Coach Jeff

Written by Coach Jeff


Why cardiac drift is important for runners who train by heart rate

In two previous articles, I’ve discussed the importance of breaking free of your Garmin dependency and three reasons not to train with a heart rate monitor. Reading both articles may make it seem like I am harsh critic of all running technology, but in reality I am a proponent of any device that helps runners train smarter.

However, these two articles do demonstrate the need for runners, beginner and experienced alike, to take the time to understand training concepts and not blindly follow the numbers and data derived from their technological gadgets.

With a better understanding of training theory you can take full advantage of the technology and make it work for you rather than running to hit subjective numbers.

That brings me to this latest article on heart rate training – the influence of cardiac drift on heart rate during long runs.

By understanding the concept of cardiac drift as it relates to effort and heart rate, you can train more effectively and maximize your potential.

What is cardiovascular drift

Cardiovascular drift refers to the natural increase in heart rate that occurs when running with little or no change in pace. Many runners mistakenly assume that if they keep their runs at a consistent pace, their heart rate will remain relatively constant as well.

However, exercise research has shown that it is common to see heart rate “drift” upward during an easy or threshold run, even with no increase in pace of effort – sometimes by as much as 10-20 beats per minute over a 30 minute period.

Here’s an illustration of how cardiac drift looks on a 20 mile long run:

cardiac drift heart rate

This is an actual Garmin report with the athletes training pace overlaid with their heart rate data. As you can see, the pace remains relatively constant (blue) while the heart rate (red) continually increases.

It is important to emphasize that cardiovascular drift results in an increased heart rate without a corresponding rise in effort, breathing rate, or calories burned.  In the long run pictured above, the athlete reported no changes in breathing rate or effort.

What causes cardiac drift

Cardiovascular drift is mostly caused by the natural  increase in core body temperature when running. This increase in core body temperature elevates heart rate the same way running in hot conditions does.

Correspondingly, the stroke volume of the heart decreases so that cardiac output and oxygen uptake remain the same, keeping your breathing and effort similar while heart rate rises.

Why is understanding cardiac drift is important for runners who train by heart rate

If you use a heart rate to measure your effort, especially during easy and long runs, you need to understand what effect cardiac drift has on your heart rate readings or you’ll constantly be under performing in workouts.

Maximizing training potential

For example, let’s assume you’re targeting a marathon paced run within your aerobic threshold training zone (80-85% of your maximum heart rate). For ease of math, let’s assume your maximum heart rate is 200 beats per minute, which puts your aerobic threshold training zone range at 160-170 beats per minute.

For the first 20-30 minutes of your run, a pace of 8 minutes per mile might put you within that goal range, which will probably also be your current marathon fitness pace.

After 30 minutes of running, cardiac drift may cause your heart rate to increase so that you have to slow down to maintain that heart rate window of 160-170 bpm. However, this decrease in pace does not correspond to your effort or fatigue levels.

Now, you’re running at 8:15 per mile pace and spending less time training at your actual aerobic threshold and thereby not getting the most out of your workouts or your training time.

Calculating calories to lose weight

Another potential issue to be aware of in regards to cardiac drift is the calculation of calories burned during a run. Many online calculators and fitness machines use heart rate to measure effort and thereby calculate calorie expenditure.

As we’ve learned by looking at cardiac drift, an increase in heart rate doesn’t necessarily correlate with an increase in effort, oxygen uptake, or calorie expenditure.  So using your heart rate as a way to measure calories burned, which is a built in feature on many heart rate monitors, could lead to false data and eating too much if you’re trying to lose weight.

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What you can do about cardiac drift

Don’t be a slave to your heart rate monitor numbers.  Rather, use your heart rate as a tool to help you learn your efforts and paces and as a guide to help get you started on the right track.

One way to accomplish this is to learn to use breathing rhythm as a way to intuitively assess your intensity. You won’t be perfect with this method at first, so wear the heart rate monitor and use it the first 10-15 minutes of your run to get you on track. Then, cover it up and practice running by feel. Look at the data only after you’re done running and compare your breathing rhythm and the effort you felt to the heart rate numbers.

By understanding the underlying concepts behind heart rate monitors and other training gadgets, you can learn to better listen to your body and apply these tools to help you train smarter in the long-term.

A version of this post originally appeared at

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13 Responses on “Why cardiac drift is important for runners who train by heart rate

  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

    • 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.

      • 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.

    • 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.

  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?

    • 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!

  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).

  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…

    Jacob V.

  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.

  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.

  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 ?


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