ΔέσεCardiac drift: why your heart rate climbs while your pace stays the same
You're fifty minutes into a long, easy run. Your pace is exactly the same as it was in minute ten — 5:45 per kilometre. Your breathing is calm. And yet your watch says 162 beats per minute, when it said 145 at the start.
Your sensor isn't broken and you haven't lost your fitness in half an hour. What you're seeing is called cardiac drift (cardiovascular drift).
What cardiac drift is
It's the gradual rise in heart rate during prolonged exercise at a constant, moderate intensity — that is, while you change nothing about your pace. It typically starts after the first 10 to 15 minutes and continues for as long as you keep running.
The key point is that it isn't just one number moving. Alongside the rise in heart rate, stroke volume — the amount of blood your heart pushes out with each beat — falls. The two go together, like two sides of the same coin.
How big is the rise? On an hour-long easy run you may see your heart rate climb by quite a few beats, without your pace having changed at all. There is no single "normal" number that fits everyone: the size of the drift depends on the heat, the humidity, your hydration, the intensity and your heat acclimation.
Same pace, higher heart rate
Why it happens
As you run, you produce heat. To avoid overheating, your body sends more blood to the skin and you sweat.
But sweating comes at a cost: along with the water, you lose plasma volume — some of the liquid part of your blood.
The result: less blood returns to the heart, so it fills less and pushes out less blood with every beat. To offset part of that drop and keep supplying your muscles, the heart raises the frequency of its beats.
There's also a detail that often gets left out. Coyle and González-Alonso argued that the relationship isn't one-way: the rise in heart rate itself leaves the heart less time to fill between beats, which pushes stroke volume down even further. The two mechanisms feed each other.
The cardiac-drift chain
Your temperature risesRunning produces heat — the body has to get rid of it
You sweat — plasma volume dropsAnd more blood is directed towards the skin
Stroke volume fallsLess blood with every beat of the heart
Heart rate risesMore frequent beats to deliver the same blood to your muscles
Heat makes it much bigger
If you've mostly noticed this in July and not in January, it isn't your imagination. A study by Lafrenz and colleagues (2008) had the same athletes perform exactly the same effort at two different ambient temperatures, and compared what had changed between minute 15 and minute 45.
Same effort, from minute 15 to minute 45
Cool conditions — 22°C
Heart rate +2% · stroke volume −2%
No significant change in VO₂max
Heat — 35°C
Heart rate +11% · stroke volume −11%
VO₂max −15%
The most interesting part isn't the heart rate. It's that last number: when the researchers measured VO₂max immediately afterwards, they found it had dropped in the heat. In other words, after 45 minutes of running in the sun, your ceiling at that moment is lower than it was at the start.
And here is the practical conclusion that matters for you: if the ceiling comes down while your pace stays the same, then the same run has become a larger percentage of your current capacity. Your heart rate isn't lying to you — it's showing you that the same speed now costs you more. A run that started out genuinely easy can gradually become more demanding and pull you out of the relative intensity you originally chose, without you ever pressing the accelerator.
📌 Did you know? Dehydration isn't the only culprit. Montain and Coyle
(1992) showed that the greater the fluid loss, the greater the rise in heart rate — so
drinking does help. But in a study of 45 minutes of exercise at 35°C, fluid intake
did not eliminate the drift: you reduce it, you don't cancel it.
What it means for your training
First, the most important thing: cardiac drift is not a sign of poor fitness. It happens to everyone — beginners and elites alike. It isn't something you "fix", it's something you understand.
It does have one very specific consequence, though: the average heart rate of a long run is not a reliable basis for comparison. If in the second half of an easy run you drift into Zone 3, even though your pace is unchanged and the effort still feels controlled, you're most likely looking at drift — not necessarily a mistake in your training.
The cleanest way to compare yourself with yourself is to look at a consistent segment after the initial warm-up — say from minute 10 to minute 20 — at a similar pace, on a similar route and in similar weather. If after a few weeks your heart rate is lower in that same segment at the same pace, that can be a sign of improvement.
And a dose of realism for the summer months: if in July you're running with 10 beats more at the same pace than you were in March, you haven't lost your fitness. You're simply running in harder conditions.
How to limit it
3 things that really make a difference
FluidsStart hydrated and drink on long runs — fluid loss makes the drift bigger
Time & routeEarly morning or late evening, with shade — temperature matters more than anything
Heat acclimationAfter a few weeks of consistent running in the heat, the body adapts
💡 Practical rule: a gradual rise in heart rate on a long run in the
heat is expected. A sudden spike together with dizziness, chills, nausea or stopping
sweating is not: that's when you stop, cool down and drink.
Frequently asked questions about cardiac drift
Is cardiac drift bad?
No. It's a normal response to prolonged exercise and it happens to every runner, regardless of level. It isn't a sign that something is wrong with your heart or your fitness.
Should I slow down to stay in my zone?
It depends on the purpose of the session. Because the rise in heart rate reflects a genuinely higher relative intensity, slowing down on an easy run isn't wrong. In a workout or race aimed at a specific pace, pace and perceived effort lead — not the number on your watch.
Is dehydration the only cause?
No. Fluid loss clearly makes the drift bigger, but body temperature and blood flow to the skin play their own part. That's why the drift shows up even when you're drinking normally.
Will it stop as I get fitter?
It will get smaller, not disappear. A better aerobic base, heat acclimation and good hydration all reduce its size, but the drift remains part of the physiology of every prolonged run.
How do I see it in my own data?
Compare the first and second half of a long run at the same pace: if heart rate rose noticeably while pace stayed flat, you're looking at drift. Platforms like TrainingPeaks and Intervals.icu can calculate this pace-to-heart-rate "decoupling" for you, which you'll come across as aerobic decoupling or Pa:Hr. It's a useful trend indicator, not a laboratory measurement.
3 things to remember
- Heart rate up, pace flat — that's normal.
- Heat and fluid loss make it bigger.
- Compare the same steady segment, not just the average.
Sources
The main scientific references behind this article:
- Coyle, E. F., & González-Alonso, J. (2001). Cardiovascular drift during prolonged exercise: new perspectives. Exercise and Sport Sciences Reviews, 29(2), 88–92. ESSR →
- Montain, S. J., & Coyle, E. F. (1992). Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise. Journal of Applied Physiology, 73(4), 1340–1350. PubMed →
- Wingo, J. E., Lafrenz, A. J., Ganio, M. S., Edwards, G. L., & Cureton, K. J. (2005). Cardiovascular drift is related to reduced maximal oxygen uptake during heat stress. Medicine & Science in Sports & Exercise, 37(2), 248–255. PubMed →
- Lafrenz, A. J., Wingo, J. E., Ganio, M. S., & Cureton, K. J. (2008). Effect of ambient temperature on cardiovascular drift and maximal oxygen uptake. Medicine & Science in Sports & Exercise, 40(6), 1065–1071. PubMed →
- Wingo, J. E., Ganio, M. S., & Cureton, K. J. (2012). Cardiovascular drift during heat stress: implications for exercise prescription. Exercise and Sport Sciences Reviews, 40(2), 88–94. PubMed →
