I wear a heart rate monitor when exercising on a treadmill.
Pre jog, my heart rate is say 61. I set off at 6 miles an hour. My heart rate climbs to 122 ish. This I get, I am now exerting myself so my HR has to increase to circulate more blood to supply oxygen etc.
40 minutes later at the same pace my HR is now 150, yet I am going the same 6 miles an hour. Why the subsequent increase?
My guess is you are transitioning from anaerobic to aerobic, if I understand your situation.
Are you saying in the first minute your HR steps up to 122, then 40 minutes later it is 150? What is your HR doing between the first minute and 40 minutes later? I’d bet it ramps up to around 150 within 5 minutes or so.
Assuming an accurate heart rate reading then another explanation could be that your form suffers and your stride less efficient as you proceed, or that you are doing something different with your upper body (not necessarily related to real running efficiency).
“Cardiac drift” (it has wikipedia entry). Your body is getting hot and dehydrated. This is normal. Try running in 50 deg temps. It helps a lot! Your body doesn’t have to work as hard cooling itself off.
Related to 1: you’re getting tired. You can’t hold that heart rate forever (eg. what if you ran 24 hours? You wouldn’t expect to stay at 122 would you?) At some point your body is going to fatigue. I’ll suggest that if you keep at it you can extend that 40 minutes to 50 min, etc.
On an elliptical trainer it takes about 5-7 minutes for me to ramp up to steady state training heart rate, which for me is 136. I could start sprinting from a dead stop and my heart probably wouldn’t even realize it for 20-30 seconds.
I get up to the 122 stage by 3 minutes or so. It will stay there for 10 minutes or so then start gradually rising, such that it will be 150 40 minutes in.