Beyond Sport
Metabolic health from heart rate.
TrueZone extends into the low-intensity domain where most people spend most of their lives. The same framework that models elite performance also reads metabolic fitness, substrate balance, and energy dynamics—from ordinary wearable data.
The Opportunity
The strongest predictor of mortality is almost never measured.
Cardiorespiratory fitness (CRF) predicts all-cause mortality more powerfully than smoking, hypertension, or diabetes. But measuring it requires a maximal exercise test with gas exchange analysis in a clinical lab—making it inaccessible to the populations who need it most.
Endurance (E) is the first wearable-derived CRF proxy. It captures the aerobic endurance that underlies cardiorespiratory fitness, estimated from ordinary heart rate data during everyday activity. No treadmill. No gas mask. No lab visit.
- Graded exercise test to exhaustion
- Metabolic cart and gas exchange analysis
- Clinical supervision required
- Single-point measurement, no longitudinal tracking
- Ordinary heart rate data from any wearable
- No maximal effort required
- No clinical supervision needed
- Continuous tracking, session by session
MFI
Metabolic Fitness Index.
A single score (0–5) that combines metabolic thresholds, cardiovascular bounds, and body size into one trackable number. MFI correlates inversely with glycemic response markers, supporting its use as a wearable-native proxy for metabolic flexibility and glycemic control.
Threshold speed
Speed at the lactate threshold (V2), reflecting oxidative capacity. Higher threshold speed at the same cardiac cost indicates greater metabolic efficiency.
Cardiovascular bounds
Resting heart rate and HRmax define the cardiac range. A wider range relative to threshold placement indicates better cardiovascular reserve and autonomic function.
Allometric scaling
Body size adjustment using allometric BMI reduces bias across different body types, making MFI comparable across populations without penalising larger or smaller individuals.
Metabolic Zones
Where daily life sits on the metabolic scale.
Metabolic zones map the slow domain (rest to moderate intensity) into three personalised heart rate bands anchored to your individual thresholds. The Metabolic Gradient (0–3) and Resting Zone Index (RZI) provide a simple, individualised window into metabolic state at rest—where two people with the same resting heart rate may sit in entirely different metabolic zones.
Below P0. Deep basal metabolism, predominantly fat-dominant. A young, fit individual rests deep in Z0 with low RZI—indicating strong fat-oxidation reserve and autonomic efficiency.
P0 to P0.5. Mixed oxidative metabolism with rising glucose flux. As fitness declines or metabolic load increases, resting HR shifts upward into Z1—a higher RZI and increasing carbohydrate reliance at rest.
P0.5 to P1. Upper slow domain with fast-oxidative onset. When even easy walking places someone in Z2, it signals significant metabolic strain—common in deconditioning, obesity, and ageing.
Resting Zone Index (RZI): Where your resting heart rate falls on the Metabolic Gradient (0–3). Lower RZI indicates greater fat-dominant reserve and better metabolic fitness. Higher RZI suggests a shift toward carbohydrate reliance at rest. Because thresholds are personalised, RZI is individualised—not just a heart rate number.
Energy Expenditure
Endurance-aware calorie estimation.
Current wearable calorie estimates use generic HR–VO₂ regressions that treat every user the same. A high-endurance individual and a sedentary person at the same heart rate have very different metabolic costs—but generic models cannot distinguish them.
TrueZone incorporates the endurance parameter (E) into energy expenditure estimation, separating basal, activity, and feeding components. The result is more physiologically informed than methods that ignore individual aerobic capacity.
Energy Intake (Pilot)
Postprandial HR dynamics.
After eating, heart rate rises as the body directs blood flow to digestion—the thermic effect of food. TrueZone includes pilot-scale modelling of these postprandial heart rate responses to estimate energy intake from HR dynamics alone.
This is early-stage research, but the signal is detectable. Combined with energy expenditure, it opens the door to continuous, non-invasive energy balance estimation from a single wearable sensor.
Applications
Where metabolic health meets scale.
These capabilities are designed for platforms that serve populations—not just athletes. The same SDK, the same parameters, applied to health instead of performance.
Digital health platforms
Integrate CRF and metabolic flexibility screening into consumer health apps. Replace step counts with physiologically grounded metrics.
Corporate wellness
Track workforce metabolic health at scale using existing wearable data. Identify metabolic risk trends before clinical symptoms appear.
Preventive medicine
Provide clinicians with continuous, non-invasive CRF monitoring. Endurance (E) as a trackable vital sign for cardiovascular risk.
Ageing research
Longitudinal tracking of metabolic flexibility decline. Quantify the effect of interventions on metabolic health trajectories over months and years.
Obesity and insulin resistance
Metabolic zones reveal substrate inflexibility. Track changes in fat oxidation capacity and metabolic state through lifestyle interventions.
Clinical trials
Continuous, objective metabolic endpoints from wearable data. Reduce reliance on periodic lab visits for metabolic assessment.
The same heart rate signal. A new layer of insight.
Endurance, MFI, metabolic zones, and energy balance—all derived from data your users already collect. No hardware changes. No lab tests.