Guides

Metabolic Health

MFI, RZI, metabolic zones, and energy tracking for health applications.

TrueZone extends beyond sports into metabolic health monitoring. The same three parameters (E, V, P) that drive training zones also define metabolic fitness indicators that track glucose control, fat oxidation capacity, and cardiovascular reserve.

Metabolic Fitness Index (MFI)

"mfi": 2.49

A dimensionless 0–5 score that combines:

  • V2 (speed at lactate threshold) — aerobic capacity
  • P2 (HR at threshold) and RHR — cardiovascular bounds
  • BMIa (allometric BMI = kg/m^2.5) — body size normalization

Formula: MFI = (V2 × 1000 × HRmax) / (60 × (P2 − RHR) × BMIa × 6)

Higher MFI = more distance per heartbeat at threshold after accounting for body size. MFI correlates inversely with glycemic response markers (R² = 0.98 in validation studies), making it a proxy for metabolic flexibility and glucose control.

How to read it:

  • 4–5: Excellent metabolic fitness (young, athletic)
  • 2–3: Average (healthy adult)
  • 1–2: Below average (sedentary, aging, or overweight)
  • <1: Poor (metabolic syndrome risk)

Resting Zone Index (RZI)

"rzi": 0.35

RZI = where your resting heart rate falls on the Metabolic Gradient (MG), a 0–3 scale across the slow-domain metabolic zones (MZ0–MZ2).

To compute RZI: provide measuredRHR in settings (actual resting HR from the wearable). The SDK maps it to the MG scale using the individual's threshold boundaries.

  • RZI < 1.0: Resting in MZ0 (fat zone) — healthy metabolic reserve
  • RZI 1.0–2.0: Resting in MZ1 (glucose zone) — declining flexibility
  • RZI > 2.0: Resting in MZ2 (lactate zone) — elevated metabolic cost at rest

Lower RZI over time = improving metabolic fitness. Track weekly to detect trends.

Metabolic zones (MZ0–MZ2)

"metabolic_zones": [
  {"name": "MZ0", "label": "Fat zone", "min": 47, "max": 114, "mg_range": "0-1"},
  {"name": "MZ1", "label": "Slow glucose", "min": 114, "max": 127, "mg_range": "1-2"},
  {"name": "MZ2", "label": "Slow lactate", "min": 127, "max": 141, "mg_range": "2-3"}
]

Metabolic zones cover the slow domain only (0–P1) — the intensity range where most of daily life occurs. They are anchored to personal thresholds at P0, P0.5, and P1.

For a fit individual, walking stays in MZ0. For an aging or obese individual, the same walk may push into MZ2 — exposing elevated physiological strain that's invisible to step counts or generic HR monitoring.

Energy tracking factors

The metabolic_factors section provides coefficients for continuous all-day HR monitoring:

FactorUseDescription
BF (base_factor)Resting caloriesDivide resting HR beats by BF to get kcal
AF (activity_factor)Exercise caloriesDivide exercise HR beats by AF to get kcal
FF (food_factor)Energy intakeDivide post-meal thermogenic beats by FF to estimate meal kcal

These factors are personalized from the individual's E, Vmax, P and demographics. They enable any platform with continuous HR data to compute a 24-hour energy balance without accelerometry.

Fiber zones (Z0–Z11)

The fiber_zones array provides all 12 motor-unit recruitment zones, from Z0 (fat-dominant slow oxidative fibers) to Z11 (sprint zone, PCr/neuromuscular). Each zone has HR boundaries and V-scale ranges.

These are the finest-grained intensity classification available — useful for research, clinical applications, and advanced training prescription.

Walking analysis

Walking HR analysis is particularly powerful for metabolic health. Even without running data, TrueZone can extract meaningful information from walking:

  • Walking in MZ0 = strong aerobic base, fat-dominant metabolism
  • Walking pushing into MZ1–MZ2 = declining fitness, elevated metabolic cost
  • Walking HR trend over weeks = metabolic fitness trajectory

This makes heart rate during everyday walking a diagnostic signal for populations that don't exercise — the exact group most at risk for metabolic disease.

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