How to Use This Glossary
HRV can feel like learning a new language. This glossary covers the terms you'll encounter in apps, research papers, and online discussions.
Terms are organized into categories: - Basic Concepts: Foundational understanding - Time-Domain Metrics: The most common measurements - Frequency-Domain Metrics: Advanced spectral analysis - Other Metrics: Additional measures you may encounter - Physiology Terms: How the body produces HRV - Measurement Terms: How HRV is captured
Basic Concepts
Heart Rate Variability (HRV) The variation in time between consecutive heartbeats. Contrary to intuition, a healthy heart doesn't beat like a metronome—it speeds up and slows down moment to moment. This variability reflects autonomic nervous system function.
Interbeat Interval (IBI) / RR Interval / NN Interval The time between consecutive heartbeats, measured in milliseconds. RR specifically refers to the time between R-peaks in an ECG signal. NN intervals are "normal-to-normal" intervals—RR intervals with ectopic beats removed.
Autonomic Nervous System (ANS) The branch of the nervous system that controls involuntary functions like heart rate, digestion, and breathing. Consists of sympathetic and parasympathetic divisions.
Sympathetic Nervous System (SNS) The "fight or flight" branch of the ANS. Activation increases heart rate, decreases HRV, and prepares the body for action.
Parasympathetic Nervous System (PNS) The "rest and digest" branch of the ANS. Activation decreases heart rate, increases HRV, and promotes recovery. Mediated primarily through the vagus nerve.
Vagus Nerve The main nerve carrying parasympathetic signals to the heart. "Vagal tone" refers to parasympathetic activity level—higher vagal tone generally means higher HRV.
Time-Domain Metrics
RMSSD (Root Mean Square of Successive Differences) The most common HRV metric. Calculated by squaring the differences between successive heartbeat intervals, averaging them, and taking the square root. Primarily reflects parasympathetic activity. Typical values: 20-100ms (wide individual variation).
SDNN (Standard Deviation of NN Intervals) The standard deviation of all normal heartbeat intervals during a measurement period. Reflects overall HRV including sympathetic and parasympathetic contributions. 24-hour SDNN below 50ms is associated with health risks.
pNN50 The percentage of successive intervals that differ by more than 50 milliseconds. Like RMSSD, this primarily reflects parasympathetic activity. Values typically range from 1-50%.
Average RR / Mean NN The average time between heartbeats during measurement. Inversely related to heart rate—longer intervals mean slower heart rate.
HR / Heart Rate Beats per minute. While not technically an HRV metric, it's often reported alongside HRV data. Lower resting heart rate generally correlates with higher HRV.
HRV Score (App-Specific) Many apps convert raw HRV into proprietary scores (e.g., 0-100). These can't be compared between apps—they use different algorithms and scaling.
Frequency-Domain Metrics
Power Spectral Density (PSD) A mathematical technique that breaks down HRV into frequency components. Reveals how much variability occurs at different speeds (frequencies).
VLF (Very Low Frequency): 0.003-0.04 Hz The slowest oscillations in HRV, corresponding to cycles longer than 25 seconds. Influenced by thermoregulation, hormones, and possibly renin-angiotensin system. Requires longer recordings (5+ minutes) to measure accurately.
LF (Low Frequency): 0.04-0.15 Hz Oscillations corresponding to roughly 7-25 second cycles. Previously thought to reflect sympathetic activity, but now understood to be mixed sympathetic/parasympathetic and heavily influenced by baroreceptor function. LF interpretation is controversial.
HF (High Frequency): 0.15-0.4 Hz Oscillations corresponding to roughly 2.5-7 second cycles, aligned with respiratory rate. Primarily reflects parasympathetic (vagal) activity. Also called "respiratory sinus arrhythmia" power.
LF/HF Ratio Once thought to indicate sympathetic/parasympathetic balance, this interpretation is now considered unreliable. Many researchers recommend against using this metric. Current science (2025) suggests avoiding this measure.
Total Power (TP) The sum of VLF, LF, and HF power. Represents overall HRV magnitude in the frequency domain.
Other Metrics
HRV4Training Score A coefficient of variation of RR intervals used by the HRV4Training app. Calculated as (RMSSD / Mean RR) × ln(RMSSD). Normalizes for heart rate.
PNS Index (Parasympathetic Nervous System Index) A composite score reflecting parasympathetic activity, used in Kubios. Combines multiple metrics into a single indicator.
SNS Index (Sympathetic Nervous System Index) A composite score reflecting sympathetic activity, used in Kubios. Derived from multiple HRV parameters.
Stress Index (SI) A measure developed by Baevsky reflecting the geometric distribution of RR intervals. Higher values indicate more sympathetic dominance. Used in some apps and research.
Morning Readiness Score App-specific score (Elite HRV, Oura, Whoop, etc.) combining HRV with other factors like sleep quality, resting heart rate, and recent activity. Usually 0-100 or 0-10.
Recovery Score Similar to readiness score—an app's assessment of your recovery status based on HRV and other variables. Not standardized between apps.
Physiology Terms
Respiratory Sinus Arrhythmia (RSA) The natural increase in heart rate during inhalation and decrease during exhalation. This is normal and healthy—it's a major source of HRV in the HF band.
Baroreceptor Reflex The body's mechanism for regulating blood pressure moment-to-moment. Baroreceptors in blood vessels detect pressure changes and adjust heart rate accordingly. Contributes to LF-band HRV.
Vagal Tone The activity level of the vagus nerve. Higher vagal tone means more parasympathetic influence and generally higher HRV. Can be enhanced through training (exercise, breathing practices).
Autonomic Balance The relative activity of sympathetic vs parasympathetic systems. Healthy autonomic balance means appropriate responses to stress and effective recovery.
Cardiac Coherence A state where HRV, breathing, and blood pressure rhythms are synchronized. Associated with resonance frequency breathing (~6 breaths/minute). Promoted by HeartMath and similar programs.
Measurement Terms
ECG / EKG (Electrocardiogram) The gold-standard method for measuring heart rhythm using electrical signals. Chest straps use this method. Provides the most accurate HRV data.
PPG (Photoplethysmography) Optical method that detects blood volume changes using light. Used by smartwatches, rings, and camera-based apps. Generally 6-11% lower accuracy than ECG but adequate for trend tracking.
Ectopic Beats Irregular heartbeats that originate outside the normal pacemaker. Must be filtered out for accurate HRV analysis—they appear as artifacts.
Artifact Any erroneous data point in HRV recording—from movement, poor sensor contact, ectopic beats, or electrical interference. Good apps filter these automatically.
Ultra-Short Recording HRV measurement under 5 minutes, typically 1-3 minutes. Standard for consumer apps. Trade-off between convenience and statistical reliability.
24-Hour Recording Clinical standard for comprehensive HRV assessment. Captures circadian variation and provides most complete picture but impractical for daily use.
Orthostatic Test HRV measurement protocol where you lie down, then stand, comparing the two readings. The heart rate and HRV response to standing reveals autonomic function. Used by Polar and in clinical settings.
Quick Reference Table
Common Metrics at a Glance:
| Metric | What It Measures | Typical Range | Higher = | |--------|------------------|---------------|----------| | RMSSD | Parasympathetic activity | 20-100ms | Better recovery | | SDNN | Overall HRV | 40-150ms | More adaptable | | pNN50 | Beat-to-beat variation | 1-50% | More relaxed | | HF Power | Vagal activity | Varies | More parasympathetic | | LF Power | Mixed/unclear | Varies | Contested meaning | | LF/HF | Unreliable | N/A | Don't use | | Resting HR | Overall fitness | 40-70 bpm | Better fitness |
Measurement Methods Compared:
| Method | Accuracy | Convenience | Best For | |--------|----------|-------------|----------| | ECG chest strap | Excellent | Moderate | Morning readings | | PPG smart ring | Very good | Excellent | Sleep tracking | | PPG smartwatch | Good | Excellent | All-day awareness | | Camera app | Adequate | High | Getting started |
Related Guides
- How HRV Is Calculated — Detailed explanation of methods
- Getting Started — Setting up HRV tracking
- HRV Myths — Common misconceptions clarified