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⚙️ Configure Heart Rate Zones
❤️ Physiological Cardiology Science

Physiological & Heart Rate Metrics

Evaluating internal cardiovascular load, cardiac drift (aerobic decoupling), Heart Rate Recovery (HRR), and Heart Rate Stress Scores (hrSS).

1. Heart Rate Stress Score (hrSS)

When training without a power meter (e.g. running, swimming, or riding a commuter bike), hrSS serves as the primary metric for tracking training load in your Performance Management Chart (PMC). It is derived from Banister's TRIMP model and normalized so that:

🎯 The 100 hrSS Calibration Rule

Exactly 1 continuous hour at your exact Lactate Threshold Heart Rate (LTHR) yields exactly 100 hrSS, creating complete numerical parity with 1 hour at 100% FTP (100 TSS).

// Mathematical TRIMP Engine:
HR Reserve Ratio: D = (HRavg - HRrest) / (HRmax - HRrest)
Exponential Weighting: Factor = D × e(1.92 × D)  (male: 1.92, female: 1.67)
hrSS = (Duration_minutes × Factor × 100) / (60 × FactorLTHR)

2. Aerobic Decoupling (Cardiac Drift %)

During long endurance rides at steady power, your heart rate naturally tends to creep upward. This phenomenon is known as cardiac drift. Ability.ICU measures this by calculating the Efficiency Factor (EF = NP / HR) across the 1st and 2nd halves of steady aerobic workouts:

🟢 Decoupling < 5.0% (Aerobically Coupled)
Your aerobic system is well-conditioned for this duration. Cardiac stroke volume remains constant, thermoregulation is stable, and slow-twitch muscle fibers are not fatiguing prematurely. You are ready to lengthen duration or increase tempo targets.
🔴 Decoupling > 5.0% (Uncoupled Drift)
Your heart rate is rising disproportionately to mechanical power. Common root causes include dehydration, elevated core body temperature, depletion of muscle glycogen, or exceeding your current aerobic endurance foundation.

3. Heart Rate Recovery (HRR): 60-Second Drop

The speed at which your heart rate drops in the first 60 seconds after terminating a hard interval measures parasympathetic nervous system reactivation:

60-Second HR Drop Rating Clinical Autonomic Diagnosis
> 35 bpm Elite Exceptional vagal reactivation. Highly responsive cardiovascular recovery.
25 – 35 bpm Good / Healthy Normal athletic adaptation. Safe autonomic balance.
15 – 24 bpm Moderate / Fatigued Sympathetic tone remains elevated. Likely carrying accumulated training fatigue.
< 12 bpm Poor / High Stress Autonomic depression. Hallmark sign of acute overtraining, dehydration, or illness.

4. Joe Friel 5-Zone Heart Rate Calibration

Based on your Lactate Threshold Heart Rate (e.g. LTHR = 165 bpm):

Zone Name % of LTHR Beats / Min (165 LTHR) Training Purpose
Z1 Recovery < 81% < 134 bpm Active recovery; stimulates lymphatic flushing without stress.
Z2 Aerobic Endurance 81% – 89% 134 – 147 bpm Fat metabolism optimization; long-distance foundation.
Z3 Tempo 90% – 93% 148 – 153 bpm Aerobic efficiency at race pace; glycogen-sparing.
Z4 Sub-Threshold 94% – 99% 155 – 163 bpm Sweet spot work; increases threshold speed without full fatigue.
Z5a Super-Threshold 100% – 102% 165 – 168 bpm Direct LTHR training; expands lactate clearance capacity.
Z5b Aerobic Capacity 103% – 106% 170 – 175 bpm VO2 Max intervals; maximal oxygen uptake stimulus.
Z5c Anaerobic > 106% 176+ bpm All-out anaerobic repeats and sprint finishes.