The parameter that no wearable measures

Open any fitness app connected to a modern wearable and you will find a VO2max estimate. Garmin calls it "VO2 Max." Apple calls it "Cardio Fitness." COROS calls it "Base Fitness." Polar has "Running Index." WHOOP derives a similar metric. The entire wearable industry has converged on the same idea: your aerobic capacity, expressed as maximum oxygen uptake in ml/kg/min, is the number that defines your fitness.

There is just one problem. VO2max tells you almost nothing about how far you can go.

What VO2max actually is

VO2max measures the maximum rate at which your body can take in, transport, and utilize oxygen during exercise. It is primarily constrained by cardiac output -- how much blood your heart can pump per minute -- and secondarily by oxygen extraction at the muscles.

It is a legitimate physiological measurement. Higher VO2max generally means a higher ceiling for aerobic work. It correlates with cardiovascular health, all-cause mortality, and athletic potential. As a health metric, it has real value.

But as a performance metric for endurance sport, it has a fundamental limitation: VO2max measures capacity, not economy.

Knowing someone's VO2max tells you the size of their engine. It tells you nothing about their fuel efficiency.

The marathon problem

Consider two runners, both with a VO2max of 55 ml/kg/min. By every wearable on the market, they are equally fit. But one runs a 3:05 marathon and the other runs a 3:38 marathon. A 33-minute gap -- at the same VO2max.

This is not a hypothetical. It is a pattern that shows up repeatedly in the research literature and in real-world data. VO2max explains roughly 30-40% of the variance in marathon performance among trained runners. The rest comes from factors that VO2max does not capture.

The Nike Breaking2 project made this strikingly visible. The three elite runners selected for the sub-two-hour marathon attempt -- Eliud Kipchoge, Zersenay Tadese, and Lelisa Desisa -- had an average VO2max of approximately 71 ml/kg/min. That number is impressive, but it is not extraordinary. Many competitive recreational runners achieve similar values. What separated the Breaking2 athletes was not their oxygen ceiling. It was what they could do with it.

The missing parameter

What separates a 3:05 marathoner from a 3:38 marathoner at the same VO2max is endurance -- the ability to sustain a high fraction of maximum capacity over time.

In TrueZone's physiological model, this is captured by the parameter E, which scales from 0 to 1 (or equivalently, 0% to 100%). E is derived from how an individual's exercise thresholds align on the intensity scale, and it encodes several interrelated physiological properties:

Fat oxidation efficiency. A high-E athlete burns more fat and less glycogen at any given intensity. This extends the duration they can sustain before glycogen depletion forces a slowdown. Fat is a nearly unlimited fuel source; glycogen is not. The crossover point between fat and carbohydrate metabolism shifts higher with better endurance.

Threshold alignment. In a high-E individual, the aerobic threshold, lactate threshold, and anaerobic threshold are compressed toward the top of the intensity scale. This means a larger fraction of their total range is aerobic -- they can work harder before crossing into unsustainable metabolic territory.

Fatigue resistance. High E correlates with slower rates of peripheral fatigue, better lactate clearance, and more efficient oxygen utilization at sub-maximal intensities. The same pace feels easier and costs less metabolically.

A runner with E = 0.85 and VO2max of 55 might run their marathon at 82% of VO2max and hold it comfortably. A runner with E = 0.55 and the same VO2max might only sustain 68% of VO2max before lactate accumulation forces them to slow. Same engine, vastly different fuel economy.

Why no wearable measures it

The short answer: the field has been obsessed with VO2max for over 50 years.

VO2max was first described by A.V. Hill in the 1920s. It became the gold standard for aerobic fitness assessment in exercise physiology labs through the mid-20th century. When wearable technology emerged, VO2max was the obvious target -- it was well-defined, well-studied, and could be estimated from heart rate and pace data using established regression equations (most notably the Firstbeat algorithm that Garmin and others license).

Endurance, by contrast, has never had a clean, single-number representation. The exercise science literature refers to related concepts -- "fractional utilization of VO2max," "lactate threshold as a percentage of VO2max," "running economy" -- but these have remained lab measurements requiring gas analysis or blood sampling. No one had distilled them into a parameter that could be estimated from wearable data alone.

That is what E does. It is not a lab measurement translated to a watch. It is a parameter that emerges from a physiological model fitted to the data that wearables already collect: heart rate and pace, session after session.

Capacity vs. economy

Think of it this way. VO2max is like the horsepower rating of an engine. It tells you the maximum power output. But if you want to know how far a car can drive on a tank of fuel, horsepower is not enough. You need to know the fuel efficiency -- miles per gallon.

Two cars with 300 horsepower can have wildly different ranges. A heavy SUV with a thirsty V8 will drain its tank long before a lightweight hybrid with the same peak power. Same capacity, different economy.

In human physiology:

• VO2max is oxygen supply -- the maximum rate of delivery
• E is oxygen economy -- how efficiently that supply is used

Both matter. An athlete with high E but low VO2max has great economy but a low ceiling. An athlete with high VO2max but low E has a high ceiling but burns through their fuel reserves too quickly. The best endurance athletes have both -- a big engine and great fuel economy.

But of the two, only VO2max gets measured by your wearable. E is invisible.

E is trainable

This is perhaps the most important point. E is not fixed. It responds to training -- specifically to the kind of training that endurance athletes have always known works: consistent aerobic volume, long runs, tempo work at or just below threshold.

E changes slowly, over months and years. It reflects deep physiological adaptations: mitochondrial density, capillarization, fat oxidation enzyme activity, slow-twitch fiber development. These are not things that shift in a week. They are the cumulative result of sustained aerobic training.

This is precisely why E is so valuable to track. VO2max can swing by 3-5% in a few weeks based on training load. E moves more glacially, reflecting genuine structural adaptation. When your E rises from 0.65 to 0.72 over six months of base building, that represents a real and durable improvement in your endurance capacity -- not a transient fluctuation.

Conversely, when E declines, it signals a loss of aerobic base that may not be apparent from VO2max alone. A runner returning from injury might maintain their VO2max through cross-training but lose significant endurance. Their wearable would show them as equally fit. They are not.

The gap in the market

Every major wearable company has invested heavily in VO2max estimation. None of them measure E. This is not because E is unmeasurable -- it is because the modeling framework to extract it from wearable data did not previously exist.

The data is already on your wrist. Heart rate and pace from every run, every ride, every session. The raw information needed to determine your endurance parameter is being recorded and discarded, or at best used to estimate the one metric the industry has agreed upon: VO2max.

The result is that millions of athletes are navigating their training with half the picture. They know the size of their engine. They have no idea about their fuel economy. And fuel economy is what determines whether you hold your pace in the last 10K of the marathon or watch it slip away.