Cardiac drift

What it is

During prolonged steady exercise, heart rate tends to creep upwards while the amount of blood pumped per beat (stroke volume) falls, even if your pace or power stays the same. This is called cardiovascular or cardiac drift. Coyle and González-Alonso’s review describes it starting after roughly 10 to 20 minutes, and links the drop in stroke volume mainly to the rising heart rate itself, as body temperature climbs.

Why it matters for training decisions

Drift explains why a long run that starts in Zone 2 can finish in Zone 3 at the same pace. If you hold heart rate strictly in zone, you’ll slow down over the session, which is often fine for easy days. If you hold pace, accept that heart rate will rise.

It also matters when you compare sessions. Heat and dehydration make drift bigger, so a hot-day run can look like lost fitness when it isn’t. Only compare drift between sessions done in similar conditions.

How wearables measure it, and the limits

Most watches don’t report drift directly. You can see it by comparing heart rate in the first and second halves of a steady session. Optical sensors can add their own errors, and hills, wind or stops make the comparison meaningless.

How P247 uses it

P247 doesn’t calculate a separate drift score.

This glossary is general information to help you understand your training data. It isn't medical advice, diagnosis or treatment. For injury, illness, medication, blood test results or any health concern, speak to a qualified health professional.

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