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Cardiac Drift: Why Your Heart Rate Climbs at the Same Pace

Jeff Gaudette, MS   |

Cardiac drift is the slow rise in heart rate during a steady run, even though your pace stays the same, and it usually begins after about 10 minutes.

Cardiovascular drift is much bigger in the heat, where heart rate rose 11% over 30 minutes at 95°F (35°C) versus 2% at 72°F (22°C).

Heat and dehydration cause most heart rate drift, and replacing sweat losses halved the rise in heart rate during a 2-hour session.

In warm weather, a drifting heart rate means the run has gotten harder, because VO2max falls as the drift builds.

On hot easy runs and long runs, slow down to keep your heart rate in its zone, and for pace workouts set a slower pace goal before you start.

Start hot long runs early, because cooling off after the drift has built up doesn’t restore lost VO2max.

You start an easy run at 9:30/mi (5:54/km) with your heart rate at 140 beats per minute, and an hour later your watch reads 155 at the same pace.

That climb is called cardiac drift, or cardiovascular drift, and it’s biggest on long runs in warm weather.

If you train by heart rate, it leaves you with a real decision: slow down to bring your heart rate back into its zone, or hold your pace and ignore the number.

The right choice depends on why your heart rate is drifting, and the research on heat and dehydration answers it for each type of run.

In this article, you’ll learn the research on cardiac drift, including:

  • How much your heart rate climbs in cool weather compared with hot weather
  • What causes the drift, and which cause you can control on a long run
  • When a rising heart rate means the run has gotten harder
  • How to adjust easy runs, long runs and marathon pace runs for it

What Is Cardiac Drift?

Cardiac drift is the slow rise in your heart rate during a steady run, even though your pace stays the same.

It usually begins after about 10 minutes of steady exercise, and as your heart rate climbs, the amount of blood your heart pumps with each beat falls.

Here’s what it looks like on a real 20-mile (32 km) long run, with pace in blue staying steady while heart rate in red keeps climbing.

Garmin chart of a 20-mile long run showing pace holding steady while heart rate rises steadily, an example of cardiac drift

How far your heart rate drifts depends mostly on the temperature.

research
Research has shown that heart rate rose 11% over 30 minutes of steady exercise in 95°F (35°C) heat, compared with just 2% at 72°F (22°C), with sweat losses replaced in both.

For a runner starting at 150 beats per minute, that’s about 16 extra beats in the heat and 3 in cool weather.

Running is no exception, because a 2020 study found heart rate rose 19% between minutes 15 and 45 of steady running in 95°F (35°C), about the same as during cycling.

That means on a hot summer run, your heart rate can climb from your easy zone into the next zone up without you speeding up at all.

What Causes Your Heart Rate to Drift Upward?

Heat and sweat loss cause most of the rise in your heart rate, and both increase with every minute you run.

How Does Heat Raise Your Heart Rate?

Researchers still debate the exact chain, but both explanations start with your core temperature rising as you run.

The first explanation is that your body sends more blood to your skin to release heat, so less blood returns to fill your heart between beats.

The second, proposed in a 2001 review, is that the rising temperature speeds up your heart rate, which leaves your heart less time to fill between beats.

Either way, you end up with less blood pumped per beat and a faster heart rate at the same pace.

How Does Dehydration Add to Cardiac Drift?

Sweat loss adds to the drift, because losing fluid leaves less blood volume for your heart to pump.

A 1992 study had cyclists ride for 2 hours in 91°F (33°C) heat while drinking enough to replace different amounts of their sweat.

The more body weight they lost to sweat, from about 1% up to 4%, the higher their heart rate and core temperature climbed.

Drinking helps even in mild weather.

research
A 1991 trial found that drinking enough to replace sweat losses cut the rise in heart rate from minute 20 to minute 120 of steady exercise at 72°F (22°C) in half, from 10% to 5%.

That means how much you drink on a long run directly changes how far your heart rate drifts in the second hour.

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Does Cardiac Drift Mean Your Run Is Getting Harder?

In warm weather, a drifting heart rate is an accurate sign that your run has gotten harder, even if your pace hasn’t changed.

That’s because the drift comes with a drop in your VO2max, the most oxygen your body can use, so the same pace takes a bigger share of what your body can handle.

research
Researchers found that after 45 minutes of steady exercise in the heat, heart rate rose 12% and VO2max fell 19% between minutes 15 and 45, so the same workload went from 63% to 78% of the athletes’ aerobic capacity.

That means a pace that felt easy at the start was a much harder effort by the 45-minute mark.

Bar charts: during 45 minutes of steady exercise in 95°F (35°C) heat, heart rate rose from 151 to 169 beats per minute and the same pace went from 63% to 78% of aerobic capacity, showing cardiac drift makes the same pace a harder effort.

The 2020 running study found the same pattern, with the drop in VO2max matching the rise in heart rate whether the athletes ran or cycled.

Cool weather with fluid replaced is different, since in the 2008 study heart rate rose only 2% at 72°F (22°C) and VO2max didn’t drop significantly.

Effort also rises during a long, steady session in mild conditions.

In a 2024 study, endurance athletes ran six 10-minute intervals at a moderate pace, and over the second half of the session their heart rate averaged 168 beats per minute in the first half and 173 in the second.

Their blood lactate and their rating of how hard the running felt went up over the same stretch.

Should You Slow Down When Your Heart Rate Drifts?

On easy runs and long runs in warm weather, slow down and keep your heart rate in its zone.

Holding your pace while your heart rate climbs turns a planned easy run into a harder session than your training called for.

Even during hard intervals in 95°F (35°C) heat, a 2025 cycling study found that people who eased off to hold their target heart rate had no significant drop in VO2max, compared with a 15.6% drop when they held the same perceived effort instead.

On a hot day, let your pace slow so your heart rate stays in its easy zone, because the heart rate is the more accurate measure of how hard that run is.

What About Marathon Pace Runs and Workouts?

For workouts with a pace goal, like a marathon pace long run, run by pace and effort in cool weather and expect your heart rate to rise a few beats in the second half.

In the heat, set a slower pace goal before you start rather than trying to hold your normal pace while your heart rate climbs.

This guide on adjusting your pace in hot and humid weather shows how much to slow down for the temperature and dew point.

How Should You Set Your Heart Rate Zones With Drift in Mind?

Check whether your easy pace matches your easy heart rate zone during the first 15 to 20 minutes of a run in cool weather, before most of the drift has built up.

If you haven’t set your zones yet, this guide to heart rate training zones for running explains what each one is for.

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How Can You Reduce Heart Rate Drift on Long Runs?

Your heart rate will always rise a little on a long run, but these habits reduce how far it climbs:

  1. Drink to replace more of your sweat on long runs. In the 1991 trial, replacing sweat losses cut the heart rate rise over the second part of a 2-hour session in half.
  2. Run in the coolest part of the day. Drift at 72°F (22°C) was a fraction of the drift at 95°F (35°C), so an early start on a summer long run keeps your heart rate lower for longer.
  3. Keep building your endurance. A 2023 study found that 10 weeks of interval training in previously untrained adults pushed back the point where heart rate and other measures began to drift during a 3-hour ride, from about 108 to 137 minutes.

Prevention matters more than cooling off once the drift has happened.

In a 2019 study, turning a fan on near the end of 45 minutes of hot cycling didn’t stop VO2max from falling, with a 15% drop with the fan and 17% without it.

Start hot long runs early in the day, because cooling down after your heart rate has drifted doesn’t restore what you lost.

Heart Rate Drift at a Glance: What Should You Do on Each Run?

This table puts the research and the adjustment for each situation side by side.

Situation What the research found What to do
Easy run in cool weather Heart rate rose 2% over 30 minutes with sweat losses replaced Run by your easy heart rate zone and expect a few extra beats late in the run
Easy or long run in the heat Heart rate rose 11% to 19%, and the same pace became a harder effort Slow down to keep your heart rate in its zone
Long run without drinking Heart rate rose 10% from minute 20 to 120, compared with 5% when sweat losses were replaced Drink on long runs to replace more of your sweat
Marathon pace run or workout Heart rate, lactate and perceived effort all rose in the second half of a moderate session Run by pace and effort, and set a slower pace goal before a hot workout
Cooling off mid-run A fan after 45 minutes didn’t restore lost VO2max Start early in the day, before the heat builds
  • Frequently Asked Questions
What is cardiac drift in running?

Cardiac drift, also called cardiovascular drift, is the gradual rise in your heart rate during a steady run while your pace stays the same. It usually begins after about 10 minutes of steady exercise. As your heart rate climbs, the amount of blood your heart pumps with each beat falls. The drift is small in cool weather, about 2% over 30 minutes at 72°F (22°C), when you replace your sweat losses, and much larger in the heat, where heart rate rose 19% between minutes 15 and 45 of steady running at 95°F (35°C).

What causes cardiovascular drift?

Heat and dehydration cause most cardiovascular drift. As your core temperature rises, your body sends more blood to your skin and your heart rate speeds up, which leaves your heart less time and less blood to fill between beats. Sweat loss adds to it by lowering your blood volume. In a 1992 study, the more body weight athletes lost to sweat during 2 hours of exercise, from about 1% up to 4%, the higher their heart rate and core temperature climbed.

How much does heart rate drift on a long run?

It depends mostly on the temperature and how much you drink. Over 30 minutes of steady exercise, heart rate rose 2% at 72°F (22°C) and 11% at 95°F (35°C). From minute 20 to minute 120 of steady exercise at 72°F (22°C) with no fluid, heart rate rose 10%, compared with 5% when athletes drank enough to replace their sweat. For a runner starting at 150 beats per minute, a 10% rise is about 15 extra beats at the same pace.

Should I slow down when my heart rate drifts?

On easy runs and long runs in warm weather, slow down to keep your heart rate in its zone. Research shows that cardiac drift in the heat comes with a drop in VO2max, so the same pace takes a bigger share of what your body can handle. In one study, the same workload went from 63% to 78% of aerobic capacity between minutes 15 and 45 in the heat. Slowing down keeps the run at the effort you planned. For marathon pace workouts, set a slower pace goal before a hot session.

Does cardiac drift happen during zone 2 running?

Cardiac drift happens during any steady run longer than about 10 minutes, including easy zone 2 runs. In cool weather it’s small, so a zone 2 run might only rise a few beats. In hot or humid weather, or if you aren’t drinking on a long run, your heart rate can drift out of zone 2 without any change in pace. Keep zone 2 runs in zone 2 by letting your pace slow on hot days rather than holding your usual easy pace.

Is cardiac drift a sign of poor fitness?

Some cardiac drift happens to every runner on a long, steady run, and it’s driven mostly by heat and sweat loss. Fitness does change how long you can run before the drift starts. In a 2023 study, 10 weeks of interval training in previously untrained adults pushed back the point where heart rate and other measures began drifting during a 3-hour ride, from about 108 to 137 minutes. A bigger drift than usual in mild weather is a reason to check your hydration and fatigue.

How can you test your heart rate drift?

Run at a steady, easy pace on a flat route for at least an hour in cool weather, then compare your average heart rate in the first half with the second half. Some training apps call this aerobic decoupling. Compare it only against your own earlier runs in similar weather, because temperature changes the drift more than anything else. A smaller rise at the same pace over several weeks suggests your endurance is improving, and a larger rise can point to heat, dehydration or fatigue.

Does cardiac drift affect the calories my watch shows?

It can make your watch overestimate them. Many watches use heart rate to estimate calories burned, but your oxygen use, which sets the calories you burn, rises much less than your heart rate during drift. In a 1991 study, heart rate climbed 10% from minute 20 to 120 of steady exercise without fluid while oxygen use climbed only 6%. If you track calories to manage your weight, treat the number on a long run as a high estimate.

Jeff Gaudette, M.S. Johns Hopkins University

Jeff is the co-founder of RunnersConnect and a former Olympic Trials qualifier.

He began coaching in 2005 and has had success at all levels of coaching; high school, college, local elite, and everyday runners.

Under his tutelage, hundreds of runners have finished their first marathon and he’s helped countless runners qualify for Boston.

He's spent the last 15 years breaking down complicated training concepts into actionable advice for everyday runners. His writings and research can be found in journals, magazines and across the web.

  • References

Coyle, Edward F., and José González-Alonso. “Cardiovascular Drift During Prolonged Exercise: New Perspectives.” Exercise and Sport Sciences Reviews, vol. 29, no. 2, 2001, pp. 88-92.

Hamilton, Marc T., et al. “Fluid Replacement and Glucose Infusion During Exercise Prevent Cardiovascular Drift.” Journal of Applied Physiology, vol. 71, no. 3, 1991, pp. 871-877.

Kjøsen Talsnes, Rune, et al. “Comparison of Acute Physiological Responses Between One Long and Two Short Sessions of Moderate-Intensity Training in Endurance Athletes.” Frontiers in Physiology, vol. 15, 2024, p. 1428536.

Lafrenz, A. J., et al. “Effect of Ambient Temperature on Cardiovascular Drift and Maximal Oxygen Uptake.” Medicine and Science in Sports and Exercise, vol. 40, no. 6, 2008, pp. 1065-1071.

Matomäki, Pekka, et al. “Durability Is Improved by Both Low and High Intensity Endurance Training.” Frontiers in Physiology, vol. 14, 2023, p. 1128111.

Montain, Scott J., and Edward F. Coyle. “Influence of Graded Dehydration on Hyperthermia and Cardiovascular Drift During Exercise.” Journal of Applied Physiology, vol. 73, no. 4, 1992, pp. 1340-1350.

Wingo, Jonathan E., et al. “Cardiovascular Drift Is Related to Reduced Maximal Oxygen Uptake During Heat Stress.” Medicine and Science in Sports and Exercise, vol. 37, no. 2, 2005, pp. 248-255.

Wingo, Jonathan E., et al. “Cardiovascular Drift During Heat Stress: Implications for Exercise Prescription.” Exercise and Sport Sciences Reviews, vol. 40, no. 2, 2012, pp. 88-94.

Wingo, Jonathan E., et al. “Fan Cooling After Cardiovascular Drift Does Not Reverse Decrements in Maximal Oxygen Uptake During Heat Stress.” Temperature, vol. 6, no. 3, 2019, pp. 260-270.

Wingo, Jonathan E., et al. “Cardiovascular Drift and Maximal Oxygen Uptake During Running and Cycling in the Heat.” Medicine and Science in Sports and Exercise, vol. 52, no. 9, 2020, pp. 1924-1932.

Yoder, H. A., et al. “Work Rate Adjustments Needed to Maintain Heart Rate and RPE During High-Intensity Interval Training in the Heat.” Frontiers in Physiology, vol. 16, 2025, p. 1506325.

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13 Responses

  1. Whilst this observation is often true, the underlying causes may have more to do with our ability to use fat sources over sugar. If one is sugar dependant (as many, if not most of us are) then we will observe this phenomena. However, if we train ourselves to utilize more fat (truly aerobic running) then the drift is much less or even almost non existant. I have 6 hour bike rides which show this and possibly some 15-20 mile runs with similar data. I think Maffetone et al would question your assumptions

    Reply

    1. Cardiac drift is caused by an increase in body temperature as you exercise – not what type of fuel you’re burning. I am very curious to hear how burning fat impacts core body temperature? While I appreciate you sharing your thoughts, this is complete nonsense.

      Reply

      1. It is true that the more developed your aerobic system becomes, the less the drift will happen. Many people have observed this, myself included. Sticking to the proper heart rate zone can accomplish this. It takes time and patience, though, to reach this level of efficiency. Many do not want to take this time, so they continue to run outside the aerobic zone. The more outside the aerobic zone you train, the more stress you put on your body. Running by pace is good for infrequent speed sessions, but long runs are meant for purely aerobic development. This will make for a healthier and more efficient and faster runner.

        Reply

    2. Maffetone actually assumes that the pace will slow down during exercise while the heart-rate stays in the MAF zone and that’s part of the MAF test so I don’t think it can be right that Maffetone et al would question these assumptions.

      Reply

  2. Interesting article, I have noticed this drift effect while running and wondered why it was happening and if it was something to worry about. I also noticed on your HR data that there were large drops in the HR. I have also observed this during my runs and was concerned since I don’t know the cause is – would you know?

    Reply

    1. Hi Gordon, thanks for reaching out. That is just where the transmitter loses signal. Make sure your strap is adjusted correctly and that the sensors are working correctly. Hope this helps!

      Reply

  3. Hello, I also noticed this drift on long runs and had looked into it before reading this article.
    Most common reason I picked up was that it was due to a decrease in body fluid volume/ blood volume. As you continue runnig some mild progressive body fluid/blood volume decrease would occur (from sweating etc) requiring your hard to beat more frequently in order to move the same volume of blood.
    I recon this explanation is compatible with (or at least coralates with) the core temperature increase. On long runs I notice that sweating (I’m a heavy sweater!) decreases over time and that the ‘pulse temperature’ of my polar shows an increasing temperature from about 30 minutes into my rund (first 30 min it decreases).

    Reply

  4. Any reason Heart Rate would go down in the last third of a long run? I was pretty “pooped” but still going at least close to the pace I did the first 2/3’s. I could show you my data but won’t leave it here…
    Thanks,

    Jacob V.

    Reply

    1. Hi Jacob, thanks for reaching out, it may have been because your legs were fatigued enough to slow your body down, and with that slowing down, it allowed your heart rate to calm down a little. We actually do not usually recommend training by heart rate, and instead recommend runners use alternative sources, which we wrote about in this article https://runnersconnect.net/coach-corner/why-i-dont-recommend-training-by-heart-rate/ Your findings are further proof of this, that listening to how you are feeling is often more important. Thank you for sharing!

      Reply

  5. Having done 50 Marathons (including 18 ironmans) with HR monitor, I reckon the cardiac drift is minimised by a decent pre-race warm-up and careful attention to fueling, hydration, salt and cooling (from ice/water next to skin etc).
    Fundamental is also to have sufficient aerobic base training on a low carb high fat diet.
    Also, need to avoid spikes in effort during the first 3/4 of race, in order to avoid deploying the anaerobic system too early. It’s ok to go to maximum effort towards the end of the race, but in fact I rarely find I can get the HR near to maximum in the last couple of miles despite feeling like I am giving it 100% effort.
    Marathon PB 2:52 age 50, Ironman PB 9:45 age 52.
    I reckon there are a number of factors influencing cardiac drift and a race with less drift probably means we got more things right on that occasion.

    Reply

  6. Cardiovascular drift is believed to be caused by multiple effects from a rise in core temperature. The first is simply triggered by increased work for thermoregulation. The second is caused by a reduction in plasma volume from perspiration, making it harder for your heart to maintain arterial pressure. Both effects are countered by hydration, as well as any available means for cooling.

    Reply

  7. Hi,

    I am Harish from India. Following are my details:

    Age: 38 yrs
    Weight: 86 kg.
    Height: 165cm
    Type-II Diabetic and Hypertension

    On 8th Oct’17, I had participated in my first ever marathon run in 10KM power run category. It took me 1hr 21m (not a good time 🙂 ) to complete the 10km run and following are the more details of that run:

    Distance: 10.08km
    Time taken: 1:21:34 min
    Pace: 8’05”
    Avg. Heart Rate: 185
    1hr 21min peak and heart rate was in between 110-201

    I am worried about the heart rate as it was at 201 during the run at one point and I want to understand/learn how I can get it down at acceptable level during my run. Also what has exactly caused 201 heart rate? . I was hydrated enough during the run. I was unable to increase my speed.

    What do you suggest me ? should I take risk as far as the heart rate is concern ?

    Regards,
    Harish

    Reply

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