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Physiological Science

Physiological & Heart Rate Metrics

Using cardiac metrics to estimate training strain, efficiency progression, and cardiovascular recovery.

1. Heart Rate Stress Score (hrSS)

When training without a power meter, **hrSS** serves as the primary metric for tracking training load. Based on the Karvonen heart rate reserve method and Banister's TRIMP model, hrSS estimates total physiological stress on your cardiovascular system.

The Mathematical Engine

D = (HRavg - HRrest) / (HRmax - HRrest)
raw_hrSS = (durationMinutes × D × ec × D × 100) / 60
*Where c is the gender coefficient (1.92 for males, 1.67 for females). The score is automatically calibrated so that exactly **1 hour of continuous effort at your Lactate Threshold Heart Rate (LTHR)** yields **100 hrSS**.

2. Aerobic Decoupling (Cardiac Drift)

Aerobic Decoupling compares your physical output (Power) against your physiological cost (Heart Rate) between the first and second halves of an activity. Over a long, steady workout, your heart rate naturally rises even if power remains identical. This is called **cardiac drift**.

How It's Calculated

1. We calculate the **Efficiency Factor (EF)** for the first half of the activity: EF1 = NP1 / HR1avg
2. We calculate the EF for the second half: EF2 = NP2 / HR2avg
3. The percentage difference is your decoupling rate:

Decoupling (%) = ((EF1 - EF2) / EF1) × 100
Decoupling < 5.0% (Aerobic Fit) Your aerobic system is well-conditioned. Your heart is operating efficiently without major cardiac drift during endurance sessions.
Decoupling > 5.0% (Uncoupling) Indicates aerobic fatigue, dehydration, heat stress, or that the duration exceeds your current cardiovascular endurance capacity.

3. Heart Rate Recovery (HRR-1m)

**Heart Rate Recovery (HRR)** measures the drop in heart rate (beats per minute) exactly 60 seconds after peak exertion. It is a key indicator of your autonomic nervous system's health, specifically showing how quickly your parasympathetic nervous system can calm the cardiovascular system.

HRR Drop (1 Min) Status Interpretation
< 12 bpm Poor / Fatigued High risk indicator. Suggests overreaching, severe fatigue, or cardiorespiratory system under load.
12 - 20 bpm Fair Normal range for recreationally active individuals or after high anaerobic depletion.
20 - 30 bpm Good / Trained Healthy autonomic response. Typical of regular aerobic athletes.
> 30 bpm Excellent / Peak Fit Exceptional cardiovascular conditioning. Typical of highly fit endurance athletes.

4. Heart Rate Zones (Joe Friel Model)

Your heart rate zones are calibrated using your **Lactate Threshold Heart Rate (LTHR)**. LTHR is the maximum average heart rate you can sustain for an hour of hard running or cycling.

Zone Name % of LTHR Target System / Effort
Zone 1 Active Recovery < 85% Easy spins, active flushing of metabolic waste. Conversation is completely effortless.
Zone 2 Aerobic Threshold 85% - 89% **Foundation Zone**. Boosts mitochondrial density and capillary growth. Essential for base building.
Zone 3 Tempo 90% - 94% Moderate-hard intensity. Teaches the body to buffer aerobic waste over longer durations.
Zone 4 Lactate Threshold 95% - 99% Hard threshold effort. Maximum sustainable speed for ~1 hour. Conversations are restricted to single words.
Zone 5a Superthreshold 100% - 102% VO2 Max work. Clearing heavy accumulation of lactate.
Zone 5b Aerobic Capacity 103% - 106% Extreme intensity anaerobic work. Developing maximum cardiac output.
Zone 5c Anaerobic Capacity > 106% Maximal sprint, neuromuscular capacity. Zero conversation capacity.
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