Why can’t I compare my power and heart rate?

Cycling Tips

Why can’t I (always) compare my power and heart rate?

This is an important question that is not very easy to answer. Several factors play a role in both power and heart rate.

The problem with heart rate

When the heart rate monitor was introduced for consumers, the main goal was to increase training efficiency. By measuring the heart rate, you got an idea of the impact and intensity of a workout on the body. In the era before the heart rate monitor, speed was monitored. This is obviously a very inaccurate way to determine the intensity because the circumstances play a major role. However, heart rate is also an indirect way of measuring the intensity of a workout. Heart rate is the body’s response to an effort. If this reaction is always the same, then heart rate could perfectly reflect the intensity of a workout. But it turns out that a large number of factors can influence the heart rate. Examples include stress, caffeine, altitude, recovery, sleep and time of day. All these factors affect your heart rate. We explain this concept a little further. We take as an example a rider with an FTP of 250 watts. When this rider cycles on a power of 200 watts, this is below his maximum lactate steady state and he can maintain this intensity for quite a long time. Now the same rider is at high altitude (2500 meters above sea level). Contrary to what many people think there is the same amount of oxygen in the air here, only the air pressure is lower so the oxygen uptake in the lungs is lower. This means that the body has to work harder to supply the muscles with the same amount of oxygen. For this reason his heart rate is higher at the same power, and because less oxygen reaches the muscles, his FTP is also lower at altitude. The same goes for fatigue (lower heart rate at the same intensity) or caffeine consumption (higher heart rate).

The effect of blood volume on heart rate

This example is fairly easy to understand, but there are more examples where power and heart rate no longer seem to be related. For example, if you haven’t cycled for a while, your heart rate is probably high for the first few rides without realizing that you are riding at high intensity. This is because you are now completely fresh and rested, but also because your blood volume decreases rapidly after a few days of not training. With lower blood volume, the harder your heart has to work to pump the same amount of blood to your muscles.
Does this mean that heartbeat is totally worthless and that you only have to look at power to determine the intensity of a workout? No, certainly not!

The body’s response to exercise

The beauty of training with a heart rate monitor is that it is not a subjective way to measure intensity. On the contrary, it is an objective way to measure the body’s response to exercise! Combining heart rate and power is a good way to see what your body does (power) and how your body reacts to it (heart rate). It gets even better if your feeling during a workout matches the numbers that appear on your bike computer! When you do an extensive endurance training, your heart rate is higher than normal and you don’t feel great? Then it could be that you are starting to get sick. Or maybe you have eaten too little or drank too much coffee. But if your heart rate is a bit higher than normal and you haven’t cycled for a few days it could mean that you are well rested.

Looking at power and heart rate

The combination of power, heart rate and your feeling gives the complete picture. Even power can vary due to the environment and conditions that day. When all external factors are the same, such as your weekly indoor workout one hour after dinner, it gives you a reliable way to measure your workout load and see if you’ve made any progress. Be aware that your FTP will not change in a day. Don’t overload yourself when your heart rate is a bit lower. Of course, you may have progressed due to a lower heart rate at the same power, but it may also be due to other factors. The same goes for a higher heart rate at a certain power.
As soon as you notice over a longer period of time that you can deliver more power at a certain heart rate, we can assume that this is the effect of training. See if there is an upward trend in your workouts and don’t be fooled by a single workout in which you had a higher/lower heart rate at certain abilities. Heart rate can’t be compared 1 to 1 with power, so don’t be too premature with conclusions and always listen to your feelings.
Do you want to get better and learn something at the same time?
Consider working with a trainer for a while. If you work with one of our trainers you not only make more and faster progress, but you also learn something for it. You already train under the guidance of one of our movement scientists for €59,- per month. Or take it easy with two tests and guidance in our introduction package. If you have any questions, you can always send us an email at info@cyclinglab.cc.