What is cardiac drift?

The gradual rise in heart rate at a constant pace during prolonged running, driven by dehydration, rising core temperature and falling stroke volume.

Run at a steady pace for long enough and your heart rate climbs even though your speed has not changed. The mechanism is mostly cardiovascular: as you sweat, plasma volume falls, which reduces the volume of blood the heart ejects per beat. To maintain the same cardiac output — and therefore the same oxygen delivery at the same pace — heart rate has to rise to compensate.

Rising core temperature adds a second driver. Blood is diverted toward the skin for cooling, which further reduces central blood volume, and the thermal load itself raises heart rate independently. This is why drift is much larger on hot days than in cool conditions at identical pace.

The rate depends on intensity, not just time. At an easy effort in thermoneutral conditions drift can be under 1 beat per minute per hour. At an effort near your second lactate threshold it is several times that, which is one reason racing a marathon at threshold heart rate does not work — the drift alone pins you against your maximum before the finish.

For a marathoner the useful reading is what drift says about preparation. Small drift over a long steady run means your aerobic system and your hydration strategy are adequate for the pace. Large drift means one of them is not, and it will show up in the closing 10 km of a race.

Glossary: Cardiac Drift, Aerobic Decoupling