i've effectively figured out that my LT heartrate is 165bpm. i did this by doing a 30 mintue time trial and taking the average heartrate for the last 20 mintues. it feels pretty accurate for me.
during longer rides i notice this:
after about 60-70 miles of riding at an average much less than 165 (but shorter periods above that for hills, windy sections, etc.) i had a hard time keeping my heart rate down. at some points i was going around 14mph on the flats and going anaerboic. stopping and resting would bring my HR rate down quickly, but then shortly afterward it would skyrocket after the smallest effort.
is this near bonking? it would seem my LT level for shorter distances (time trial) and longer distances do not match up.
An explanation for this that I have seen calls this `cardiac creep'. After a few hours of exercise (i.e., your 60-70 miles of riding), a number of things start to happen that tend to drive up heart rate a bit. One of them is a bit of dehydration, which reduces blood volume. In order to deliver the same amount of blood to the body, brain, etc. the heart has to pump a bit faster to compensate.