This was a “fat burner” ride that I started with five repeats (upper Alpine Road for locals). Without referring to my power meter, I just tried to ride at what seemed a consistent pace. See the graph and numbers further below.
Power: 243, 244, 240, 244, 242 watts, green line
HR: 128, 125, 123, 122, 122 BPM, red line
Time: 698, 701, 712, 702, 705 seconds
It’s interesting how my heart rate asymptotically approached 122 on the least repeat. This is something I’ve observed as my body “settles into” a workout. It’s as if the aerobic and anaerobic energy systems + cardiovascular system all settle into their jobs at some ideal agreed-upon level, and then things become very steady-state in terms of power production, at least for these not-so-hard repeats which were mainly aerobic in nature. But this seems to hold true for double centuries; see in particular the Solvang Spring Double Century graph.
The cool temperatures also greatly reduce heart rate (red line) because the body does not have to cool itself by pumping more blood. The first repeat started around 75°F, the last repeat at 64°F, to 63°F at the end of the last repeat, with rest of the ride varying from 68° down to 60°F at the end (near sunset). Heart rate reserve is more than ample (my max is about 172, but hard to reach when highly trained, unless quite hot).
Were the temperature 95° F, I’d expect heart rate to be 15-20 beats higher, just to cool the body. That’s why heart rate is such a poor metric for training; it’s highly dependent on temperature (and humidity + temperature).
Also, observe just how low the HR is later in the workout, even as the power levels approach 300 watts. The relatively low BPM indicates that the stroke volume is very powerful—highly conditioned. I’ve seen this each season as I reach very strong aerobic condition; generally there is a 15-20 beats per minute (BPM) drop in heart rate from poorest fitness to best fitness at the same power output. Such are the effects of training. Assuming similar conditions, this is one way to assess fitness gains.