Heart Rate v Power ~ Again

Someone asked me if I still monitor heart rate now that I use a power meter. The answer is" yes and never not really". I occasionally put my chest strap on, as much for entertainment value as for anything else. Sometimes, for longer base rides I find it useful to see that heart rate is between145-155bpm. I know then that regardless of the stochastic nature of the power meter, I'm maintaining a pretty even overall pace that will equate to my upper level 3 power zone. It just helps me to maintain a level of intensity for two to three hours without constantly watching power. But when I get home and download my data, it's the power values that I analyse. Not only am I interested in the workout's training load as a function of normalised power, but also as to whether I achieved my specific power goals within the workout.

The above chart is a segment from last Monday night's workout. It shows a 20min effort where I wanted to work at 100% of my FTP. I've smoothed the data considerably to illustrate the relationship between heart rate and power over the 20mins. On this occasion it was a cold night and was raining steadily. I rode though a couple of villages, round some fairly tight slippery corners and up some hills. The power trace reflects this and is not as smooth as I would manage on my normal threshold training circuit. The horizontal yellow line indicates my current FTP level. Average normalized power for the 20mins was 101% and I managed to go slightly harder in the last 10mins too which was another objective.

Now look at the heart rate trace. My maximum bpm during the 20mins did not occur until the end and it rose steadily throughout, yet power output was relatively steady. I could not have paced my effort paced on heart rate alone, and in fact at the time I didn't even watch what it was doing.

The situation is even more complicated if you take into account all the other factors. On Monday I was very fresh after having not ridden since Saturday morning, but it was only just above freezing. When I am in a proper race on a warm day and at threshold for any length of time, I can expect my heart rate to be at between 170 and 175bpm. In training under the same conditions, even if well rested and at the same power levels my heart will only be between 165 and 170bpm.

The situation gets even more frustrating when interval durations are measured in minutes and impossible when they are in seconds. With a heart rate monitor you simply have no way of knowing which energy system you are loading. Hence the reason why is bought a power meter other than the fact that I like new toys         :)