While hiking uphill at a normal pace my heart rate will often increase rapidly. I can feel that it is very high yet my Fitbit only shows a heart rate of about a hundred. Using a stopwatch I counted my heart rate at around 90 beats in 30 seconds. While standing still I watched my Fitbit finally catch up with my actual heart rate once it was down to about 174 bpm. I checked my arm, it wasn't exceptionally sweaty, and the Fitbit was properly fitted with good contact. It would be very helpful to get accurate readings so that I can stop when the heart rate is excessive.
Moderator Edit: Clarified subject