Heart-rate drift: what the second half of a steady run is telling you

Run an honest easy pace for an hour on a flat road, and your heart rate should barely change from the start of the run to the end. The fact that it usually does is information.

The phenomenon has a name: aerobic decoupling, or heart rate drift. Same pace, but the heart works harder and harder to keep producing it. The causes stack — core temperature rises, sweat shifts your blood volume, glycogen stores deplete, postural muscles fatigue — and each one taxes the cardiovascular system a little more to deliver the same speed.

The useful part is that drift is a measurable window into how aerobically fit you actually are. The standard test is to compare the average heart rate of the first half of a steady-state effort against the average of the second half, run at the same pace, in moderate conditions. Roughly:

• Under 5% drift — strong aerobic base at that intensity. The pace is honest.
• 5-10% — moderate decoupling. Either the pace was a notch too hot, conditions were rough, or the aerobic engine needs more work.
• Over 10% — the effort exceeded what your aerobic system can sustain. That wasn't an easy run, whatever it felt like.

This is why "just run easy" is bad advice without a check. Plenty of runners think they're running easy because their effort feels comfortable for the first 30 minutes, but their drift quietly says they're closer to a tempo than a true aerobic run. Compound that across a training block and you end up with junk-mileage syndrome — too tired to hit workouts hard, not aerobic enough to absorb volume.

Over a training block, falling drift at the same pace is one of the clearest signs your aerobic fitness is moving in the right direction.

Source: TrainingPeaks — Aerobic Decoupling and Heart Rate Drift Explained