HRV (Heart Rate Variability)
Heart rate variability (HRV) is the natural fluctuation in the time interval between consecutive heartbeats, measured in milliseconds. Unlike a metronome, a healthy heart does not beat at perfectly regular intervals — the gap between one beat and the next varies continuously, and the magnitude of that variation provides critical information about the state of the autonomic nervous system and overall recovery status.
Deeper Explanation
At any given heart rate, the time between successive heartbeats — called the RR interval — fluctuates constantly. At a resting heart rate of 60 bpm, beats do not occur precisely every 1,000 ms. One interval might be 940 ms, the next 1,060 ms, the next 980 ms. HRV quantifies the degree of this variability.
The most commonly used HRV metric in sport science and consumer wearables is RMSSD (Root Mean Square of Successive Differences). RMSSD is calculated by taking the difference between each consecutive RR interval, squaring each difference, averaging them, and then taking the square root. It is highly sensitive to parasympathetic nervous system activity and is widely regarded as the most reliable beat-to-beat HRV metric for assessing recovery state.
Autonomic nervous system and HRV: The autonomic nervous system controls HRV through two opposing branches. The parasympathetic nervous system (rest-and-digest) increases heart rate variability when dominant — a high-HRV state indicates the body is calm, recovered, and ready for stress. The sympathetic nervous system (fight-or-flight) reduces variability when activated — low HRV reflects increased sympathetic tone from hard training, poor sleep, illness, psychological stress, or alcohol consumption.
Population norms for RMSSD:
- Age 20–29: approximately 55–75 ms
- Age 30–39: approximately 45–65 ms
- Age 40–49: approximately 35–55 ms
- Age 50–59: approximately 25–45 ms
- Age 60+: approximately 20–35 ms
Trained endurance athletes typically run 10–30% above age-matched non-athletes due to chronic parasympathetic adaptation from aerobic training.
How HRV Relates to Training
HRV is used in three primary ways in athletic training:
1. Daily readiness assessment: Morning HRV compared to a rolling 7-day and 30-day personal baseline indicates whether the body is well-recovered (elevated HRV → green light for intense training) or under systemic stress (suppressed HRV → reduce intensity or rest).
2. Overtraining detection: A sustained multi-day depression in HRV — typically 10–20% below personal baseline for 5 or more consecutive days — reliably indicates accumulated fatigue consistent with functional overreaching. This signal often appears 1–2 weeks before subjective performance decline becomes apparent.
3. Long-term fitness tracking: Chronic aerobic training raises baseline HRV over months and years as the parasympathetic nervous system adapts. An upward trend in HRV baseline over a training block is a sign of genuine aerobic adaptation.
Measurement protocol for accurate results:
- Measure immediately upon waking, before getting out of bed
- Remain lying down (supine) throughout the measurement
- Measure for 3–5 minutes at the same time each day
- Avoid caffeine, food, or significant movement prior to measuring
Single readings are minimally meaningful; trend data over 7–30 days is what drives actionable insight.
How Fitiv Uses HRV
Fitiv Pulse measures HRV through Apple Watch (photoplethysmography) or any Bluetooth Low Energy or ANT+ chest strap that transmits RR interval data, including the Polar H10, Wahoo TICKR X, and Garmin HRM-Pro.
Each morning, Fitiv calculates RMSSD from a 3–5 minute guided measurement session. This feeds into the daily readiness score, which also incorporates resting heart rate trend and sleep score. HRV contributes approximately 50% of the readiness score weighting.
Fitiv's readiness scale:
- Score 75–100: High HRV relative to baseline — hard training recommended
- Score 50–74: Moderate HRV — aerobic or moderate sessions appropriate
- Score below 50: Suppressed HRV — recovery or rest recommended
Fitiv also displays HRV trends over 7, 30, and 90-day periods, enabling athletes to track how aerobic training is shifting their parasympathetic baseline over time.
Frequently Asked Questions
Q: Is higher HRV always better? A: In general, yes — within an individual's personal range. A higher HRV relative to your personal baseline indicates greater parasympathetic dominance and better autonomic recovery. However, extremely high HRV can sometimes indicate cardiac issues in clinical contexts. For healthy athletes, trending upward within your personal range is the goal. Do not compare absolute values to other athletes; HRV is highly individual.
Q: What is the difference between HRV and resting heart rate? A: Resting heart rate (RHR) measures how many times the heart beats per minute at rest — a measure of cardiac efficiency. HRV measures the variability between those beats — a measure of autonomic nervous system balance. Both decline with stress and improve with aerobic fitness, but HRV responds faster to daily changes in recovery state and is a more sensitive short-term marker. RHR is better for tracking long-term aerobic fitness trends.
Q: Does stress at work affect HRV? A: Yes, significantly. Psychological stress activates the sympathetic nervous system and suppresses HRV through the same pathway as physical training stress. Athletes under significant occupational or personal stress will often see HRV decline even during reduced training periods. Fitiv's readiness score reflects total systemic load — not just workout-derived fatigue — which is physiologically accurate: your body does not distinguish the source of the sympathetic activation.