Heart Rate Drift: A functional measure of fitness


By Uphill Athlete co-founder Scott Johnston

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.

Rather than “heart rate drift,” TrainingPeaks calls this phenomenon “decoupling.”

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 0.5km/hr in speed will result in the same jump in heart rate whether the speed increase is from 6 to 6.5 km/hr or from 9.5 to 10 km/hr, as long as these speeds are within your aerobic domain. Once your speed or intensity exceeds your aerobic threshold, this linear relationship breaks down—decouples—and becomes nonlinear.  Each small increase in speed will cause a smaller and smaller heart rate increase. Above your anaerobic threshold 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 collecting they allow, this decoupling is easily seen. We at Uphill Athlete have extolled the virtues of the data crunching that TrainingPeaks offers, and here is another, useful, functional metric that you can use to monitor your fitness progress as you accumulate training hours.

How to use the Aerobic Decoupling feature

Do a workout at what you think is at aerobic intensity (below the upper limit of Aerobic Threshold, or 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 is under 5%, 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% 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.


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.

Photo: Acclimatizing near Makalu. By Steve House.