After I did my Heart Rate Variability measurements (check out my earlier post, if you haven’t), I suspected that RMSSDs had wide range. For example, when I compared my RMSSD just before the long run and 45-hours after the long run, I ended up getting 64% higher number. This led me to suspect that RMSSD varies quite a bit depending on multiple factors.
So I calculated RMSSDs using different number of heart beats. Earlier I used 336 heart beats (about 5 minutes worth of heart beats) to measure my HRV. (Note that these are the data that I collected between December 28th and 30th, 2015; the same set of data that I discussed on my earlier post.)
- Measure 1: 25 minutes before the run
- 60 Heart Beats: 50.64 msec
- 100 HBs: 40.962 msec
- 336 HBs: 25.551 msec
- Measure 2: 1 hour 45 minutes after the run
- 60 HBs : 5.534 msec
- 100 HBs: 5.196 msec
- 336 HBs: 4.505 msec
- Measure 3: 23 hours after the run
- 60 HBs : 17.606 msec
- 100 HBs: 20.348 msec
- 336 HBs: 14.396 msec
- Measure 4: 45 hours after the run
- 60 HBs : 93.512 msec
- 100 HBs: 73.093 msec
- 336 HBs: 42.032 msec
Looking at these numbers, it is easy to see that HRVs are not about exact measurement, but instead it’s about relative scale. For example, Measure 4 could have more than twice as large HRV if I took the first 60 HBs only as opposed to looking at the 336 HBs.
Why? Because when the heart rate had other reasons to vary, such as external stress or change in posture, HRV would be greater to include the increase in heart rate.
In fact I think what might have happened on Measure 4 was exactly that. While hurrying to take the RR recordings, I got up, got my chest monitor on, and lied back down on the bed quickly to start the measurement. That explains why the heart rate was dropping from 80.
The below graph shows my heart rate changes on Measure 4.
So the first point to note is that it’s important to minimize the external factors that may change the heart rate. I think the best way is to lie down for a few minutes before the measurement, and stay lying down during the measurement.
The other point is that HRV should be thought of as a range. From multiple measurements I took, it looks like there are three ranges that I could identify for myself:
- Relaxed: 25 msec or greater
- Recovering: 10 – 24 msec
- Stressed: 9 msec or less
It would make sense to compare 50 msec (60 HBs) and 25 msec (336 HBs) in Measure 1 to conclude that I was under physical stress (because I was not).
What I do not know: How much psychological stress would affect my HRV, and how long the effect would be sustained in my HRV. I’m sure experimental psychologist would have many sample points in this area.
So what’s the bottom line?
HRV definitely works in measuring the physical stress, but to properly measure I’ll need to lie down and spend at least 5 minutes of measurements to get a relative idea of how stressed I am.
I wonder if there are any experts who can share their findings. Please feel free to chime in.