Cardiac Drift in Running: Why Your Heart Rate Climbs Even When Pace Stays the Same

This post is about cardiac drift, the gradual rise in heart rate during a steady effort, and why it matters to ultrarunners trying to pace well, interpret training data, and avoid turning an easy long run into a moderate one.

If you have ever started a long run at conversational effort, held the same pace for two hours, and watched your heart rate rise 10 to 20 beats per minute by the end, you have seen cardiac drift. Many runners assume that means they got fitter or less fit, or that their watch was wrong, or that they simply needed more willpower. Usually it means something more specific. The cost of maintaining the same external output, in this case pace on a given terrain, rose over time. Your body had to work harder internally to produce the same result.

That distinction between external output and internal cost is central. External output is what you can see from the outside, such as pace, power, or speed. Internal cost is what your body is spending to produce it, such as heart rate, ventilation, glycogen use, and thermal strain. In a fresh, cool, well-fueled state, a given pace may cost one heart rate. Ninety minutes later, the same pace may cost more.

In endurance science, this is often discussed as cardiovascular drift or heart rate drift. In applied coaching, runners also talk about decoupling. Decoupling means two signals that were moving together begin to separate. The common example is pace and heart rate. Early in a run, 8:30 per mile might correspond to 140 beats per minute. Later, you are still at 8:30 pace, but heart rate is 151. Pace stayed flat, heart rate drifted upward, and the two became less tightly coupled.

For ultrarunners, this matters because races are long enough for drift to become one of the main stories of the day. A pace that feels easy at mile 5 may be too costly at mile 35. A heart rate cap that works in cool weather may be unrealistic in heat. A long run with major drift may tell you less about your threshold and more about your hydration, glycogen status, or aerobic durability.

What is actually drifting

Heart rate rises during prolonged exercise because stroke volume tends to fall. Stroke volume is the amount of blood the heart ejects with each beat. Cardiac output, the total amount of blood pumped per minute, equals heart rate multiplied by stroke volume. If your muscles still need roughly the same oxygen delivery to hold pace, and stroke volume drops, heart rate often rises to compensate.

Why would stroke volume drop during a run that is not getting faster? One reason is fluid loss. As you sweat, plasma volume, the liquid part of blood, declines unless you replace enough fluid. With less circulating volume, venous return drops. Venous return is the amount of blood returning to the heart. If the heart fills a bit less between beats, each beat can eject less blood.

Heat adds another layer. As body temperature rises, more blood is directed toward the skin to shed heat. That helps cooling, but it also creates competition for circulation. Working muscles still need blood flow, the skin needs blood flow, and the heart has to manage both. The classic review literature on cardiovascular drift has shown that prolonged exercise in the heat magnifies the effect, especially when dehydration is present.

There is also a simple mechanical point. Over time, you become less economical at a given pace. Running economy means how much oxygen and energy you need to hold a speed. As fatigue accumulates, stride mechanics can get a little worse, muscle stiffness can change, and you may bounce more or brake more. Even small losses in efficiency raise energy demand. If the pace remains the same but the oxygen cost rises, heart rate often follows.

Fuel availability matters too. As glycogen stores fall, your body relies more heavily on fat oxidation. Fat oxidation is the use of fat as fuel in the presence of oxygen. That is useful and necessary in ultra distances, but fat produces energy more slowly than carbohydrate. As carbohydrate availability drops, the same pace can feel harder. The body may need to recruit more muscle fibers or increase physiological strain to maintain output. The result can be another push upward in heart rate.

None of this means drift has a single cause. It is usually a combined effect of thermoregulation, fluid balance, fuel status, fatigue, and the limits of your aerobic system.

Why pace and heart rate separate over time

Early in a run, pace and heart rate often have a fairly stable relationship. If conditions are controlled, a runner may settle into a narrow band where aerobic metabolism is meeting the demand efficiently. Aerobic metabolism means producing energy with oxygen, mainly inside the mitochondria, the structures in muscle cells that help convert fuel into usable energy.

As the run continues, several small changes stack up. Core temperature rises. Sweat loss accumulates. Muscle glycogen declines. Neuromuscular fatigue builds. Ventilation may increase. Perceived effort creeps up even if pace does not. Each factor is modest on its own. Together, they change the internal cost of the work.

That is why heart rate-based training can feel straightforward for 40 minutes and much messier for three hours. The signal is still real, but it is no longer describing only intensity in the simple sense of how close you are to threshold. It is also reflecting accumulating stress.

This is where runners often get confused about zones. In much coaching language, easy running sits mostly in Zone 1 and low Zone 2 of a five-zone model, below the first major metabolic turn point. In exercise physiology research, the naming can differ, but the basic idea is similar. There is a lower-intensity domain where lactate remains relatively stable and effort is sustainable for a long time. The problem is that if heart rate drifts upward enough during a steady run, you can move from truly easy toward moderate internal strain even without changing pace.

That does not mean the run was a mistake. It means the later part of the run was not the same session physiologically as the first part.

What decoupling tells you, and what it does not

Many runners use aerobic decoupling as a marker in long runs. They compare heart rate to pace, or heart rate to power, in the first half versus the second half of a steady effort. If heart rate rises substantially relative to pace, decoupling is high. If the relationship stays stable, decoupling is low.

This can be useful, but only if you are careful with interpretation.

Low decoupling during a long steady run usually suggests that the pace was well within your aerobic capacity for that day and in those conditions. It may also suggest decent fueling, hydration, and heat management. High decoupling suggests the opposite, but it does not tell you which factor was responsible. Maybe your aerobic base is underdeveloped. Maybe you started too fast. Maybe the weather was warm. Maybe you had poor sleep and low glycogen. Maybe the route had more climbing late in the run than you realized.

Recent marathon field data has reinforced that internal and external workload can separate substantially over long events, and that the degree of separation relates to fatigue and performance. Newer research is also trying to identify which physiological traits are associated with more or less heart rate-speed decoupling, but this is still an evolving area. The practical takeaway is strong even if the exact weighting of causes remains unsettled. A widening gap between heart rate and pace means the same output is getting more expensive.

That is information you can use.

Why ultrarunners should care more than shorter-distance runners

In a 5K, drift matters little because the race is over before thermal and metabolic strain have time to accumulate in the same way. In a marathon, it matters a lot. In an ultra, it can dominate pacing decisions.

Ultramarathons extend every vulnerability. Small pacing errors become large glycogen costs. Mild dehydration becomes measurable cardiovascular strain. Moderate heat becomes a central limiter. Mechanical inefficiency that barely matters in a 90-minute run becomes expensive after five hours.

Cardiac drift is one reason smart ultrarunners pace by effort first, with pace and heart rate as supporting tools rather than dictators. If your watch says the pace is still acceptable but your breathing, form, and heart rate all say the cost is rising quickly, that matters. Late-race success often depends on managing cost, not just defending pace.

This is also why uphill hiking can be a rational choice in ultras. On steep grades, running may push internal cost up sharply for little gain in speed. Hiking can reduce the metabolic and mechanical price enough to preserve overall race performance. Drift reminds you that not all forward motion costs the same.

How training changes cardiac drift

The good news is that cardiac drift is trainable, at least partly. You cannot eliminate it, especially in long events or hot weather, but you can reduce how fast it appears and how severe it becomes.

The first adaptation comes from aerobic base training. Easy running with enough volume increases mitochondrial density, capillary density, and the muscle machinery that supports steady aerobic work. Capillaries are the tiny blood vessels that deliver oxygen and remove byproducts at the muscle level. More capillaries and more aerobic enzymes improve your ability to produce energy at a given pace with less disturbance. Over time, that usually lowers the heart rate cost of submaximal running and helps keep pace and heart rate more stable for longer.

The second adaptation is fuel handling. Well-trained endurance athletes store more glycogen, use fat more effectively at submaximal intensities, and spare carbohydrate better at easy and moderate efforts. That does not mean they run ultras without carbohydrate. It means they can preserve stability longer before fuel limitations push cost upward.

The third adaptation is muscular endurance. As your legs become better at producing force repeatedly with less breakdown in mechanics, running economy deteriorates less late in the run. If your stride changes less after two hours, the oxygen cost of the same pace changes less too.

The fourth adaptation is heat acclimation. Repeated training in heat, done carefully, can expand plasma volume, improve sweat response, and reduce cardiovascular strain at a given workload. Heat adaptation does not make hot racing comfortable, but it often makes drift less severe for the same temperature.

Threshold and moderate-intensity training can help indirectly as well. If your lactate threshold, the highest intensity you can sustain with relatively stable lactate for a prolonged period, moves upward, then an easy pace sits farther below it. That makes the same long-run pace less stressful. But for most ultrarunners, the biggest gains in drift resistance still come from consistent volume, sensible long runs, fueling practice, and heat management.

How to use drift in real training

The simplest place to watch for drift is a steady long run on a route with minimal interruptions. Keep terrain as consistent as possible. Start conservatively. Fuel as you would for an important training day. Then compare the relationship between pace and heart rate from early to late in the run.

Suppose you run two hours at roughly 9:00 per mile on flat terrain. In the first hour, your average heart rate is 138. In the second hour, it is 149 at the same pace. That is meaningful drift. If the weather was cool and your fueling was normal, it may suggest that this pace was near the upper edge of your true easy range for that duration. If you repeat a similar run six weeks later and the second hour is only 143, that is a useful sign of improved aerobic durability or better execution.

But context matters. If one run was in 50 degree weather and another in 75 degree weather, the comparison is weaker. If one followed a hard workout the day before, that changes the picture. Drift is a clue, not a verdict.

A few practical uses stand out.

First, use drift to calibrate easy intensity for long runs. If heart rate climbs steadily on a run that was meant to be easy, the answer is often to start slower, not to force the same pace next time.

Second, use drift to test fueling and hydration strategy. If your heart rate stays more stable when you take in 60 to 75 grams of carbohydrate per hour and drink appropriately for conditions, that is not abstract physiology. It is direct feedback on race execution.

Third, use drift to respect heat. A heart rate cap that works in winter may be too aggressive in summer. In warm conditions, you often need to slow down earlier to preserve the same internal load.

Fourth, use drift to evaluate progression across a training block. If the same long aerobic run produces less separation between pace and heart rate over time, that usually means something good is happening.

One caution is worth stating clearly. Do not chase zero drift. In very long runs, some drift is normal. Trying to hold heart rate perfectly flat for four hours can turn into an overcorrection where you slow too much early and lose the purpose of the session. The useful question is not whether drift exists. The useful question is whether the amount of drift matches the goal of the day and the conditions you were in.

The mistake runners make with heart rate caps

Heart rate is attractive because it feels objective. The common mistake is treating it as equally objective in all circumstances. In reality, heart rate is responsive to many factors besides pace, including heat, dehydration, altitude, caffeine, fatigue, stress, and illness.

That does not make it useless. It makes it contextual.

For example, on a cool day an ultrarunner may aim to keep a long run under 145 beats per minute. On a hot day, trying to stay under 145 might force a dramatic slowdown that is actually appropriate. The wrong conclusion would be that fitness vanished. The right conclusion is that environmental cost rose. In racing, the same logic applies. The internal effort you can afford at mile 10 is not always the one you can afford at mile 60.

Heart rate works best when paired with perceived exertion, breathing, terrain awareness, and knowledge of the conditions. When all four line up, pacing gets much better.

Cardiac drift is not a bug in the data. It is the data telling you that the body you started the run with is not the body you are finishing it with. As hours pass, the same pace can ask a different physiological question. Ultrarunners who understand that tend to pace earlier, fuel sooner, respect heat more, and read their long-run files with better judgment. That is the practical value of the concept.

Written by Wade Wegner. Train Ultra is a private AI coach that reads every workout you post to Strava. Try it free.