“Long before the days of GPS watches and heart rate monitors, coaches and endurance athletes observed that during prolonged training, an athlete’s heart rate would gradually climb while maintaining a steady pace.”

“What those coaches and athletes were observing back in the day was the linear relationship between heart rate and pace during aerobic metabolism.”

“The concept is simple – when you maintain an aerobic pace (<AeT), your heart rate will remain nearly constant for as long as an hour.”

“If your Pa:Hr is between 3.5% and 5%, you have successfully determined your AeT heart rate, which is your starting heart rate for the test (not your average heart rate, but your starting heart rate).”

“Keep in mind that, just like heart rate and pace, your AeT is not fixed. It will change day to day based on your recovery status and overall fitness.”

Long before the days of GPS watches and heart rate monitors, coaches and endurance athletes observed that during prolonged training, an athlete’s heart rate would gradually climb while maintaining a steady pace. Although anecdotal (the lack of tech necessitates that these “observations” were entirely subjective), the evidence suggested that the faster the pace, the more an athlete’s heart rate would “drift” during the second half of the workout.

This phenomenon is now widely acknowledged – coaches call it heart rate drift, scientists call it cardiac drift, and TrainingPeaks calls it decoupling.

“Long before the days of GPS watches and heart rate monitors, coaches and endurance athletes observed that during prolonged training, an athlete’s heart rate would gradually climb while maintaining a steady pace.”

📈 Heart rate drift

There are a few different physiological effects that explain heart rate drift but, primarily, it comes down to your body trying to maintain the same cardiac output as you exercise.

Cardiac output  =  stroke volume  x  heart rate

If your stroke volume is the amount of blood your heart can pump in a single beat, and your heart rate is how many times your heart beats in a minute, then your cardiac output is simply the amount of blood your heart can pump out in one minute.

As you exercise, your stroke volume is likely to decrease due to fluid loss caused by sweating and muscle pump. If your stroke volume decreases, your heart rate must increase in order to maintain the same cardiac output (and, in turn, maintain the same steady pace).

To a lesser degree, heart rate drift is also influenced by:

🔹 Hormones – exercise stimulates the sympathetic nervous system which releases norepinephrine and epinephrine, the build-up of which in your bloodstream can cause your heart rate to increase.
🔹 Thermoregulation – as you exercise your body has to work harder and harder to remove the excess heat generated causing a slight rise in heart rate.

So, during prolonged training my heart rate will most likely increase even though my pace remains the same, and that’s normal. Great, but what use is understanding heart rate drift to me?

Well, if we look a bit deeper into the linear/nonlinear relationship between heart rate drift and metabolic exercise intensity, we can actually measure an athlete’s rate of heart rate drift and use this measurement to very accurately predict (with >95 percent precision) an athlete’s aerobic threshold (AeT).

📉 Linear and non-linear heart rate drift

What those coaches and athletes were observing back in the day was the linear relationship between heart rate and pace during aerobic metabolism. When an athlete’s aerobic system is providing the bulk of their energy requirements (RPE 1-5, or Zones 1 and 2), their heart rate will increase in tandem with their pace (provided their movement economy is uniform).

“What those coaches and athletes were observing back in the day was the linear relationship between heart rate and pace during aerobic metabolism.”

For example, an increase of 0:30/km in pace will cause the same increase in heart rate regardless of whether the pace increase is from 05:30/km to 05:00/km or from 06:30/km to 06:00/km, as long as these paces are within the athlete’s aerobic domain.

Once an athlete’s pace exceeds their aerobic threshold (AeT), this linear relationship breaks down (decouples) and become non-linear. From this point onwards, each small increase in pace will cause a smaller and smaller increase in heart rate, eventually plateauing at the athlete’s maximum heart rate.

Herein lies the practical value of understanding heart rate drift. If we measure the rate of this linear/non-linear decoupling, we get a functional metric that we can use to: 1) Ensure we are training the aerobic system most effectively and; 2) Monitor the development of aerobic capacity.

⏱️ Heart rate drift test

The concept is simple – when you maintain an aerobic pace (<AeT), your heart rate will remain nearly constant for as long as an hour. If your heart rate rises more than 5% at that steady pace, your starting heart rate was higher than AeT. If your heart rate rises less than 3%, your starting heart rate was below AeT. And if your heart rate drift is between 3.5-5%, we can take your starting heart rate as an accurate predictor of your AeT heart rate.

The data does become a little more complex to analyse when conducting the test outside, as we need to take into account the slight changes in pace. Fortunately, there are programs (and coaches!) out there to help us interpret the data.

You will need:

☑️ A GPS-enabled watch or phone
☑️ A chest strap or armband heart rate monitor that pairs with your watch or phone (wrist-based heart rate readings are NOT accurate enough)
☑️ A way to analyse the data (such as Runalyze, TrainingPeaks Premium, or a good mathematician!)

The test should be conducted on flat (or very gently rolling) terrain. A running track works well if you don’t have a suitable flat route near you. Do NOT do this test on an uphill/downhill out-and-back course, even if the gradient is slight (the pace-to-heart-rate ratio of each half will be too different to meaningfully interpret).

“The concept is simple – when you maintain an aerobic pace (<AeT), your heart rate will remain nearly constant for as long as an hour.”

👨‍💻 Analysing the data

Once you’ve completed the test, you need to establish your pace-to-heart-rate ratio (Pa:Hr). Pa:Hr is calculated as the difference between the aerobic efficiency of the first half and the second half of the workout.

If you use TrainingPeaks Premium, once the activity has uploaded you can click “Analyze” and TrainingPeaks will calculate Pa:Hr for you. You can also use Runalyze to read your workout and automatically generate the Pa:Hr of your test.

Alternatively, you can calculate your Pa:Hr manually using the below formula (OR, you can hire a coach who will do it all for you!!)

Pa:Hr [%] = (Pa:Hr2 – Pa:Hr1) / Pa:Hr1

First Half:
Avg. Pace = 6:00/km (360 seconds/km)
Avg. HR = 130
Pa:Hr1 = 360/130 = 2.77

Second Half:
Avg. Pace = 6:20/km (380 seconds/km)
Avg. HR = 132
Pa:Hr2 = 380/132 = 2.87

Pa:Hr = % change in Pa:HR ratio from first half to second half = (2.87-2.77)/2.77 = 3.6%

“If your Pa:Hr is between 3.5% and 5%, you have successfully determined your AeT heart rate, which is your starting heart rate for the test (not your average heart rate, but your starting heart rate).”

💯 Results

3.5–5 percent: If your Pa:Hr is between 3.5% and 5%, you have successfully determined your AeT heart rate, which is your starting heart rate for the test (not your average heart rate, but your starting heart rate).

0–3.5 percent: If your heart rate drift is below 3.5%, the workout was within your aerobic capacity, but you should repeat the test with a starting heart rate that is 5 bpm higher.

>5 percent: If your heart rate drift is above 5%, then your initial heart rate/pace was above AeT and you should repeat the test using a lower starting heart rate.

Note: It may take several attempts to nail a decoupling rate that is slightly less than or equal to 5 percent.

⏭️ What next

Once you’ve established your aerobic threshold (AeT), you can utilise the corresponding AeT heart rate and AeT pace in conjunction with feel/effort (RPE) to ensure you’re conducting your aerobic capacity training at the correct intensity.

Keep in mind that, just like heart rate and pace, your AeT is not fixed. It will change day to day based on your recovery status and overall fitness. Heart rate drift is a convenient means of double checking whether you are training within your aerobic capacity (and can be used as an occasional progress check on your aerobic fitness development) but it should not be taken as gospel or relied upon in isolation.

Remember that your aerobic threshold (AeT) is the uppermost intensity of exercise where ATP production is still taking place predominantly through the oxidation of fats. Above AeT, the metabolism of carbohydrates (glycolysis) becomes the primary means of ATP production. AeT is important because it marks the upper level of the most important training zone for developing aerobic capacity. With consistent and disciplined aerobic training, both your pace and heart rate at AeT will increase.