May 18, 2025 bb-projects

Heart Rate Normalized HRV: reflections

Exploring the relationship between HRV and Resting Heart Rate (RHR) and why normalization matters

A single insight from a response on a tweet sent me down an interesting and relatively unexplored rabbit hole: looking at HRV values directly in proportion resting heart rate (RHR). Why would there be a single variability metric when resting heart rates can easily vary by twofold (say 40 and 80 bpm)? Is there a scientific term for this phenomenon? Is there any literature behind it?

My angle

I am in no way qualified to talk about these topics, but since it’s not well researched or broadly talked about I wanted to look into this and share my findings.

I am approaching this research mainly from the viewpoint of the nervous system. Specifically, what do these findings and values say about someone’s sympathetic/parasympathetic states and its flexibility, if anything at all.

Normalized HRV

Mathematically, HRV should be higher with people with lower resting heart rates. Indeed, there are correlative effects as well, since improving one’s cardiovascular fitness lowers both one’s resting heart rate and increases one’s HRV value.

Let’s take person A and B, both healthy and fit. Both have a HRV of 55 ms (RMSSD). However, person A has a resting heart rate of 35 but person B of 70. Why would there be a single HRV metric when the actual variability baseline is twice as large? Let’s do the calculations.

Formula to calculate Heart Rate Normalized HRV

Person A has a RHR of 35 bpm, or (60000/35 =) 1714 ms per beat for RHR. To normalize HRV for person a -> 55/1714 = 3.2%

Person B -> 60000/70 = 857 ms per beat -> 55/857 = 6.4%

These percentages show the normalized HRV ratios, which give the proportional beat-to-beat variability.

For quick comparison, I created a table roughly indicating what values are expected, and when it’s relatively low or high. The colored values are based on nothing other than my subjective perspective.

Table 1: lookup table for normalized HRV values by RHR and HRV

Interpreting Normalized HRV

Considering our two example persons, can we draw any (loose) conclusions about their (baseline) physiology?

Person A (low RHR, medium HRV)

Has a very low (athletic) resting heart rate and a good to medium HRV. But in the perspective of normalized HRV, his HRV can actually be considered on the low side.
This is indicative of a strong parasympathetic (vagal) tone, but his CSN adaptability is on the lower side.
Very simply put, this person is fit and relaxed by default, but cannot handle (unplanned) stressors very well.

This person is efficient rather than flexible.

Person B (high RHR, medium HRV)

Has a high resting heart rate, but a good to medium HRV, which in relative terms is on the high side.
This means that this person’s baseline stress levels are higher, but also means that his ability to adapt to stressors is quite good.

This person is flexible rather than efficient.

Table 2: Interpretation of HRV values in proportion to resting heart rate (RHR)

Heart Rate Variability Variability

In addition, I did a little research into the daily variation of HRV, which can be called the derivative of HRV, heart rate variability variability or its more scientific term, coefficient of variation of HRV. My findings concluded that a greater baseline variability was not indicative of greater nervous system adaptability. The opposite seems to be true, a more stable daily HRV is in line with a better nervous system state and faster recovery.

Sources

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