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, the anecdotal evidence told these early observers that the faster the pace, the more the athlete’s heart rate would drift upward during the second half of the workout. This phenomenon became known as heart rate drift.
TrainingPeaks refers to it as decoupling.
From Linear to Nonlinear
It has long been known that there is a 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, and eventually plateaus at your maximum heart rate.
With modern combination heart rate monitor–GPS watches and the precision of data collection they allow, it is easy to see this decoupling. We at Uphill Athlete have extolled the virtues of the data crunching that TrainingPeaks offers, and here is another helpful, functional metric that you can use to monitor your fitness progress as you accumulate training hours.
How to Use the Aerobic Decoupling Feature in TrainingPeaks
To use the decoupling feature in TrainingPeaks, do a workout at what you think is aerobic intensity (below the upper limit of AeT). You must exercise for an extended period—we usually recommend a minimum duration of 1 hour—and at a steady effort. This means that you need to run on a fairly flat or gently rolling loop course or hike uphill on a consistent grade while recording heart rate and GPS data.
Upload the data to TrainingPeaks, then open the workout and click the “Analyze” button. In the upper right corner you will see Pa:Hr= X.XX%. This metric compares the pace (Pa)–to–heart rate (Hr) ratio of the first half of the workout to that of the second half. When the ratio—heart rate drift—is under 5 percent, the workout was within your aerobic intensity zones.
An uphill/downhill hike, run, or ski will not work because the Pa:Hr ratio of the two halves will be very different: during the first half you will be going up and therefore doing more work.
Another way to do this test is on a treadmill or stair machine. First, find a pace that brings your heart rate up into what you feel is your aerobic zone. Hold that pace for an hour or so and notice if your heart rate begins to climb. If your heart rate climbs less than 5 percent over the course of the workout, that heart rate was within your aerobic intensity zones.
Keep in mind that your AeT is not fixed. It changes day to day based on your recovery state and overall fitness. The decoupling 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.
Conducting the Heart Rate Drift Test
This test can be done on on a treadmill, a stair machine, or a flat to very gentle loop course outdoors. It cannot be done on an uphill/downhill out-and-back course.
NOTE: This test is used to determine your Aerobic Threshold. It is more accurate than the nose-breathing test suggested elsewhere on the site, especially if you are aerobically deficient.
Testing on a Treadmill
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. Gradually build speed over the first 10–15 minutes until your heart rate stabilizes at what you feel is an easy aerobic effort. If you have a good idea of what your AeT is, then target that heart rate for the beginning of the test. Now you are ready to begin the test.
NOTE: If hiking, you may need to use a steeper grade (10+ percent) in order to get your heart rate up sufficiently.
Once you’ve dialed in the speed and grade, do not adjust them again during the test. Run or hike continuously for 60 minutes at that speed/grade while recording your heart rate. Then upload the data to TrainingPeaks.
Since GPS does not work indoors, the pace part of the Pa:Hr will not be accurate. You cannot use the TrainingPeaks Pa:Hr metric on a treadmill. That’s why it is so important that you hold the pace and grade constant once you start this test on a treadmill. It is very likely that you will see an upward trend in your heart rate over the course of the hour. To calculate heart rate drift, you need to select the first half of the test in the graph of HR/Pace?elevation. Note the average heart rate for each half. Compare them to see if your average heart rate rose more than 5 percent.
Run, preferably on a flat (or very gently rolling) course, at what feels like an easy aerobic pace. Once your heart rate stabilizes, start the recording feature on your GPS-enabled heart rate monitor watch. Record for 1 hour while doing your best to keep your heart rate close to that initial heart rate number. Upload the data to TrainingPeaks.
If the Pa:Hr is greater than 5 percent, your initial heart rate/pace was above AeT. You should do the test again at a lower heart rate. It may take several attempts to find a Pa:Hr decoupling of less than 5 percent.
Once you determine your AeT heart rate, set that as the top of your Zone 2 in your TrainingPeaks zones. Subtract 10 percent from this and set that as the top of your Zone 1.
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 utilize the feature outlined here.
-by Uphill Athlete co-founder Scott Johnston
Photo: Acclimatizing near Makalu. By Steve House.