This question gets asked all the time. The quick answer is Yes and No and here’s why…
We all start out cycling and measure our fitness level by the amount of time we can cycle or the distance covered. If you start out and can only cycle for 20mins then one day you realise that you can go for 40mins then surely this is an indication of fitness? Yes, that is true.
But it is only an indication until you get to a point where you can go on for hours without stopping. At that point we need some other measure.
If we look at the forces of nature, then what if I weigh twice as much as you and we ride up a hill. You take exactly half the time that I do to get there. So did you win? In science we learnt that Work = Power / Time.
Yet, for me to make it up the hill I needed twice as much power as you. Yes, your time was half mine, but in the end we both used the same Power to get to the top.
So given all variables of weight etc, we both worked the same.
During the 1980’s the portable heart rate monitor was introduced and it revolutionized the fitness industry. If we look at our hill example above, let’s say we cycle up that hill with an average heart rate of 150. A month later after some training we go up the same hill with an average heart rate of 130 and in the SAME time then surely we’re getting fitter.
I emphasized SAME here because if you go up with an average of 130 and your time was slower then one can’t necessarily say you’re getting fitter. It’s hard to say what the story is…and that’s the problem with heart rate.
Your heart rate varies with factors such as humidity, stress, fatigue, temperature. If a car turns in front of you unexpectedly it will go up from fright.
Heart rate is also a lagging indicator. If you start cycling up a hill it takes a minute or two to increase, and similarly when you get to the top. It takes a few seconds before it recovers. This is why it is not such a good “work” indicator when doing short intervals of up to two minutes or so.
Power on the other hand is always constant and “instant”. As soon as you hit that hill it goes up. It doesn’t matter what the temperature is the same power will be needed.
One nice factor with power is that we can compare it when we take weight out of the equation. We do this by taking the Power and dividing it by the Weight.
Thus if you lose weight over time and yet your Power is the same then you are getting fitter.
The big problem with Power is that the meters are still expensive for the average cyclist. Your weekend warrior can’t really justify the spend.
The good news is that for longer intervals of a few minutes Heart Rate is a good indicator of “effort”. You just need to bear in mind factors such as stress, temperature etc. So don’t just decide based on one ride that you’ve lost fitness. Look at the big picture. You’ll know if you’re getting fitter.