My first run on the Big Island, Hawaii prior to the Ironman World Championships last year taught me one thing: my race pace in 35 deg heat with high humidity and strong sun is different to my race pace in the much cooler Cape Town which I had left. How did my body tell me this? First my heart rate at race pace was 20 beats higher than it was in Cape Town and second, it told me this by wanting to shut down and seek shade!
Now image I had a training program which told me to run at a given pace – but didn’t take into account the conditions on the day. That session would have been much tougher in the conditions in Hawaii than in Cape Town and would have placed me in completely different training zones, working on different energy systems. It would have been a completely different workout. While doing a number of those sessions per week in Cape Town would have been fine, doing them in Hawaii would have lead me to exhaustion and over-training.
The same is true on a bike. Training to a given power can vary the intensity of the same session from a day to day basis – depending on a number of factors including how tired I am, the weather, if I am dehydrated or don’t fuel the session properly or may be even if I have an underlying infection and I am getting sick but don’t feel ill yet. What should be a relatively easy session can become much harder due to these factors.
To overcome these external factors I train to heart rate – when it’s hot that means my pace slows down and my power is lower – but I work the right systems and illicit the correct training response. When I am tired or sick or my nutrition is off the mark the same thing happens – I slow down but work at the right heart rate. I do use power and pace to measure progress: over time I can compare similar sessions to see what power (or pace) a given heart rate resulted in – which in my eyes, is the real measure of progress.
To go faster in a long distance race you don’t need to produce huge amounts of power, you have to produce that power efficiently, that is with less effort, and I use my heart rate as a proxy for effort. If I can produce the same power at a lower heart rate then, all other things being equal, I can hold that power for longer. So I train to improve my efficiency by making sure I am training the right systems as often as possible.
So in short, I train to heart rate and use power and pace to measure progress.