Why Cardiac Drift Is Important For Runners Who Train By Heart Rate

Audio Transcript

Jeff: Welcome to the Runners Connect, Run to the Top Extra Kick Podcast. Today’s question comes from Michael.

Michael asked, “I enjoy training by heart rate but I’ve noticed recently that my heart rate always rises throughout a run, even when I keep the pace the same. Since I’m training by heart rate zones, by the end of a long run, I always need to slow down to stay in the right zone, even though I’m feeling fine. Am I doing this correctly or might something else be going on?”

Michael, the simple answer is that’s what’s happening here with your heart rate, is something that we call ‘cardiac drift’.

Cardiac drift is basically the scientific term for the natural increase in heart rate that occurs when you’re running, when you have little or no change in your pace.

Like you’ve noticed, Michael, you kind of assume that if you keep your runs and your run pace at a consistent pace, then your heart rate is going to stay consistent as well.

But research has shown that this is not common; it’s actually common for the heart rate to drift upwards.

Basically, you can actually drift upwards by as much as 10 to 20 beats per minute, over a 30 minute period, according to some of the research. So, it’s definitely significant.

In terms of what causes cardiac drift, it’s basically just the natural increase in your core body temperature when you’re running.

It doesn’t have to be hot out. As you run, your core body temperature is going to heat up no matter what. It’s obviously exasperated when you’re running in hotter conditions.

Basically, the stroke volume of the heart decreases, so the cardiac output in the options and uptake remain the same.

This is why you’re keeping your breathing an effort similar while your heart rate is actually rising.

This is a very important point.

If you are training by heart rate and you’re using heart rate on your long runs, you need to keep this in mind.

You need to estimate how much of your heart rate increase is because of effort or just feeling fatigued, et cetera, and how much is a factor of a cardiac draft.

Basically, you’ve got to know yourself.

If you’re using a heart rate, then you’re somebody that likes data, so you’re going to need to probably make a spreadsheet or just keep track.

But basically, what I would do is just estimate that your heart rate is going to rise anywhere from 5 to 15 beats per minute, every 30 minutes.

Maybe to start conservatively, I would factor in a 5 beat per minute increase, every 30 minutes during your long run.

If you find that towards the end of the run, your heart rate still isn’t matching where your pace is at the beginning, and you felt completely fine, you’re recovering fine, everything is normal otherwise, like you noticed in your training log, then for the next run, you should look at doing a 10 beats per minute increase, every 30 minutes.

And again, keep a spreadsheet. And then over time, maybe after two, three, or four runs, you’ll have pretty good data about how much your cardiac drift is going to impact you.

The other thing that you want to keep in mind, is that it’s going to change in different weather conditions.

So, if it’s 50 degrees outside, your cardiac drift is probably going to be somewhat minimal, whereas if it’s hotter out, your cardiac drift is going to be a little bit more significant.

Again, this is where keeping a chart or some type of log is going to be helpful.

You can say, “Well, it’s normal and I ran at 50 degrees, and my heart cardiac drift is about five beats per minute,” then if it’s 70 degrees, you may want to go up to ten beats per minute. You kind of see how that works.

Then, what I like to do in these types of situations, and I do this with hydration, et cetera, is I keep a chart.

I know it sounds a little dated, but I like to do that.

Most runners do, but I’ll just keep a chart and it will be like cardiac drift, 50 degrees, five beats per minute. Cardiac drift, 70 degrees, 10 beats per minute.

That gives me a good range that, when I go on my runs, if I need to use heart rate, then I have a pretty good idea of how much that’s going to factor in.

The other thing about cardiac drift is, you’re probably not going to notice it for your easy runs, and it’s typically there, probably 60 to 90 anyway.

You’re probably not going to drift too much in that timeframe, but it’s going to make a big difference when you do your 20 mile long runs, or 18, 19 mile long runs, when you’re out there for three hours or so.

That can make a significant difference in your overall pace.

So, that’s my recommendation. It’s definitely something that you need to factor in, and you need to be very cognizant of, if you’re using heart rate to train.

Because it’s going to be very important that you factor this in, so that you’re training at the right effort levels all the time.

Again, make a chart, look at what works for you, and go from there.

I actually recommend doing this for heart rate in general. I know it’s a little bit of an aside here on this particular question, but this is one of the reasons that I’m not a huge fan of training by heart rate.

There are so many variables that you need to plan into, that can affect your training, that are hard to estimate. It’s hard to know exactly how much of your training is being impacted by variations in heart rate.

What I mean by that is that your heart rate can not only be affected by cardiac drift, but it can be impacted by things outside of running.

For example, your heart rate can warn you when you’re running, and can increase if you have a lot of caffeine, if you are low on sleep, or if it’s very hot outside.

Those are just a few of the things that can impact your heart rate.

What ends up happening is that, let’s say that you go into a workout or a long run or whatever your day may be, and you had a bad night’s sleep, had a lot of caffeine, or were really stressed at work. Maybe you were a little bit dehydrated.

Each one of those individually, we may be talking about two or three beats per minute, maybe up to five beats per minute impact on your training.

So, one individually isn’t a big deal, but you factor all of those in. Let’s say that, like I said, you have one of those days or it’s a combination of everything, you could be talking about a 15 to 20 beats per minute difference, in what you would normally be and what you are in this particular day.

All because of these factors that are outside of your fitness level, that have nothing do with your fitness level. So, that’s one of the reasons that I don’t like it.

The other reason is that in the research that I’ve read, finding the correct heart rate zones is really difficult.

Training by heart rate zones is an inexact science, from everything that I’ve read.

So, the first part is that you have to calculate your max heart rate. Almost all of the heart rate zone training, goes on the fact that you know your maximum heart rate.

What ends up happening with maximum heart rate, is that most people do the really simple one, where it’s like 220 minus your age, and then that’s going to be your max heart rate.

But if you’ve ever done a real max heart rate test or you train by heart rate, you know that that’s pretty off.

Or if you’ve gone a race and you’re coming down the rest part of a 5K, and you look at your watch, and your heart rate is higher than 220 minus your age, then you know that that’s not your maximum heart rate.

First, you need to establish that number which can be a little difficult, especially for newer runners.

Secondly, there’s a lot of research that shows there’s a big difference between, for example, lactate threshold, which would be a zone that you’d want to be in for your heart rate.

There’s actually a couple of research studies that show that in well trained runners, lactate threshold can come between 80 and 90% of your maximum heart rate in well trained runners.

But for beginner runners, it can come between 50 and 60% of your maximum heart rate. So, if you’re looking at heart rate training zones, if you’re a beginner and you’re an experienced level runner, that can be a very big difference in what zone you’re in.

Any case, that was an aside on heart rate.

That’s not to say that I think heart rate training is bad or that I don’t think you should do it. I think it’s a valuable tool in your arsenal, so it’s something that I use as a backup data point or a data point where I compare things in previous train blocks too.

Again, nothing against heart rate training. I hope that people that use heart rate training understand all of the different things that can impact it, and make sure that you’re factoring those into your training, and using it as a data source and data point, instead of blindly following a heart rate zone type of thing.

Or as in Michael’s case, not realizing that cardiac drift is something that’s going to impact how you go and do your long runs.

Michael, I hope that answered your question and I hope that gave you a little bit of background on some other factors that you may want to consider when using heart rate.

Finally, I want to thank our sponsor, Aftershokz Headphones. Now, when you’re training outdoors, hearing your surroundings like traffic and other runners around you is really important. But we also know that you want to listen to your favorite podcast like, Runners Connect and also listen to music to keep your feet moving.

Now, some runners choose not to run with headphones because they don’t want to turn out the world around them while they listen to the music.

Others don’t listen or use headphones because they’re really uncomfortable, especially when you’re out running for an hour, 90 minutes or two hours, because the ear buds don’t fit into yours really well.

The nice thing about Aftershokz is they’ve actually found a solution to both of these problems.

Aftershokz Headphones use bone conducting technology that delivers premium audio through your cheekbones to your inner ear, without blocking or covering your ears.

The unique open ear design allows you to listen to your favorite music while you’re running, while you’re chatting with your running partner, even at the office, while being completely in tune with the environment around you, because there’s nothing in your ear with these headphones.

Your ears are also not going to get sore, which is a really big bonus.

The Aftershokz Headphones are lightweight, comfortable, sweat resistant, and they’re durable.

Aftershokz allowed us to cook up a really great deal with you, and all you need to do is visit runnersconnect.net/shokz, and you can be able to claim your free gift with your purchase. That’s runnersconnect.net/shokz. You can take these headphones with you everywhere so you can hear your adventures, hear your music, and hear it all.

Thanks again for listening to this podcast. I hope you enjoyed the thorough answer and I hope to see you back tomorrow. Thank you so much for listening.