What is HRV?
HRV measures the micro-variations between each heartbeat, expressed in milliseconds (RMSSD). High HRV indicates good parasympathetic recovery. Low HRV signals accumulated stress or fatigue. It's a scientifically validated marker for detecting overtraining.
After 15 years of coaching, I've seen dozens of athletes plateau despite serious training. The problem was never the program. It was timing: they pushed hard on the wrong days and recovered when they could have performed.
HRV changes that. But beware: 90% of people tracking it don't know how to interpret it correctly.
🔬 What HRV Actually Measures
Your heart doesn't beat like a metronome. Between each beat, the interval varies slightly:
- Beat 1 → 2: 950 ms
- Beat 2 → 3: 1020 ms
- Beat 3 → 4: 880 ms
This variability is controlled by your autonomic nervous system. The more recovered your body, the higher the variability. The more stressed or fatigued, the lower it drops.
Metrics You'll See in Your Apps
- RMSSD: The reference metric, measures parasympathetic (rest) activity
- lnRMSSD: Logarithmic version, used by Whoop and HRV4Training
- SDNN: Overall variability, combines sympathetic and parasympathetic
Research shows RMSSD is most reliable for daily tracking due to its low day-to-day variation.
📊 How to Interpret Your Data
Mistake #1: Comparing Your HRV to Others
An RMSSD of 40ms might be excellent for you and insufficient for someone else. Age, sex, training level, and genetics influence your baseline. What matters: your variations relative to your own 7-day average.
What research shows:
- After intense training, RMSSD drops by approximately 40%
- After 2 hours of recovery, it remains 18% below baseline
- Overtrained athletes show persistently low HRV over several days
- Stable HRV (low coefficient of variation) indicates good training adaptation
In practice:
- HRV above your 7-day average → Intense session possible
- HRV within your normal zone (±5%) → Planned training OK
- HRV 10-15% below your average → Reduce intensity
- Very low HRV for several consecutive days → Warning sign, rest needed
📱 Reliable Measurement Protocol
Your HRV accuracy depends on measurement consistency:
- Upon waking, before getting up
- Same position every day (lying or sitting)
- 1 minute stabilization, then minimum 1 minute recording
- No movement, no talking during measurement
- Before coffee/stimulants
Research-Validated Devices
A meta-analysis of 23 studies shows that chest straps (Polar H10), watches (Apple Watch, Garmin), and even some smartphone apps (via camera) provide acceptable results compared to medical ECG.
⚠️ What Skews Your HRV (And Nobody Tells You)
Your HRV doesn't just reflect your training. It reacts to all stress:
- Alcohol: Even 1-2 drinks the night before can drop your HRV by 20-30%
- Poor sleep: Sleep deprivation suppresses parasympathetic activity
- Mental stress: A work conflict affects your HRV as much as leg day
- Dehydration: Reduces blood volume and stresses the cardiovascular system
- Incubating infection: Your HRV can drop 24-48h before first symptoms
That's why HRV alone isn't enough. Studies recommend combining it with subjective indicators: sleep quality, muscle soreness, perceived energy level.
🤖 Analyzing Your HRV with Claude (via Smart Rabbit)
Smart Rabbit generates an expert prompt for Claude. Concretely, you can:
- Export your data from your app (CSV or screenshot)
- Upload to Claude with Smart Rabbit context
- Request analysis of your trends and correlations
Concrete Example
You: "Here's my HRV data from the last 2 weeks. My average is 52ms but I had 3 consecutive days at 38-42ms. I also noted I slept poorly those days."
Claude + Smart Rabbit: "Your HRV dropped 23% below baseline for 3 days, coinciding with your sleep deficit. This pattern suggests incomplete recovery. I recommend maintaining light sessions until your HRV returns to your normal zone (49-55ms). Did you also consume alcohol or experience particular stress those days?"
The advantage: Claude can cross-reference your HRV with other data (training load, sleep, nutrition) to identify patterns you wouldn't see alone.
🎯 My Take as a Coach
HRV is a powerful tool, but it's not a crystal ball. I use it as one signal among others:
- Low HRV + you feel good → Train normally, observe
- Normal HRV + you feel exhausted → Listen to your body, not the number
- Low HRV + felt fatigue → Rest, no hesitation
The real power of HRV is forcing you to measure and observe. Many athletes ignore fatigue signals until overtraining hits. HRV makes these signals visible, objective, impossible to ignore.
🐰 Analyze Your Data with Smart Rabbit + Claude
Upload your HRV exports and get personalized analysis crossing recovery, training load, and stress factors. Free via Claude.
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