In August 2024, I performed several 400m interval sessions, and the wrist sensor provided reasonably accurate heart rate (HR) readings. I understand that wrist-based sensors are not ideal for interval training, but they still delivered some useful data.
However, since the beginning of October 2024, I’ve started running intervals again (40s to 90s), possibly with an older software version, and the HR readings have become almost useless. Occasionally, the HR is accurate, but for most intervals, it’s far from relevant. Maybe 1 out of 4 intervals shows the correct HR. So it's little bit funny at the end that this type of run was for Garmin "Base training" and I was exhausted :D
What’s even more concerning is that during rest periods, my HR drops below 100 bpm within a minute after a run. This is unrealistic, as I’m still breathless, and my actual HR should be around 130 bpm or higher. I’ve tried tightening the watch, repositioning it, and wiping off sweat, but none of these have helped.
For tempo runs, the HR readings work perfectly fine.
In my opinion, it seems that Garmin FR 965 doesn’t trust such rapid increases in HR and instead applies some kind of correction through the algorithm. This correction then carries over into the rest period as well. Unlike regular running, such as transitioning to an uphill segment, Garmin smoothly shows the HR change. Although slightly delayed, this behavior is understandable due to the nature of wrist-based optical heart rate measurement.
Of course, I don’t know the exact algorithm, but I would expect that during interval training, especially during the run portion, the device would prioritize measuring HR as accurately as possible, even at the expense of battery life. Intervals typically aren’t very long. Alternatively, Garmin could focus on the first 1–2 minutes of measurement during intervals to ensure accuracy and then manage HR data in the usual mode afterward.
