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. One physiological framework, three parameters, every metric derived from the same structure hidden inside the heart rate signal.
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 (0–100%)
The first direct, wearable-accessible measure of aerobic endurance. E reflects fat-oxidation capacity, mitochondrial density, and slow-twitch muscle fiber ratio. It captures how much of your aerobic machinery is active at submaximal intensity—the ratio of threshold alignment in the model's characteristic speed curve.
A marathoner typically scores E > 90%. A sprinter may score below 20%. The parameter is derived geometrically from the relationship between lower and upper threshold speeds.
Maximum Speed (Vmax)
The maximum neuromuscular speed capacity. Vmax defines the top of the intensity axis and determines where every threshold and zone sits on the speed scale. It reflects the fastest speed the model predicts an individual can recruit motor units for, regardless of metabolic capacity.
In cycling, this maps to maximum power output rather than speed. The same model architecture adapts to both modalities.
HR Peak (Model-Predicted HRmax)
The model-predicted maximum heart rate, determined geometrically from submaximal heart rate kinetics. No maximal effort test is required. P anchors the heart rate axis, enabling all derived HR zones to be placed precisely.
Unlike age-based formulas (220 − age), P is individualized and refines with each session. Two people of the same age and fitness may have very different P values.
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.
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.
Fiber Zones
Twelve motor-unit pools, recruited sequentially.
The fiber zone system models 10 overlapping motor-unit pools (Z1–Z10) recruited sequentially as intensity rises, plus a base zone Z0 (fat-dominant at low intensities) and a sprint zone Z11 (supramaximal, beyond HRmax). Each zone represents a band of the V-scale where a specific fiber population dominates.
Fat-dominant base
Recovery / easy aerobic
Aerobic base
Moderate aerobic
Tempo / Fatmax ceiling
Threshold onset
Lactate threshold
VO₂max
Anaerobic power
Neuromuscular
Max neuromuscular
Sprint / supramaximal
The fiber zone framework gives coaches and athletes a higher-resolution view of intensity than traditional five-zone models. Because each zone maps to a specific motor-unit recruitment band, it connects directly to the physiology of adaptation.
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.
True 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 for distances from 1 km 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 converges from ordinary training sessions. Parameters typically stabilize within 1–3 sessions—a median of 1 session for E and P, and 3 sessions for Vmax.
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.
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.