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I was messing with my Zwift controller thingies during an erg workout inside. I had it in low z3 for like 30min at some point.
My hr didn’t drift higher during the period, but had a sine wave increase decrease with a “period” of exactly 3min 40sec for like several “waves” in a row. I skipped drinking after noticing just to make sure it wasn’t when I chose to take swigs of hydration.
Why this sine wave? Odd controller PID nature of the smart trainer? Communication protocol to f the HRM?
This is fascinating. How well did it fit a sinusoidal curve? Was it more of a step function or more of a smoothed curve? And the amplitude?
I can't imagine it's the pid controller in the trainer as that should be completely independent of heart rate.
My first intuition was to think that there's an overlap between the sample capture rate and broadcast rate. Like at the low point the broadcast rate of the HRM and sample capture rate of the computer are only overlapping minimally, but then as they get more in sync the overlap grows to a maximum. It's a 220 second cycle so the rate difference of any multiple of 221 v. 220 should in theory produce the same result. The computer would have to be reading the 'no signal's' as zeroes tho. I'm way out of my depth with software so hoping more experienced minds will chime in.
While I don't get that type of waveform, I do notice a dip in my HR (Garmin) about every 6 minutes while riding steady state on Zwift.
Guy at the group ride I mentioned this to noted I had recently swapped from a Garmin strap to an optical armband also. It never did this with the chest strap. Guessing something to do with the optical armband perhaps and sample rate and transmit rates.
The optical armband is interesting. Is it a knock-off brand? Some of them are really good and some are not. An issue with the optical pickup could be the most likely. They're just backwards-looking waveform calculators, it could be as simple as the software having a wrong multiplier in it somewhere that is divisible by 3:40 in whatever time standard they are using.
I also googled just out of interest and there is the Nyquist-Shannon theory about aliasing. I have to assume though that Ant protocol sends data packets and not a continuous signal. I'm not sure if a similar thing can happen when a digital signal is received, seems unlikely but idk.
Wild guess: Were you listening to music during the ride? Many songs are about 3:40 long, and maybe your playlist had several songs back to back that had up-tempo sections in similar places that raised your HR?