So I am on a cardio machine at the gym - cycle, running, stairs, whatever - wearing a telemetry strap around my chest to pick up my heart beat and transmit it. I am also wearing a watch that was sold with the telemetry strap, that picks up the transmitted signal and gives a readout of my heart rate.

Meanwhile the machine itself is picking up the same signal and showing my heart rate. (I am not touching the sensors, so it is not reading my pulse directly; it has detected the telemetry strap.)

The two heart rate displays regularly differ, normally by 2-10 bpm.

Is there a general rule about which one I should believe?

For example:

  • The watch is the same brand as the telemetry strap, and is better tuned to it. Trust the watch over the cheap electronics they put in gym machines.

  • The machine has a much larger antenna, and is plugged into mains so it can be more sensitive. Trust the machine over the low-power processing they put in watches.

  • One of them is missing beats. Trust whichever reads higher.

  • Like uncalibrated scales, they all have their biases, so use your watch because at least the bias is always consistent, no matter which machine you are on.

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    I have a sneaking suspicion that the difference might be less one of accuracy and more one of timing, with one machine giving heart rates a second or so later due to how it receives the signal. Is one consistently higher or lower? Can you provide the brand of telemetry strap so we might take a stab at how it communicates? Aug 17 '17 at 16:38
  • I haven't noticed any consistent different but (a) I haven't really looked and (b) I keep using different machines, so if it is machine specific it'll take a while to notice. Aug 17 '17 at 17:22
  • The strap is a Suunto brand, with an IND logo (which I believe is 5.3 kHz and fairly standard, but I don't know a lot about it.) Aug 17 '17 at 17:23

The standard protocol for networked fitness devices is is ANT, with ANT+ typically implemented for interoperability. The standard for heart rate devices is to send full pages of compiled data from the device. Thus, it is extremely likely that both the treadmill and your watch are receiving the same data from your monitor. Unless they are changing that data (for example, smoothing out the data to guard against infrequent messages, or modifying the displayed heart rate data based on other information) or displaying different fields (the standard page only has the timing, count, and computer heart rate, but you could have a sensor that sends additional information), the odds are that both devices are accurately displaying the data given.

The most likely case I can think of is data latency. Either your watch or the treadmill may be operating at a different speed, either delaying the readout of your heart rate or displaying a smoothed value based on multiple inputs during the time frame between updates. If you were to catalog the values from both devices, I suspect that the answer would probably jump out in the data, but ultimately, I think the biggest takeaway here is that the data you are being given is accurate, just not necessarily in the same time frame.

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    I'm not certain what the downvote is for... Aug 18 '17 at 15:35
  • I dont know either re the dv, but you could expand a little on the sampling. Its also possible on or the other samples at different time intervals and extrapolates.
    – JohnP
    Oct 17 '17 at 1:08

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