Heart Rate Drift: A Functional Measure of Aerobic Fitness

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Back in the day, coaches and endurance athletes noticed that during prolonged training, an athlete’s heart rate would slowly climb while the athlete maintained a steady pace. Bear in mind that this was long before modern GPS watches, so much of this “noticing” was very subjective. Nonetheless, to these early observers the anecdotal evidence suggested that the faster the pace, the more the athlete’s heart rate would drift upward during the second half of the workout—a phenomenon that became known as heart rate drift.

TrainingPeaks refers to this as decoupling.

From Linear to Nonlinear

What these athletes and coaches were observing was the linear relationship between heart rate and pace in the aerobic metabolic domain. In other words, when you are operating in Zones 1 and 2 (intensities at which your aerobic metabolism is providing the bulk of the energy needed), your heart rate increases in lockstep with your pace if your movement economy is uniform throughout these lower speeds. For instance, an increase of 0.5 kilometer/hour in speed will result in the same jump in heart rate whether the speed increase is from 6 to 6.5 kilometers/hour or from 9.5 to 10 kilometers/hour, as long as these speeds are within your aerobic domain.

Once your speed or intensity exceeds your aerobic threshold (AeT), this linear relationship breaks down—decouples—and becomes nonlinear. Each small increase in speed will cause a smaller and smaller increase in heart rate. Above your anaerobic threshold (AnT) it becomes highly nonlinear, eventually plateauing at your maximum heart rate.

With modern GPS watches and heart rate monitors and the precision of data collection they allow, it is easy to see this decoupling between heart rate and pace. We at Uphill Athlete have extolled the virtues of the data crunching that TrainingPeaks offers, and decoupling (heart rate drift) is another helpful, functional metric that you can use to monitor your fitness progress as you accumulate training hours.

How to Conduct the Heart Rate Drift Test

The heart rate drift test is used to determine your aerobic threshold (AeT). It is more accurate than the nose-breathing test suggested elsewhere on the Uphill Athlete website, especially if you are aerobically deficient. The goal is to do the workout at what you think is aerobic intensity (below your AeT). You must exercise for an extended period—about one hour—and at a steady effort while recording heart rate and GPS data.

Where to do the test
  • Outside: On a flat to very gently rolling course (running)
  • Inside: On a treadmill or stair machine
  • Do not do this test on an uphill/downhill out-and-back course. During the first half you will be going uphill and therefore doing more work, meaning the pace-to-heart-rate ratio of the two halves will be very different.

Watch this video tutorial on how to properly conduct a Heart Rate Drift Test.

What you will need
  • A GPS-enabled watch or phone
  • A chest strap heart rate monitor that pairs with your watch or phone (we recommend against using your watch’s built-in wrist heart rate monitor)
  • A TrainingPeaks Premium account 

The various metrics TrainingPeaks provides are invaluable to athletes and coaches, and we gladly pay our fee to them (we receive no price break or kickbacks). However, you do need to have a TrainingPeaks Premium account to access the decoupling feature outlined here.

FAQs

Should I run or should I hike? If you are going to be doing a lot of running in your training, do this test as a run. If you are primarily going to be hiking for your training, do this test as a hike on a treadmill or stair machine.

What should the effort feel like? This effort should feel easy and relaxed—a conversational pace. In other words, you should be able to carry on a conversation in full sentences.

Any tips on doing this test on a running track? Yes! Read this short post for tips on getting the best and most accurate data.

Testing Outdoors

  1. Run, preferably on a flat (or very gently rolling) course at what feels like an easy aerobic pace. (See above for more detail on the desired effort.) If you have a good idea of what your AeT is, then target that heart rate for the beginning of the test.
  2. Once your heart rate stabilizes for 2–3 minutes after at least a 15-minute warm-up, start the recording feature on your GPS watch.
  3. Record for 60 minutes while doing your best to keep your heart rate close to that initial heart rate number.
  4. Upload the data to TrainingPeaks.
  5. Open the workout in TrainingPeaks and click the “Analyze” button. In the window to the right of your workout graph you will see Pa:Hr X.XX%. This decoupling metric compares the pace-to-heart-rate ratio of the first half of the workout to that of the second half. Note the number and skip ahead to “Reading the Results.”

Testing Indoors

  1. Set the treadmill to 10 percent and begin hiking slowly. If you are training for flatter runs, set the treadmill to 3 percent and run. (NOTE: If hiking, you may need to use a steeper grade—10+ percent—in order to get your heart rate up sufficiently.)
  2. Gradually build speed over the first 15 minutes until your heart rate stabilizes at what you feel is an easy aerobic effort for 2–3 minutes. (See above for more detail on the desired effort.) If you have a good idea of what your AeT is, then target that heart rate for the beginning of the test. Once you’ve dialed in the speed and grade, do not adjust them again during the test.
  3. Now you are ready to begin the test: Run or hike continuously for 60 minutes at that speed/grade while recording your heart rate.
  4. Upload the data to TrainingPeaks. Since GPS does not work indoors, the pace part of Pa:Hr will not be accurate. That is why it is so important that you hold the pace and grade constant once you start this test on a treadmill.
  5. Open the workout in TrainingPeaks and click the “Analyze” button. You will see a graph of your heart rate, pace, and elevation. To calculate heart rate drift, select the first half of the test in the graph and note your average heart rate in the window to the right of the graph. Then do the same for the second half. Compare the two numbers to determine the percentage rise of your average heart rate.

Reading and Implementing Your Results

  • 3.5–5 percent: You have determined your AeT heart rate, which was your starting heart rate for the test. Set that as the top of Zone 2 in your TrainingPeaks zones. Subtract 10 percent from this and set that as the top of your Zone 1.
  • 0–3.5 percent: The workout was within your aerobic intensity zones, but you should do the test again at a starting heart rate that is 5 beats per minute (bpm) higher.
  • >5 percent: Your initial heart rate/pace was above AeT. Redo the test using a lower starting heart rate. It may take several attempts to nail a decoupling that is slightly less than or equal to 5 percent.

Going Forward

Keep in mind that your AeT is not fixed. It changes day to day based on your recovery state and overall fitness. The decoupling (heart rate drift) metric is a convenient means of ensuring your workouts are within your aerobic capacity, and it can be used as an occasional spot check on your AeT.

-by Uphill Athlete co-founder Scott Johnston

Photo: Acclimatizing near Makalu. By Steve House.


 

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