Cardiac Drift Explained: Causes and What It Tells You

Twenty minutes into a steady long run, your watch reads 145 bpm at your usual easy pace. Two hours in, you're still holding the same effort and your heart rate is 158. Nothing about the effort feels different. Your legs are fine. Your breathing is steady. But the number keeps climbing.

That climb is cardiac drift, and almost every endurance athlete experiences it. The interesting question is whether the drift you are seeing is due to dehydration, bad sleep the night before, or an actual signal that your durability is starting to break down. This article unpacks the causes of cardiac drift, the difference between benign and deleterious drift, and what current sports science actually lets you read from a drifting heart rate.

What Cardiac Drift Is, in One Sentence

Cardiac drift is the upward drift of heart rate at a fixed workload during prolonged exercise.

If you held a constant effort, and your heart rate at the end of the bout is meaningfully higher than at the start, you've drifted. The amount of drift varies between athletes, the conditions, the fueling, and how long the bout lasted, but a 10 to 15 bpm rise over a long aerobic session is typical. In extreme conditions: hot, dehydrated, under-fueled, drifts of 20 to 30 bpm at the same power are not unusual.

Cardiac drift is the most familiar form of physiological decoupling: the progressive divergence between an internal physiological signal (HR) and external work. It's what coaches mean when they talk about "HR pulling away from pace" on a long run.

Causes of Cardiac Drift

There is no single cause. Cardiac drift is the visible end-point of several mechanisms running in parallel, some benign, some not. The combination is what determines whether your drift is meaningful or just noise.

1. Plasma Volume Reduction (Benign)

The single largest contributor to cardiac drift on most long sessions is mechanical. Sweat draws fluid out of the body. Some of that fluid comes from blood plasma. Plasma volume can fall by 10 to 15 percent during a long bout of moderate exercise even with reasonable hydration and that means there is less blood returning to the heart on each beat.

The heart's response is straightforward. Stroke volume drops because there's less blood to pump. To maintain the same cardiac output (the total volume of blood per minute), heart rate rises to compensate. The math is forced: if stroke volume falls 10 percent, heart rate has to rise to deliver the same total flow.

This is the "benign" half of cardiac drift. Nothing about your aerobic capacity has changed. You haven't crossed into a harder intensity domain. The metabolic cost of the work is the same. You're just running on lower-volume blood.

2. Thermoregulatory Demand (Benign)

Working muscle produces heat, about four watts of heat for every one watt of mechanical work. To dump that heat, the body shunts blood to the skin, where it can radiate and convect away. That circulation grows progressively as core temperature rises.

That redirected blood is blood that isn't returning to the heart. Functionally, it has the same effect as plasma volume loss: less blood return to the heart, lower stroke volume, compensatory HR rise. On a hot day or in still air, this mechanism dominates. Two athletes running the same session at 32°C versus 12°C will show substantially different drift purely from thermoregulation.

This is why drift is exaggerated in the heat, and why drift on a cold day at the same pace is much smaller. Most of the difference is thermoregulatory.

3. Dehydration (Mostly Benign, Compounds the First Two)

Dehydration is not a separate mechanism so much as an amplifier of the first two. Net fluid loss reduces total blood volume on top of the plasma redistribution from sweating, and it impairs the body's ability to dissipate heat. The result is more cardiac drift, faster, for the same workload.

A useful rule of thumb: when athletes lose more than about 2 percent of body mass to sweat, cardiac drift accelerates noticeably. Below that threshold, drift is more modest and tracks the underlying plasma-volume and thermoregulatory mechanisms. Hydration status is one of the few drift drivers that's easy to manipulate during a session.

4. Substrate Depletion and Threshold Erosion (Deleterious)

This is where cardiac drift gets interesting and where its causes shift from cardiovascular to metabolic.

As muscle glycogen depletes during a long bout, two things happen at the same time. The relative contribution of fat oxidation rises (which is fine, and to some extent protective). But the metabolic cost of producing the same external power often rises slightly because fat oxidation requires more oxygen per unit of ATP than carbohydrate oxidation. The work gets a little more expensive, oxygen demand rises, and heart rate rises with it.

Worse, the moderate-to-heavy intensity transition (VT1) drifts downward over the course of a long session in terms of power/pace output. The Rothschild 2025 study showed that after roughly 2.5 hours at 90 percent of fresh VT1, VT1 power had fallen by an average of about 10%, with individual drops ranging from 1 watt to 45 watts.

When VT1 falls, the same external workload is now closer to (or above) the threshold than it was an hour ago. HR responds to the new intensity domain. This is the deleterious half of cardiac drift: the drift you see is a signal that your physiology has degraded and the workload is no longer where it started.

5. Sympathetic Drive and Central Fatigue (Deleterious)

Cardiac drift is also pushed by central nervous system fatigue. As effort accumulates, sympathetic outflow rises, catecholamines (adrenaline, noradrenaline) increase, and HR rises directly through autonomic stimulation independent of metabolic demand. This contribution is harder to isolate, but it's real, and it tends to dominate the late-session, near-failure portion of long bouts where drift accelerates sharply.

This mechanism is mechanistically related to why breathing rate also drifts: the same rising central drive that elevates HR also drives respiratory frequency upward. We've covered the breathing-rate side of that picture in why breathing is the missing link in endurance training.

Benign vs Deleterious Drift: The Distinction That Matters

The clinical and training distinction is direct: benign drift is something to ride through; deleterious drift is something to respond to.

The unsolved problem in field sports science is that heart rate alone cannot tell you which kind of drift you are seeing. Both look identical on the wrist. As Maunder put it on Koopcast Episode 160's discussion of durability in ultrarunning:"an athlete using a fixed heart-rate ceiling on a long run is likely undertraining late in the session if their drift is mostly benign", but if the drift reflects real threshold erosion, holding a steady pace through it is exactly the wrong move.

This is the core practical limitation of HR-only training. The signal carries information, but the information is ambiguous.

Why Fixed HR Zones Break Down on Long Sessions

Cardiac drift compounds a more fundamental problem with HR-based training. Even in a fresh state, fixed percentages of HRmax do not reliably represent individual thresholds. Meyer's 1999 study showed that "85 percent of HRmax" placed 36 trained cyclists anywhere from 87 to 116 percent of their individual anaerobic threshold. We covered this in why 85 percent of max heart rate doesn't work for anyone in particular.

Add Rothschild's roughly 10 percent threshold shift to Meyer's roughly 30-percentage-point fresh-state variability, and a fixed HR zone late in a long ride can be off by more than 40 percentage points from the athlete's actual current threshold. That's not a precision problem; that's a category error. The athlete is no longer training in the zone they think they're training in.

This is why coaches who work with athletes like ultrarunners, long-course triathletes, and gravel cyclists increasingly distrust fixed HR ceilings on long runs, even ones built from individualized threshold tests rather than max-HR percentages. The threshold the test measured is not the threshold operating at hour three.

Beyond Heart Rate: Relative Drift

The reason HR drift is ambiguous is that heart rate is the least specific of the available physiological signals. It conflates metabolic cost, cardiovascular drift, thermoregulation, dehydration, and central drive into a single number.

Two other signals each isolate part of that picture, and reading them alongside HR is what lets you separate benign from deleterious drift.

Minute ventilation (V̇E) is tightly coupled to O₂ consumption and CO₂ production, i.e. coupled to metabolic demand. V̇E does not get fooled by plasma volume loss or thermoregulatory drift. Stevenson's 2024 work showed that during 2 hours of cycling at 90 percent VT1, V̇E was statistically unchanged between start and end. Rothschild similarly found that V̇E drift was not a significant predictor of durability loss. This is informative, not a null finding: V̇E is the signal that is supposed to stay stable when nothing metabolic is changing.

The implication is direct. If your HR is drifting up and your V̇E is flat, the metabolic cost of the work hasn't changed so your drift is mostly cardiovascular and benign. If V̇E is also rising at the same workload, the work has actually gotten more expensive metabolically, and the drift is at least partially harmful.

Breathing rate (BR) carries a third, complementary signal. Breathing rate is regulated by central command and perceived exertion, not by metabolic demand directly. Nicolò's work established that breathing rate correlates with RPE across various exercise protocols. 

Stevenson saw this pattern after 2 hours at 90 percent VT1: breathing rate up about 16 percent, tidal volume down about 16 percent, V̇E unchanged. Breathing rate is also the "sticky" signal, once elevated by neuromuscular fatigue, it doesn't fully reverse within a session, which is what you want in a stress indicator.

Reading the three signals together resolves most of the ambiguity in HR drift on its own:

• HR up, V̇E flat, BR flat → benign cardiovascular drift. Plasma volume, heat, mild dehydration. No threshold movement.
• HR up, V̇E up, BR flat or up → metabolic drift. The work has genuinely gotten more expensive — substrate shift, threshold erosion. This is the deleterious case.
• HR moderate, V̇E flat, BR up → neuromuscular fatigue. The metabolic cost is the same, but central drive is rising. Late-session pattern, often signals it's time to back off.

This is the framework Tymewear's VitalPro chest strap is built around: heart rate alone is the historical workhorse of endurance training, but it is the most contaminated of the available signals. Pairing HR with breathing data gives you the mechanistic decomposition needed to read drift correctly.

Where to Go Next

For the broader framework that cardiac drift sits inside, see the cornerstone on durability training, which covers the cardiovascular, metabolic, and neuromuscular systems together and the field-based methods for measuring durability without a lab.

For the wider problem of HR-based training zones — including why 220 − age and percent-of-HRmax cannot represent individual thresholds — see why 85 percent of max heart rate doesn't work for anyone in particular and the cornerstone on VT1, VT2, and VO₂max.

For the closely related question of HR creeping up at the same pace from one week to the next (rather than within one session), see why your heart rate is higher on easy runs than it used to be.

To measure drift mechanistically through HR, V̇E, and breathing rate captured second-by-second in the field see the Tymewear VitalPro chest strap and the paired-session validation against the Cosmed K5.

References

1. Rothschild JA, Gallo G, Hamilton K, Stevenson JD, Dudley-Rode H, Charoensap T, Plews DJ, Kilding AE, Maunder E. Durability of the moderate-to-heavy intensity transition can be predicted using readily available markers of physiological decoupling. European Journal of Applied Physiology 125:2911 to 2920 (2025).
2. Maunder E, Seiler S, Mildenhall MJ, Kilding AE, Plews DJ. The importance of "durability" in the physiological profiling of endurance athletes. Sports Medicine (2021).
3. Stevenson JD, et al. Ventilatory pattern shift at VT1 during prolonged cycling (2024).
4. Stevenson JD, et al. Prolonged cycling reduces power at VT1 (2022).
   
5. Nicolò A, Marcora SM, Sacchetti M. Differential control of respiratory frequency and tidal volume during exercise. European Journal of Applied Physiology (2018).
6. Nicolò A, Marcora SM, Bazzucchi I, Sacchetti M. Differential control of respiratory frequency and tidal volume during high-intensity interval training. Experimental Physiology (2015).
7. Meyer T, Gabriel HHW, Kindermann W. Is determination of exercise intensities as percentages of VO₂max or HRmax adequate? Medicine & Science in Sports & Exercise (1999).
8. Smyth B, et al. Decoupling of internal and external load in marathon runners (2022).
9. Coop J, Maunder E. Koopcast Episode 160: Durability in Ultrarunning.
10. Coyle EF, González-Alonso J. Cardiovascular drift during prolonged exercise: new perspectives. Exercise and Sport Sciences Reviews (2001).
11. Tymewear vs Cosmed K5 paired-session validation analysis (internal, 2026).