Here are two sessions you can only assess with a power meter.
Session one: 5 x 30s sprint
Warm-up 20 minutes in zone two
5 x 30s sprints with six minutes zone one recovery between efforts
Cool down zone one
A 30-second sprint is such a short effort that heart rate cannot respond to it in real time. Often with a 30-second sprint your maximum heart rate will be after the sprint is finished. In fact, in a well-trained athlete, the maximum heart after each sprint may only be 80 per cent of the rider’s true maximum heart rate (despite the fact you are ready to collapse!). In addition, during the first five to ten seconds of the sprint your heart rate may not rise at all.
As a result, a 30-second sprint cannot be accurately assessed with a heart rate monitor, whereas with a power meter the exact wattage of each sprint can be recorded and analysed. What’s more, with a power meter you can see if your power is still good after 30 seconds, if you are flagging, or your initial kick needs work etc. That’s a lot more information that you can get with heart rate monitor.
Session two: 3 x 20 minutes zone four hill efforts
Warm up 20 minutes zone two
3 x 20 minutes zone four (uphill) with 20 minutes recovery between efforts
Cool down zone one
These sorts of consistent efforts seem perfect for measuring through heart rate data. However, this isn’t the case because as you become more fatigued, often a higher heart rate is required to be able to produce the same power.
Therefore, if you went out and did 3 x 20 minutes at 165bpm, then chances are the third effort wouldn’t be as good as the last. However if you did 300 watts, 299 watts and 265 watts then you know that the first two efforts were of the same quality but the third one wasn’t as good.
You can then look back and compare the power outputs to your heart rate data. This might tell you that the first effort was relatively easy, the second was quite tough and the third was very difficult. Knowing this means you can go away and work on making the efforts more repeatable. This might pay dividends come the last mountain of the Etape du Tour!
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