To understand what to rely on in our calculations, we have thoroughly analyzed normal ranges and metrics to measure and evaluate heart rate variability (HRV) from an array of scientific sources.
First, there are Guidelines on Heart rate variability: Standards of measurement, physiological interpretation, and clinical use (Task Force of The European Society of Cardiology and The North American Society of Pacing and Electrophysiology).
Then, there are recent research papers on reference ranges for HRV in healthy adults, for example:
- Determinants and interindividual variation of R-R interval dynamics in healthy middle-aged subjects
- Short-term heart rate variability in healthy young adults: the Cardiovascular Risk in Young Finns Study
- A quantitative systematic review of normal values for short-term heart rate variability in healthy adults
- Healthy lifestyle and heart rate variability in young adults
- Detailed heart rate variability analysis in athletes
- Heart rate variability and heart rate in healthy volunteers. Is the female autonomic nervous system cardioprotective?
However, the above papers have their drawbacks:
- They are small-scale. When performing data analysis, the researchers considered groups of up to 100 subjects only.
- Scientists still argue what “normal” really means. Health is a relative and not an absolute state, and a lot of different factors can impact a healthy (i.e., normal) or unhealthy status for a particular individual. That’s why researchers still argue about what is meant by “normal ranges” or “population reference ranges.”
- Some studies ignored additional yet critical factors that can affect your health, such as chronic illnesses or stress levels.
- It’s next to impossible to dig up credible reference ranges for some rare scores such as AMo50 and MxDMn, because these scores are mostly used in studies focused on elite athletes, not healthy non-athletes.
Let’s take a closer look at RMSSD values (Root mean square of successive RR interval differences) to illustrate the issue. If we take a measuring time of 2.5–5 minutes, researchers have proposed the following normal ranges in different studies:
- 13–48 ms — healthy adults aged 38–42 years
- 35–107 ms — elite athletes
- 53.5–82 ms — healthy men
- 40.5–71 ms — men
- 29–65 ms — women
- 23–72 ms — men
- 22–79 ms — women
These standard ranges vary considerably, and obviously can’t apply to everyone. If we try to unify them, the reference range for RMSSD would be 13–107 ms, which seems like a pretty broad generalization.
Welltory uses the reference range of 20–89 ms, which falls into the range mentioned above. Even though it's possible to get an RMSSD score that falls out of our range, it would most likely mean that your body is working overtime.
The same generalization issue happens with other not-so-common HRV scores like SDNN, pNN50, or Mean RR.
Such ranges seem too broad to be useful for everyday measurements because heart rate variability is an extremely sensitive metric. That’s why we’ve narrowed the typical normal ranges up to more specific ones.
To do so, we’ve analyzed over 60,000 RR intervals of more than 600 users and derived all the data from photoplethysmography signals in keeping with the following criteria:
- All RR intervals shall be artifact-free and contain no arrhythmias or ectopic beats. This criterion is essential because these things can significantly distort both time- and frequency-domain measurements;
- No data shall contain unwanted noises.
Plus, our study included only the data of people who measured their HRV for at least 90 days. We then conducted data analysis to determine how the same person’s measurement results can vary depending on the time of the day, a healthy or unhealthy state, work, exercises, caffeine & alcohol intake.
It is worth pointing out that our narrowed normal ranges are also not personalized enough.
However, we realize how crucial it is to understand what’s going on with your body right now in the view of your previous trends and measurements. That’s why we offer you Productivity, Energy, and Stress scores that can reflect how you feel more accurately than reference ranges.