Blog

News, research, and training insights.

Updates on TrueZone development, validation studies, and insights from exercise physiology and metabolic health research.

Field notes

Physiology, validation, product thinking, and the arguments behind the model.

Science

The tokenizer for heart rate

Foundation models in health are training on noise. Heart rate traces without physiological labels teach models statistics, not biology. TrueZone changes that.

April 21, 2026
Science

We tested 297 cyclists. Here's what FTP gets wrong.

FTP assumes every rider can sustain their lactate threshold for 60 minutes. We tested 297 cyclists and found that's only true for a narrow slice of the population.

March 28, 2026
Science

3 parameters vs 270,000 runs: how TrueZone matches Apple's model

Apple trained a deep neural network on 270,000 runs from 7,465 users. TrueZone matches their accuracy with a physiological ODE and three parameters.

March 15, 2026
Science

The parameter that no wearable measures

Every fitness tracker estimates VO2max. None of them measure endurance. That's a problem, because endurance is what actually determines how far you can go.

February 20, 2026
Science

What GPS vests miss about your players

Every professional team tracks distance, speed, and sprint counts. Almost none of them track the physiological cost of that work — and the data to do it is already there.

February 5, 2026
Training

Your zones are wrong — and it's not your watch's fault

Fixed-percentage heart rate zones ignore the one variable that determines where your thresholds actually fall: endurance.

January 18, 2026
Science

Metabolic fitness: the health metric hiding in your heart rate

Metabolic flexibility — the ability to switch between fat and carbohydrate as fuel — is one of the most important markers of metabolic health. It's also never been trackable from a wearable. Until now.

December 10, 2025
Training

Why maximum speed is a limiting factor for endurance performance

Maximum speed has not been widely used as a fitness metric and especially not for endurance athletes but is in fact, instrumental in dictating performance.

July 24, 2023
Training

Why endurance matters for your training zones

Heart rate zones are important to know if we want to train efficiently. Unfortunately it can be difficult to know our true heart rate zones.

March 15, 2023
Science

The Discovery of Endurance

If you are reading this blog then surely you must have witnessed Eliud Kipchoge's epic breaking of the 2-hour barrier in the marathon in Vienna in 2019.

March 12, 2022
Science

Your True Exercise Thresholds

Exercise thresholds have been defined as boundaries between different domains of exercise intensity. They are used by medical professionals to clinically assess patient physiological function or dysfunction, such as heart failure, diabetes, peripheral artery disease, and suitability for surgery or transplant.

March 11, 2022
Science

Cracking the Heart Rate Code

Wow, 3.6 billion! In years, that‘s about the entire history of life on Earth. In people, that‘s about the coverage of Zuckerberg’s Metaverse. But in your chest, that's about the number of times your heart might beat should you be so blessed to live into your old age!

March 10, 2022