I bought my first HRM in around 1988’ish, a Polar Sport Tester. These were quite expensive at that time, so it was an advantage that I could buy it for the club I was coaching riders in. We used it mainly to collect data in races and then use that info to set target HRs for different interval training sessions. It only took a little bit of trial and error to find the anaerobic threshold and set levels from that. (The book Training, Lactate and Pulse Rate (Peter Janssen) was very useful.) Then, after a few sessions training with HR enabled riders to more accurately judge efforts by feel alone (RPE).
This graphic shows how HR varies at constant power output during the third eight minute interval of a set of four at slightly over FTP. After two minutes the HR remains very constant (+/- 1bpm) until near the end when it increases very slightly due to a slight increase in power.
The key to successful training with a HRM is to understand oxygen deficit. Immediately on starting an effort, the muscles work harder than the rate of oxygen is supplied for aerobic respiration so the muscles work anaerobically initially until the heart rate catches up and the blood supplies enough oxygen. This takes typically about 2 minutes, after which HR may continue to rise but very gradually for a bit longer until reaching a plateau. Therefore, for the first 2-3 minutes of an effort, you should go on RPE not HR. In a TT, assuming you didn’t start too fast, you settle into your desired effort after the first mile and from then on your HR shouldn’t vary too much. If it drops below your target range you increase your effort, and if it climbs too high you ease a little.
Cardiac drift can occur if you are dehydrated, particularly in hot and humid conditions. This is when HR increases even though the power output remains constant, this is thought to be caused by decreased plasma volume and increased blood supply to the extremities to aid cooling.
Power meter evangelists point to the two effects above as reasons why training with power it so much better than HR, but I would contend than as long as you understand the practical effects of oxygen deficit and cardiac drift a PM doesn’t gain you much if anything, over a HRM unless doing intervals shorter than 8-10 minutes.
However, having both HR and PM data to compare post ride is very useful. HR can tell you hard you worked, but together they can give you a measure of current fitness.
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