the rate of lactate clearance from the blood. Below this threshold, lactate levels remain stable. Above it, lactate begins to accumulate exponentially, leading to a rapid increase in metabolic acidosis and fatigue.
The Analogy: If VO2 Max is the engine’s maximum horsepower, FTP is the highest speed you can maintain for a long road trip without the engine overheating and forcing you to slow down. It’s your sustainable, high-performance cruise control.
The central theme of the podcast is that while VO2 Max sets the potential, your FTP as a percentage of your VO2 Max is a far better predictor of real-world endurance performance.
Two athletes can have the exact same VO2 Max (the same engine size), but the athlete who can sustain a higher percentage of that maximum will be superior in almost every endurance scenario.
Athlete A: VO2 Max of 4.5 L/min, FTP at 70% of VO2 Max.
Athlete B: VO2 Max of 4.5 L/min, FTP at 85% of VO2 Max.
Athlete B will be able to ride at a significantly higher power output for a long duration, making them a much more formidable endurance athlete, despite having the same “aerobic ceiling” as Athlete A. Their superior performance comes from being able to utilize a greater fraction of their potential.
The podcast centers its discussion on a seminal paper, “Determinants of Endurance in Well-Trained Cyclists,” co-authored by Dr. Ed Coyle and a young Andy Coggan. This study was ahead of its time because it directly investigated this crucial relationship.
The researchers selected 14 well-trained cyclists who all had a very similar absolute VO2 Max (around 4.2-4.3 L/min). This was the key control. By keeping the “engine size” constant across all participants, they could isolate and study the effects of the other major variable: the percentage of VO2 Max at which their lactate threshold (FTP/MLSS) occurred.
They then divided the cyclists into two groups:
High Group: FTP occurred at a high percentage of VO2 Max (80-86%).
Low Group: FTP occurred at a lower percentage of VO2 Max (60-67%).
The first test had the cyclists ride at an intensity of 88% of their individual VO2 Max until they could no longer continue.
The Logic: This intensity was above every participant’s FTP. However, for the “High Group,” it was only slightly above their threshold. For the “Low Group,” it was significantly above their threshold.
The Findings: The results were dramatic and showed a very strong correlation.
The athlete with the highest FTP (86% of VO2 Max) lasted for 75 minutes. For him, this effort was only 2% above his threshold. His blood lactate at the end was a manageable 6.1 mmol/L.
An athlete from the Low Group (FTP at 66.7% of VO2 Max) lasted only 12 minutes. For him, this was a massive effort, more than 20% above his threshold. His blood lactate skyrocketed to a painful 18.2 mmol/L.
Conclusion: The closer the test intensity was to the athlete’s FTP, the longer they could sustain it. This demonstrates that FTP, not VO2 Max, dictates fatigue resistance at high, sub-maximal intensities.
The second test had the cyclists ride for 30 minutes at a fixed intensity of 80% of their VO2 Max.
The Logic: This intensity created a different physiological scenario for each group. For most of the “High Group,” this was a sub-threshold effort. For the “Low Group,” this was an at-threshold or supra-threshold effort.
The Findings: The metabolic data revealed profound differences in efficiency.
Glycogen Use: The Low Group burned more than twice as much muscle glycogen (65.4 mmol/kg) as the High Group (27.9 mmol/kg) to sustain the same relative intensity.
Metabolic Fuel: The High Group demonstrated a Respiratory Exchange Rate (RER) of ~0.81-0.86, indicating they were fueling their effort with a healthy mix of fats and carbohydrates. The Low Group, being over their threshold, would have had a much higher RER (closer to 1.0), indicating near-total reliance on carbohydrates.
Conclusion: A higher FTP as a percentage of VO2 Max confers a significant metabolic advantage. At the same high power output, these athletes are more efficient, spare precious glycogen stores, and rely more on fat for fuel, all of which are hallmarks of elite endurance performance.
The insights from this study have critical takeaways for both athletes and scientists.
FTP is King for Endurance: While improving your VO2 Max is important for raising your ultimate potential, the primary goal for most endurance athletes should be to increase their FTP, thereby increasing the percentage of VO2 Max they can sustain.
Fueling and Pacing: An athlete with a higher %FTP is more “economical.” They burn less carbohydrate at any given high intensity. This means they can handle surges in a race with less metabolic cost and are less likely to “bonk” from glycogen depletion.
Training Focus: This explains why so much modern training is focused on “threshold” work. The goal is to push the MLSS (and thus FTP) to a higher and higher percentage of your VO2 Max.
This study exposes a major flaw present in many exercise science studies, both old and new.
The Protocol Problem: Any study that prescribes exercise intensity only as a percentage of VO2 Max, without accounting for individual thresholds, should be viewed with skepticism.
Why It’s a Problem: As the Coyle study shows, assigning a task at “75% of VO2 Max” could be an easy endurance pace for one subject and a brutal, lactate-accumulating effort for another, even if they have identical VO2 Max values. This failure to normalize the physiological strain across participants can completely confound the data and lead to unreliable conclusions.
VO2 Max represents your physiological ceiling, but it doesn’t define your endurance capability. True endurance performance is dictated by the highest percentage of that ceiling you can sustainably hold—your FTP. An athlete with a high FTP relative to their VO2 Max is more fatigue-resistant, more metabolically efficient, and better equipped for the demands of long-duration events. This fundamental relationship, elegantly demonstrated by Coyle and Coggan over three decades ago, remains a cornerstone of modern training theory and a critical lens through which we should view exercise science.