How It Works
The unified model of human physiology.
VO₂max, thresholds, zones, endurance, recovery, metabolism—exercise science has studied these for decades as separate concepts, measured with separate tools, defined by separate models. TrueZone unifies them.
Model premise
One physiological framework, three parameters, every output derived from the same heart-rate dynamics.
The Parameters
Three numbers define the athlete.
Every person's heart rate response to exercise is governed by three physiological quantities. TrueZone extracts them from ordinary activity data using a Bayesian ODE fit. From these three parameters, the entire fitness and metabolic profile follows.
Endurance
The first direct, wearable-accessible measure of aerobic endurance. E reflects fat-oxidation capacity, mitochondrial density, and slow-twitch muscle fiber ratio.
A marathoner typically scores above 90%. A sprinter may sit below 20%. The parameter is derived geometrically from threshold alignment.
Scale
0-100%
Maximum Speed
The maximum neuromuscular speed capacity. Vmax defines the top of the intensity axis and determines where every threshold and zone sits.
In cycling, this maps to maximum power output rather than speed. The same model architecture adapts to both modalities.
Scale
Vmax
HRmax
The model-predicted maximum heart rate, determined geometrically from submaximal heart-rate kinetics. No maximal effort test is required.
Unlike age-based formulas, P is individualized and refines with each session through Bayesian memory.
Scale
bpm
Interactive
See how the parameters shape the profile.
Adjust endurance and maximum speed to see how thresholds, zones, and the full physiological fingerprint shift in real time.
Try It
Adjust the three parameters.
Move E, Vmax, and HRmax to see how every derived metric shifts. Race times, thresholds, VO₂max—all from the same three numbers.
Derived outputs
The Key Distinction
VO₂max is not endurance.
VO₂max
Oxygen supply. Maximum aerobic capacity. How much oxygen your body can consume at peak effort. Important, but it doesn't tell you how efficiently that oxygen is used, how long you can sustain effort, or how resistant you are to fatigue.
Endurance (E)
Oxygen economy. How efficiently you use available capacity. Reflects fat-oxidation efficiency, threshold alignment, and fatigue resistance. Two athletes with identical VO₂max can differ by 30+ minutes in the marathon—E captures why.
VO₂max tells you the size of the engine. E tells you the fuel economy.
The V-Scale
A mechanistic intensity axis.
The V-scale replaces traditional %VO₂max and %HRmax scaling with a mechanistic axis running from rest to maximum speed. Intensity is expressed as a fraction of the individual's own neuromuscular ceiling—not a population average.
Major waypoints on the V-scale correspond to real physiological transitions, not arbitrary percentages. These are derived geometrically from the model's ODE structure, not set by heuristic rules.
V-Scale Waypoints
The lowest intensity the model tracks. Corresponds to resting metabolic state.
The intensity at which fat burning peaks. Geometrically placed from E and Vmax.
The transition from predominantly aerobic to mixed metabolism. Talk test boundary.
Maximum lactate steady state. The highest sustainable aerobic intensity.
The speed at which heart rate reaches its model-predicted maximum (P). Beyond here, further speed recruits only anaerobic pathways.
P-Scale Waypoints & Training Zones
The floor of the P-scale. Measured or estimated from the model.
Heart rate at the first ventilatory threshold. Below P1 is Zone 1 (easy aerobic). Above it, Zone 2 begins.
Heart rate at the lactate threshold. The boundary between sustainable and unsustainable intensity. Zone 4 begins.
The model-predicted maximum heart rate. Zone 5 represents efforts at or near this ceiling.
Training Zones (Z1–Z5)
The P-Scale
Heart rate mirror of the V-scale.
Every waypoint on the V-scale has a corresponding heart rate on the P-scale. Where the V-scale describes intensity in terms of speed, the P-scale describes it in terms of heart rate. Together, they form a dual-axis framework for prescribing and monitoring training.
Training zones Z1 through Z5 map directly to the P-scale ladder. Because the thresholds are derived from the individual's own physiology, the zones are truly individualized—not percentage-based approximations.
Motor-Unit Recruitment
A continuum of fiber pools, recruited progressively.
As intensity rises, Henneman's size principle recruits motor units in order of size — small slow-oxidative units first, supplied efficiently from fat and glucose oxidation, followed by larger fast-oxidative and fast- glycolytic units that shift metabolism toward carbohydrate dependence and glycolysis.
Each pool, as it engages, leaves a distinct signature on the cardiovascular response — shifts in oxygen demand, lactate accumulation, and sympathetic drive that the heart-rate system integrates. The shape of an HR trace over a session (its rise, plateau, drift, and recovery) is the integrated record of which pools were recruited, when, and at what cost. That is why HR is rich enough to invert.
TrueZone models this recruitment as a continuum of overlapping pools spanning the V-scale, anchored to physiological landmarks rather than fixed percentages of HRmax. Endurance (E) sets where those landmarks fall for each individual; the same recruitment pattern reads as a different HR shape for a high-E athlete than for a low-E one, and recovering E from the HR shape is recovering the recruitment thresholds.
Slow oxidative
Type I fibers dominate. Fat-oxidation reserve carries low and moderate intensities.
Fast oxidative
Type IIa fibers progressively recruited. Carbohydrate flux rises; lactate clearance still keeps pace.
Fast glycolytic
Type IIx fibers engage near and above threshold. Glycolytic flux exceeds oxidative capacity.
Resolving the recruitment continuum with physiological landmarks — rather than enumerated zones — lets the same model carry across running, cycling, and team sport without bespoke zone definitions for each context.
Derived Outputs
Everything follows from three parameters.
Once E, Vmax, and P converge, the model derives the complete fitness and metabolic profile. No additional tests or inputs are required.
Endurance (E)
The first direct, wearable-accessible measure of aerobic endurance. Tracks fat-oxidation capacity and mitochondrial adaptation over time.
HRmax (P)
Model-predicted maximum heart rate derived geometrically from submaximal data. No maximal test required. Anchors the P-scale.
VO₂max
Estimated from E and Vmax without a graded exercise test. Validated against laboratory measurements across multiple datasets.
Exercise thresholds
VT1, LT2, and Fatmax are geometrically placed on the V-scale. No lactate sampling or gas exchange required.
Individualized HR zones
Five training zones (Z1–Z5) mapped to the P-scale. Boundaries shift automatically as E and P converge.
Race time prediction
Predicted finishing times from 100 m to the marathon, derived from the speed–endurance relationship.
Fat oxidation rate
Peak fat oxidation intensity and total fat utilisation derived from the endurance parameter and V-scale waypoints.
Energy expenditure
Separated into basal, activity, and feeding components. More accurate than generic HR–VO₂ regression by accounting for endurance.
Metabolic Fitness Index
A composite score quantifying metabolic flexibility—the ability to switch between fat and carbohydrate as fuel.
Recovery kinetics
Heart rate recovery speed and post-exercise drift modelled from the ODE. Tracks autonomic fitness over time.
Training load
Session intensity quantified against the individual’s own threshold ladder, not population averages.
Cardiac drift
The progressive rise in heart rate at constant effort. TrueZone models it explicitly as part of the ODE dynamics.
Bayesian Accumulation
How it learns.
Each activity session produces an error surface in three-dimensional parameter space. TrueZone accumulates these surfaces using Bayesian memory with exponential decay—recent sessions count more, older sessions fade.
No calibration test is needed. The model produces initial estimates from the first session and typically converges within 3–10 sessions, depending on data quality and activity variety.
As the athlete's physiology changes over weeks and months, the exponential decay ensures parameters track those changes. The model is always current, never stale.
Real-Time Capabilities
Not just retrospective. Real-time.
Because TrueZone is a physics model—not a statistical regression—it runs forward in time. Given your current intensity, the ODE predicts what happens next: how your heart rate will evolve, when you'll cross a threshold, and how long you can sustain the current effort.
Time-to-exhaustion
“How long can I hold this pace?” The model predicts sustainable duration at any given intensity, updated continuously as conditions change.
Fatigue trajectory
Real-time cardiac drift tracking reveals fatigue buildup during the session. The model distinguishes genuine fatigue from normal HR variability.
Substrate balance
Live estimation of aerobic vs. anaerobic contribution at the current intensity. Shows how close you are to depleting glycogen reserves at race pace.
HR prediction
Given your current speed or power, the model predicts where your heart rate will be in 5, 10, or 30 minutes—before you get there.
Pacing guidance
For a target distance or duration, the model calculates the optimal pace that avoids premature fatigue—individualized to your endurance and speed profile.
Live zone tracking
Real-time display of which physiological zone you're in, based on individualized thresholds—not fixed percentage bands that ignore your endurance.
Three parameters. No black box. Real physiology.
TrueZone is available as an SDK and API for wearable platforms, training apps, and digital health providers. Every metric described on this page is derived and returned per user.