What is Cardiac Drift?
Cardiac stands for cardiovascular. Simply put a Cardiac Drift is an upward drift/ change in your heart rate over a certain amount of time, even though the training intensity remains the same.
It is known that the Cardiac Drift exists and happens in our bodies but it hasn’t been proven when exactly it happens in your body, as this is a very individual thing.
When you first start your training, your heart rate will go up and then start to plateau (= stay at about the same beats per minute) as long as the intensity remains. This happens after about 1 to 2 minutes (or later as it depends on the athletes age).
The Cardiac Drift can start at any time during your training (it is dependent on many variables such as your resting heart rate that day, your digestion, hydration, etc.) with the same intensity.
Factors that can lead to Cardiac Drift
- increase in core body temperature: The rise in body temperature does the same thing to your body as if you went running on a hot day. It increases your heart rate.
- sweating (caused by increase in body and outside temperature, etc.): The sweating causes you to loose body fluids which need to be taken back in as it has been shown that dehydration can increase the Cardiac Drift. (read more).
- warmer outside temperatures: Automatically make you start off with a higher heart rate
- your previous training: Can have an effect on your level of fatigue and you might therefore have a higher heart rate than normal.
- there are many more factors, which all somehow correlate and can in the right combination lead to the phenomenon of the Cardiac Drift
Causes of the Cardiac Drift
- demand competition: Since your body wants to keep your body temperature in check, it automatically increases skin blood flow but your working muscles also want and need a large part of the blood flow. This can lead to a “demand competition”.
- the body decreases the volume of blood that is pumped through your heart (= stroke volume). This leads to your oxygen uptake and cardiac output (=volume of blood pumped by the heart in one minute) to stay equal. So for you pretty much nothing changes as things like your breathing pattern stay the same but your heart rate goes up.
- misinterpretation of the heart rate: always remember that your heart rate data needs to be put into the situation that they were produced in and all variables must be looked at when the training is analyzed
Don’t confuse the lag of your heart rate and the Cardiac Drift
The Cardiac Drift is a change in heart rate, even though activity levels stay the same but it is not the same thing as when your heart rate lags behind your activity. What we mean when saying this is, when you have an interval training, you might be increasing your intensity from the moment you start your stopwatch but your heart will be a little delayed with realizing this and hence increasing your heart rate. The same thing happens when you stop and decrease your intensity, your heart will keep beating a little faster for longer even though you might already be on your break part of the session. This delay isn’t the Cardiac drift we were talking about above.
The lag is rather due to the fact that heart rate isn’t a measure of power/ work but rather it displays the bodies reaction to it. It therefore depends on a lot of factors such as; what you ate, when you ate it, how the weather is, how your mental state is, etc. While using Watts to measure your training, shows just Watts and isn’t dependable on anything around it. Therefore a much more reliable source for measuring your training. We’ve covered this topic before so if you are interested in rereading our articles and getting more input, click on the following links: training with power, Training in the Power Zone (Z5)
Cardiac Drift Athlete Example
Now that we’ve understood the bodies doings behind the Cardiac Drift, we can take a look at an example from one of our athletes, that proves that this phenomenon really does exist in real life. The above example (from a runner) is based on speed and not power but since it was pretty much on a flat, it doesn’t matter. The blue line shows the speed while the red line shows the heart rate. The first ten minutes of the run are the warm up, the speed and heart rate rise.
When the first interval starts, you can see that the speed goes up immediately while the heart rate lags behind (same thing when the interval is over). Now this is the effect of the heart lagging behind (read the article here).
What is however interesting if you further examine the picture, is that while the interval speed stays about the same every time, the athletes heart rate always reaches a plateau and then after about 10 minutes begins to climb again. So the heart rate increases even though the training intensity remains the same. This phenomenon is proof of the Cardiac Drift we’ve explained in theory above, actually happening in real life.
Why is it important to know about the Cardiac Drift ?
There are several misinterpretations of heart rate (also but not only) caused by Cardiac Drift that can lead to wrong outcomes.
1. Tendency to under train: For example, if you are meant to run at 140-150 beats per minute, you will be in that zone at the beginning of your training but after time, your heart rate will go up and you will reduce your pace, trying to stay in the correct heart rate window. This decrease in pace however, doesn’t correspond to your level of fatigue or your effort levels.
2. Training to lose weight: Many online calculators and machines still use heart rate to calculate the burned calories and when the beats per minute increase, the machine assumes that you are automatically burning more calories, which with the effect of the Cardiac Drift isn’t happening.
Is there anything we can do to reduce this from happening ?
Technically the Cardiac Drift is nothing bad or hurtful, it just happens and needs to be taken into consideration when training and analyzing training. Especially when you are training with heart rate and your training are “defined” using heart rate, Cardiac Drift can be a little upsetting and misleading. There isn’t much you can do to prevent this process from happening. One option however would be, especially when you are training inside, to try to keep your body cool, using a fan, etc. so that your body temperature doesn’t rise all too fast.
A second idea is to drink more and keep yourself hydrated, as it has been shown that Cardiac Drift can be reduced if athletes consume enough fluids to keep up with their sweat rate. If you want to make your training become more precise however, maybe you should consider training by Watts instead of heart rate (as we’ve mentioned in a previous article, there are devices that make this possible for runners as well, not just cyclists).
The take away message is, that we have limited influence on the phenomenon of Cardiac Drift, so we need to take into account its existence and possibly avoid its decoupling from our training effort altogether by monitoring our training based on a more objective parameter such as power (for cyclists and now runners) or pace/speed on flat terrain for runners. And remember that we subsidize selected power sensors as well as training instrument if you bundle them with your next training plan purchase: go to the shop