Cardiac Drift: What, Why, and Training
Harry Pino, PhD
Don’t get too caught up with your numbers.
Have you ever paid close attention to your heart rate on a long run or race? You probably noticed that your heart rate gradually increased even if you were maintaining a steady pace.
Why does this happen and what should we do about it?
This naturally occurring phenomenon is called cardiac drift and is common in endurance sports. It’s nothing to worry about but, if you train by heart rate or if you check on your heart rate after your runs, it’s something to be aware of.
What is cardiac drift?
Our heart rate naturally increases as we run. Why?
Dehydration is a large factor in cardiac drift. As your body gets more dehydrated, your blood plasma is reduced and your blood becomes thicker and harder to move through the vessels. This leads to your heart having to work harder to move the same amount of red blood cells. Unless the dehydration is severe, this isn’t something to worry about. Just realize that even being slightly dehydrated can cause your heart rate to go up slightly.
Body temperature also plays a role in cardiac drift. As you get warmer, your body prioritizes maintaining a safe temperature above all else. One of the most effective ways to cool your body is to pump more blood to your skin, which is why your skin might look more pink or red after a hard workout in high temperatures. If more blood is going to your skin, your heart will have to work harder in order to provide the same amount of blood to your working muscles.
Hormones, stress, and fatigue also play a role in cardiac drift. It’s a pretty small role but fatigue stresses the body, which causes hormones to be released that affect your heart rate.
What should we do about cardiac drift?
Accept the drift. Don’t fight it.
If we’re training by heart rate and don’t account for cardiac drift, we’re going to slow throughout a run to maintain a given heart rate target. Experienced runners and coaches know we ideally want the opposite. Maintain a steady pace or even get slightly faster as the run goes on. This means don’t panic when you see your heart rate gradually increasing throughout a run. Instead, just keep doing your thing and let your body do what it needs to do.
If you want to train by heart rate, target ranges, not precise numbers. Start at the lower end of the target range and understand that, as the run goes on, you’ll naturally move toward the higher end of the range.
Especially on race day, don’t worry about the drift. Consider the factors I mentioned above. You might experience a bit (or a lot) more of each of those things when you’re pushing yourself to the limit on race day. If you’re feeling as good as should be expected and running well, don’t slow down on race day just because your late race heart rate may be higher than expected from earlier in the run.