Yesterday I raced the Tour de Sportsolid (D category). About 3 minutes in, when my heart rate had just started to stabilize at 140, my heart rate spiked to 181 within 7 seconds. In the next 7 seconds it returned to 142. (This 14 second interval would account for about 32 beats if the spike hadn’t occurred.) The spike increases smoothly to the peak and likewise decreases smoothly (no jittering).
No other such spikes were observed for the rest of the race (or ever on Zwift during 2.5 years of monitoring). I didn’t feel anything at the time and only noticed the spike later on the two monitoring apps (Zwift Companion and WhatsHeart).
Although this spike might be an accurate measurement, I wonder if there are any other possible explanations. (If the strap had lost contact with my skin then I would have expected a drop in heart rate, not a spike.)
Polar H10 with generic strap.
EDIT (12/1/2021) –
During hard efforts my max heart rate is usually upper 150s, occasionally exceeding 160. Over the past 2.5 years on Zwift my absolute max heart rate has been 164. During a supervised stress test in 2016 my max heart rate was 166. Thus, my race spike of 181 seems very abnormal.
Attached is a handmade graph that closely approximates the spike.