I’ve been using a smartwatch with heart rate monitoring for a while as well as cardio equipment with built in heart rate monitors. Something I’ve noticed is that for the same level of exertion, my heart rate is significantly increased for the same exercises if they come later in my workout. Swimming at a certain pace will plateau around 115-120 bpm during the first 15 or 20 minutes of my swim, but doing the same strokes at the same pace after I’ve been swimming for an hour will tend to run 135-140. Or using a treadmill, which is a very standardized workout because you can control the pace exactly - doing a fast uphill walk will get me up to about 125 bpm if I do it for 20 minutes at the beginning of my workout but more like 145 bpm if I do it again at the end of my workout (usually an hour to 1:15 later).
Is this normal? Why does it happen?
My guesses: the stuff becoming depleted in your muscles increases over time, and maybe more blood flow is needed to get whatever they need (or whatever waste product they’re creating). Or maybe it has to do with overheating - if you’re hotter later your heart rate increases to get more blood to the parts of your skin that will cool you off.