Same pace. Rising heart rate. What gives?
Forty minutes into a steady run, your pace hasn’t changed but your heart rate has climbed ten beats. That’s cardiovascular drift — ordinary physiology that quietly carries you out of your zone, and doubles as a free fitness test.
What’s actually happening.
After roughly 10–20 minutes of steady exercise, your heart rate climbs while the amount of blood your heart moves per beat falls — and the classic reference, Coyle & González-Alonso’s review of cardiovascular drift, argues the rising heart rate itself drives much of that fall, with dehydration making it worse.
Heat and fluid loss make it much worse. In a study of endurance athletes exercising dehydrated in the heat, hyperthermia or 4% dehydration alone each trimmed stroke volume 7–8% — together they cut it by 26 mL per beat and dropped cardiac output 13%. Your “mysteriously hard” summer runs are neither mysterious nor in your head.
Coaches read drift like a lab report.
Compare the two halves of a steady workout — heart rate against pace or power. Joe Friel calls it aerobic decoupling: under 5% drift signals strong aerobic endurance at that intensity, 5–10% suggests a moderate limitation or fatigue, and more than 10% means the effort sat above your aerobic threshold. As your base builds, the same workout drifts less — fitness you can see without a lab.
Practical countermeasures follow directly from the causes: start hydrated, drink on long sessions, and give summer efforts an honest pace cut.
Drift is why pace-based training slips.
Hold a fixed pace for an hour and drift guarantees the second half is harder than the first — often hard enough to leave Zone 2 entirely. Training by heart rate flips that: Zone2 keeps your live zone on your wrist, taps you as you drift up, and its Coach can tell you what the drift did to your week, in your own numbers.
Advice with your numbers in it.
Coach knows your zones, your goal, and what you’ve already done this week, so its answer is specific: which session, how long, what heart-rate range. You can check every number it cites against your own history — and when there’s nothing useful to say, it says nothing.
Catch the drift while it’s happening.
Zone2 is free. Your watch shows your zone in real time, so a creeping heart rate never quietly rewrites your workout.