Heart-rate drift and heat: why the same work costs more

Drift is what happens to your heart rate when the work stays the same. In long, steady, moderate exercise, heart rate rises and stroke volume, the blood pumped with each beat, falls. Researchers put the start at about 10 minutes in [1].

It is normal. Trained athletes show it too. It means that late in a long piece your heart rate reads higher for the same work than it did early on, so a rising heart rate on its own does not tell you the work got harder.

Heat, fluid loss and harder efforts all change how big the drift gets [4]. The clearest picture comes from one lab study of 10 endurance-trained men cycling at about 59% of their VO2max with their fluids replaced. From minute 15 to minute 45, heart rate rose 11% at 35 °C and 2% at 22 °C [2].

Same kind of work, fluids replaced, a different room. A review of drift in the heat describes the higher heart rate as reflecting a higher relative metabolic intensity during prolonged exercise in the heat [1]. The work on the dial is the same; the share of your capacity it takes is not.

This was a cycling study in a laboratory. It is not a 30-minute row in your garage, and it gives no numbers for the rower, so read it as the direction of the effect, not as a forecast.

Drinking changes the picture as well. In a second lab study, which kept the temperature at 22 °C, 10 endurance-trained subjects cycled for 2 hours at 22 °C, starting at 70% of VO2max. With no fluid, heart rate rose 10% between minute 20 and minute 120. With drinks that replaced what they sweated, it rose 5% [3]. Matching sweat roughly halved the rise.

Again, that was a 2-hour lab ride, not a 30-minute row. The lesson is the same: how big the drift is depends on more than the work.

On the erg, the split tells you the work: a 2:20.0 split is 127.6 watts on a cold morning and on a hot afternoon [8]. What it cost you today is a different question, and effort out of 10 answers it.

Ratings of perceived exertion track heart rate, blood lactate and oxygen uptake. A 2002 meta-analysis found correlations of 0.62 with heart rate, 0.57 with lactate and 0.64 with the share of maximal oxygen uptake, and called the Borg scale a valid measure of intensity, though with a validity that may not be as high as people thought [5]. The scale itself comes from Borg's 1982 paper [6]. It is a useful guide, not a perfect one.

That is why heat is one reason the cost moves. The watts hold still while the effort rating goes up.

Heart rate alone never changes the trend. A big drift is a prompt to ask how hard the session felt, out of 10, and to read the effort rating next to the split and the day.

Rating the whole session out of 10 is a tested way to track training: the session RPE method has been found valid for quantifying exercise training across a wide variety of exercise types [7]. Write the rating down with the split, and the day you drifted becomes a data point rather than a verdict.