If you train by heart rate, or monitor it on your running watch, you might have noticed that sometimes, towards the end of your longer runs, your heart rate seems to climb progressively higher even if you’re not picking up the pace or the exercise intensity isn’t increasing. It happens to me all the time!
The heart rate increase may be especially pronounced when it’s hot out and you’ve been sweating a lot.
What’s happening here? Why does an endurance athlete’s heart rate increase over the course of a run?
Although there can be a few potential causes, one of the main reasons you’ll see an elevation in your heart rate during the last few miles of run is due to something called cardiac drift, more scientifically known as cardiovascular drift.
Cardiac drift is a natural response of the cardiovascular system, but it can make it tricky to gauge whether you should adjust your effort when training by heart rate.
Understanding how it works can help you train smarter and avoid unnecessary frustration. In this guide, we’ll break down what cardiac drift is and how it impacts your workouts.
What Is Cardiac Drift?
Cardiac drift refers to the upward rise of your heart rate during exercise over a long, continuous workout, even though the intensity of exercise has remained constant.
A scenario that exemplifies cardiac drift running would be someone running a long run of 12 miles on a warm, summer morning. The runner maintains a steady pace of 8 minutes per mile the entire run.
However, heart rate data shows that their heart rate for the first 7 miles of their training session was an average of 145 beats per minute, but then it gradually and progressively crept up over the next five miles to 166 beats per minute.
The increased heart rate occurs even though they didn’t speed up nor encounter hillier terrain. In other words, their exercise intensity remained constant, but their heart rate drift went up over the course of the run.
Why Does Cardiac Drift Occur?
This cardiovascular response is coupled with a progressive decline in stroke volume.
Stroke volume refers to the volume of blood that your heart ejects out of the ventricles and into circulation per beat.
The higher the stroke volume, the more blood (and thus oxygen) your muscles and tissues are getting per beat.
One of the cardiovascular adaptations that occurs from consistent aerobic training is an increase in stroke volume. These changes are due to an increase in the size and strength of the muscular chambers in the heart as well as an increase in the blood blasts volume.
If you have more blood, a stronger heart, and larger ventricles to hold more blood, you’ll have a greater volume of blood squeezed out of the heart when it contracts.
Higher stroke volume is associated with greater efficiency of the heart because the cardiac output increases.
Cardiac output refers to how much blood your heart is circulating through the body in one minute. It is the product of your heart rate (how many times your heart beats per minute) multiplied by the stroke volume (how much blood is pumped per beat).
Why does cardiac output matter?
During exercise, your muscles need significantly more oxygen to fuel your activity. Therefore, cardiac output has to increase to ensure your tissues are properly oxygenated.
This increase can come in the form of increasing your heart rate, increasing your stroke volume, or both.
And, in most cases, it is indeed both.
You can clearly feel your heart rate increase, and though you can’t see it, you can probably feel that your heart is contracting more forcefully, which ejects a greater percentage of the blood in the chambers of the heart out into circulation.
This means that stroke volume increases as well.
Here’s where cardiovascular drift comes into play.
Most of the time, cardiac drift is a phenomenon that occurs during long workouts, which gets exacerbated due to heat stress.
What is happening is that exercising causes an increase in core temperature and dehydration—a loss of body fluids through sweat.
This fluid loss directly decreases the plasma volume of your blood.
Returning to our concept of stroke volume, when plasma volume decreases, there’s less blood circulating in the blood vessels and less blood to fill the ventricles of the heart.
Consequently, when the heart contracts, it’s a more measly volume of blood that’s pumped out of the heart, which is reflected in a decrease in the stroke volume.
If stroke volume drops, cardiac output—the amount of blood pumped around the body per minute—also drops.
However, if you’re still in the middle of your long run, the oxygen needs of your muscles and other tissues haven’t suddenly dropped as well; they are still working hard and consuming massive amounts of oxygen to produce energy.
Fortunately, the autonomic nervous system is smart and rather than simply let the cardiac output drop as it may, protective mechanisms kick in that stimulate an increase in your heart rate to compensate for the decrease in stroke volume.
Recall that the formula for cardiac output is Q = HR x SV (cardiac output equals heart rate times stroke volume).
Thus, if the body wants to keep cardiac output stable, if one of the two factors drops—in this case stroke volume—the other must increase—in this case heart rate.
So, why does cardiac drift occur?
As mentioned, part of the increase in heart rate is due to the decrease in stroke volume from body water losses in sweat and expired respiratory gasses.
Additionally, exercise causes an increase in core temperature, especially over long endurance exercise and when exercising in the heat and humidity.
An elevation in heart rate is associated with this increase in core body temperature, partially due to heat stress, which causes the body to increase circulation to the skin to promote evaporative cooling.
The increase in blood flow to the skin actually further increases the cardiac output demand, so even if stroke rate were to remain constant rather than decline, heart rate would still have to increase to achieve a higher cardiac output and satiate the oxygen needs of the muscles and perfuse the skin adequately to promote cooling.
One study1Hamilton, M. T., Gonzalez-Alonso, J., Montain, S. J., & Coyle, E. F. (1991). Fluid replacement and glucose infusion during exercise prevent cardiovascular drift. Journal of Applied Physiology, 71(3), 871–877. https://doi.org/10.1152/jappl.1991.71.3.871 that investigated the effect of fluid losses on cardiac drift in cyclists found that those who did not consume any fluids during their workout experienced an increase in the heart rate of about 10%, whereas those who ingested a fluid volume that matched their sweat rate experienced only a 5% increase in heart rate.
This suggests that cardiac drift is largely attributable to dehydration, but that some upward trend in heart rate also occurs in the euhydrated state.
Again, this is likely due to increases in core temperature and the resultant physiological mechanisms enacted to cool the body to combat heat stress.
How Does Cardiac Drift Affect Your Training?
Cardiac drift doesn’t have to be a completely unavoidable phenomenon.
Though it’s likely impossible to fully protect against the increase in core temperature component underlying cardiac drift, staying properly hydrated is the most effective means at minimizing the severity.
Your fluid intake should keep pace with your sweat rate. If it’s hot and humid out, having a sports drink or adding electrolytes, and a small amount of glucose to your water can increase the rate of fluid absorption.
How much cardiac drift is normal?
Depending on your sweat rate and the environmental conditions, it’s normal to see an upward drift of up to 15% or so in your heart rate.
This can actually be quite significant, as it can make it appear that you’ve switched intensity zones despite your effort level staying the same.
Let’s walk through an example of a 40-year old doing a long run in zone 2, which is 60-70% of maximum heart rate.
For simplicity, we will use the Fox Formula for age-predicted max heart rate: 220-age. 220-40 = 180.
We will take the average heart rate in zone 2, which is 65%: 180 x 0.65 = 117 bpm.
If that were to increase by 15% due to cardiac drift, 117 x 1.15 = 135 bpm.
Now, we can look at what zone this puts the runner in: 135 bpm / 180 bpm max heart rate = 75%.
That bumps up his effort one full heart rate zone, and if he had been in a higher heart rate zone initially, a 15% increase would have been an even higher increase in the heart rate number, which could have taken us up two heart rate zones.
Therefore, it’s important to be aware that if you’re training by heart rate, some amount of cardiac drift is going to occur (again, you should aim to minimize it through proper hydration).
Depending on the goals of your workout and whether you need to stay in a certain zone or not, you may need to compensate by slowing your pace2Billat, V. L., Palacin, F., Correa, M., & Pycke, J.-R. (2020). Pacing Strategy Affects the Sub-Elite Marathoner’s Cardiac Drift and Performance. Frontiers in Psychology, 10. https://doi.org/10.3389/fpsyg.2019.03026 or reducing your power output, which will drop the oxygen demand and thus need for a higher cardiac output.
However, this can impact the effectiveness of your training, again, depending on your particular goals. It is important to consider the reason why you are monitoring your heart rate in the first place.
Otherwise, you can maintain your pace and accept that your heart rate will creep up with the passing minutes of exercise during longer workouts. But no, you haven’t lost fitness, and your vo2max hasn’t hit the floor.
You will just know that you’re not really working appreciably harder; it’s just that the heart is beating faster but ejecting less blood per beat.
There are inherent flaws on most metrics we track as runners, so being a slave to your numbers is a bit of a futile effort.
Listen to your body and take everything together in a comprehensive picture about how you’re feeling and functioning while you work out.
If you are looking to forget about heart rate for a moment, and train according to your perceived effort, take a look at our RPE guide:
I’ve definitely noticed it, but had no idea about the reasons and solution. Not something I’ve heard a lot in my circles and not something I knew about before. This article explains it to the point and gives all-so-needed rational. Thanks!
Very interesting. I have always wondered what was going on here.
So, one thing that is a little confusing about this is that I thought red blood cells are responsible for carrying the oxygen. A decrease in plasma is not a decrease in red blood cells, right?
Hi John, A decrease in plasma would not mean a decrease in red blood cells, correct.
But it would mean a decrease in total blood volume (made up of plasma, RBCs etc..) so basically lesser water in your blood pipes.
Thus the need for an increased HR to compensate for the decrease in stroke volume.
This happens so that the Cardiac output (oxygen and nutrients reaching your muscles) remains the same throughout the activity.
I had this exact question as to why my HR increases over long runs, which is why I came to this article. The article was spot on!
John and Rish,
1. Yes, a decrease in blood plasma (water content) causes lower blood volume, BUT this does NOT decrease the amount of oxygen being supplied to our working muscles (mitochondria). Remember (from biology class): A reduction in plasma directly increases the concentration of red blood (oxygen carrying) cells in the blood (i.e., an increased hematocrit value). Therefore, it is NOT the reduction of oxygen that causes cardiac drift.
2. Plasma provides the nutrients (fats/carbs) to “refill/recharge” the storage tanks within the muscle cells, that feed the mitochondrial engine. A depletion of fuel, or reduced fuel capacity can cause an increase in HR (cardiac drift) so as to help compensate (increase refill/recharge fuel) to the rapidly depleting fuel supply.
To finalize my comments above: The better fueled and hydrated we are, the longer we can hold off cardiac drift.
I’d been having a nightmare trying to keep my heart rate in zone two over a few runs recently. Going back to the old run/walk strategies.
This is such a useful concept. I will keep more of an eye on hydration and making sure to manage insulation levels so I’m not running too warm.
Another ace article. 😎 Thanks.
Andrew.