Why your heart rate creeps up: the real physiology of cardiovascular drift
Run at a perfectly steady pace for an hour and your heart rate won't hold steady — it climbs. This is cardiovascular drift, and for decades the textbook explanation was subtly wrong. The corrected version, laid out in Coyle and González-Alonso's landmark review, changes how you should read your own heart rate.
Start with what's actually happening to the pump. The defining event of cardiovascular drift is a progressive decline in stroke volume — the amount of blood your heart ejects with each beat — that begins after roughly 10–20 minutes of prolonged steady exercise. To keep total cardiac output constant despite each beat delivering less, the body does the obvious thing: it raises heart rate. So the heart-rate creep you see on your watch is a compensation. Stroke volume is quietly falling; heart rate rises to cover for it.
Now the reframe. The old explanation said the falling stroke volume was caused mainly by blood being diverted to the skin for cooling as you heat up — the skin "stealing" venous return so less blood comes back to fill the heart. Coyle and González-Alonso made the case that the dominant driver is actually the rising heart rate itself. A faster heart spends less time in diastole, the filling phase, so each beat has less time to load with blood — which lowers stroke volume. Cutaneous blood flow plays a smaller role than the classic story claimed. It's closer to a feedback loop than a one-way drain.
Underneath it all sit two upstream culprits: hyperthermia and dehydration. As you sweat, plasma volume drops, so there's simply less blood returning to fill the ventricles, and rising core temperature compounds the strain. That's the practical hook. Because drift is tethered to heat and fluid status, a heart rate that climbs on a long effort can reflect thermoregulatory load rather than a decline in aerobic fitness. Read it in context — duration, conditions, hydration — and the creeping number stops looking like a verdict on your engine and starts looking like a readout of how hot and how dry you've gotten.
Understanding the mechanism changes how RunNerd reads your heart rate. Because drift is driven by heat and fluid loss — not by the run suddenly getting harder — the coach won't treat a creeping HR on a long steady effort as a fitness red flag on its own. It weighs duration, conditions, and your hydration before judging, and it expects the rise to be larger on hot days and on under-fueled long runs. When you replace fluid and run in cool conditions and the climb shrinks, that's the coach's confirmation the drift was thermoregulatory, not a fading engine.