Running Tips: Cardiac Drift in Runners

Let me start by asking you this:
Which workout do you think would provide more cardiovascular benefit—
A 30-minute run at a steady, moderate intensity?
Or
Three 10-minute intervals at the same intensity with five-minute rest breaks in between?

The answer is in something called Cardiac Drift.

What Is Cardiac Drift?

If you’ve ever kept your pace steady during a run but noticed your heart rate slowly creeping up, you’ve experienced cardiac drift. It’s a gradual increase in heart rate that happens over the course of a sustained workout, even if your effort and pace stay the same.

Here's what’s happening under the hood:
After about 10 minutes of moderate-intensity exercise, your stroke volume—the amount of blood your heart pumps out with each beat—begins to decrease. This happens because your body is directing more blood to the skin to help regulate temperature as you heat up. Since your heart still needs to deliver the same cardiac output (total volume of blood per minute), it compensates by beating faster. Hence, the drift.

This is where heart rate training, as valuable as it is, can get tricky. If you're trying to stay within a heart rate zone (say, Zone 2 for aerobic development), but your heart rate starts to drift upwards over time even though your pace is steady, are you still training in the right zone? Or are you underperforming?

What Affects Cardiac Drift?

One key factor is hydration. A study comparing cyclists who were allowed to drink during exercise versus those who weren’t found that:

  • The dehydrated group experienced a 10% increase in heart rate (cardiac drift),

  • While the hydrated group only had a 5% increase.

That means hydration accounts for roughly 50% of the magnitude of cardiac drift. So if you're noticing heart rate creep during your runs, first ask: are you drinking enough?

The Bigger Question: What Kind of Training Is Best?

Let’s go back to our original question about the 30-minute continuous run versus the 10-minute intervals.

In a study comparing these two approaches—continuous running vs interval running at the same intensity—researchers found something fascinating:

  • The continuous runners only spent about 1.5 minutes at maximum stroke volume,

  • While those doing 10-minute intervals (with 5-minute rest) spent 10 minutes at maximum stroke volume.

Why does this matter?
Stroke volume is one of the best predictors of cardiovascular fitness. Spending more time at your peak stroke volume is a key driver in improving heart health and endurance. So, breaking your workout into chunks with short rest may actually be more beneficial than a long, steady effort—especially if you're pressed for time.

What This Means for Your Training

  1. Walk/Run Intervals Are Incredibly Effective
    For beginners or anyone getting back into shape, alternating walking and running can not only make workouts feel more doable, but also improve cardiovascular fitness more efficiently.

  2. Advanced Runners Should Rethink Long Runs
    Tempo and VO₂ max workouts could benefit from shorter intervals and longer rest periods. Instead of always aiming for a long, steady-state run, consider breaking it into intervals. You may get more cardiovascular benefit in less time.

  3. Hydration Is Key
    Stay hydrated to help minimize cardiac drift and keep your heart rate zones more accurate.

  4. Use Heart Rate Training—But Use It Smartly
    Understand that heart rate is influenced by more than just effort. Environmental factors, hydration, fatigue, and cardiac drift all play a role. Use it as a tool, but don't treat it like gospel.

Final Thoughts

Cardiac drift isn't something you can eliminate completely—but once you understand it, you can train smarter around it. Whether you’re new to running or an experienced athlete, recognizing how your heart responds to exercise can lead to better training, improved fitness, and more enjoyable workouts.

Happy (and hydrated) running!

Charlotte Spangler