Heart Rate Drift in Running: Causes and Coping Strategies for Rising Heart Rate at the Same Pace Over Time

Introduction

You’ve certainly had this training experience: starting out at an “easy pace,” your heart rate is around 140 bpm for the first 20 minutes, and it feels easy; by the 60-minute mark, at the same pace, your heart rate has climbed to 155 bpm, and it starts to feel hard. The pace hasn’t changed, but your heart rate keeps rising—this is Cardiac Drift, also known as “heart drift” or the “drift effect.” This phenomenon is common in long-distance running, and understanding its causes can help you interpret your heart rate training data more precisely.

Definition and Quantification of Cardiac Drift

Cardiac drift is generally defined as the gradual rise in heart rate over 30-60 minutes during exercise at a fixed intensity (fixed pace or power).

Quantification method: Drift Magnitude = (Heart rate at end of exercise - Heart rate at start of exercise) / Heart rate at start of exercise × 100%

For the average runner during a 90-minute easy run, drift magnitude can reach 10-15%, meaning heart rate rises from 130 bpm to 145-150 bpm. Well-trained runners typically show smaller drift magnitudes (5-8%), indicating better cardiovascular stability.

Main Causes of Cardiac Drift

Cardiac drift is not caused by a single factor but results from the combined action of multiple physiological mechanisms:

1. Rising Body Temperature (Core Temperature Effect)

During exercise, core body temperature gradually rises, and the body directs more blood to the skin for heat dissipation (skin vasodilation). This reduces the venous blood returning to the heart (decreased venous return), lowering stroke volume (SV). The heart must then increase its rate to maintain the same cardiac output.

This is the primary cause of cardiac drift. Hot, humid environments significantly amplify the drift magnitude.

2. Dehydration (Reduced Plasma Volume)

As fluid is lost through sweat, plasma volume shrinks, blood viscosity increases, and venous return is further reduced, exacerbating the decline in stroke volume and causing a compensatory rise in heart rate.

Research shows that for every 1% of body weight lost as fluid, heart rate increases by approximately 2-3 bpm; when fluid loss reaches 2%, the cardiac drift effect can be more than double that of normal conditions.

3. Muscle Glycogen Depletion (Fuel Shift Over Time)

As running duration extends, muscle glycogen gradually depletes, and the body increases its reliance on fat utilization. Fat oxidation requires more oxygen (higher oxygen consumption per mole of ATP than carbohydrates), meaning that even at the same pace, oxygen demand increases slightly, indirectly pushing heart rate higher.

4. Neurohormonal Effects

Prolonged exercise keeps the sympathetic nervous system activated, and blood concentrations of catecholamines (adrenaline, noradrenaline) rise, directly increasing heart rate even when exercise intensity remains unchanged.

Impact of Cardiac Drift on Training Data

Understanding cardiac drift is essential for correctly interpreting training data:

Challenge of Heart Rate-Based Training: If you use heart rate zones to control long-run intensity, you must continuously reduce your pace over time to stay in the same heart rate zone. This is the correct approach, but it often makes runners mistakenly think they are “getting weaker.”

Drift Rate as an Aerobic Capacity Indicator: The better a runner’s training condition, the smaller the cardiac drift magnitude, indicating the cardiovascular system can better maintain a stable stroke volume. The “drift rate” from a 90-minute steady run can be used as a metric to track improvements in aerobic fitness.

Comparing Heart Rate on Hot vs. Cool Days: Heart rate from running in Taipei in summer cannot be directly compared to running in Wulai in winter. Ambient temperature is the largest external variable affecting cardiac drift magnitude.

Strategies to Reduce Cardiac Drift

1. Hydrate actively: Consume 150-250 ml every 30 minutes; add electrolytes for runs exceeding 60 minutes to significantly delay dehydration-induced cardiac drift
2. Consume carbohydrates: Take in 30-60g of carbohydrates every 60-75 minutes during long runs to maintain glycogen availability and reduce the decline in oxygen efficiency from fat-dominant metabolism
3. Choose cooler times: Cardiac drift accelerates in hot environments. Taiwanese runners choosing morning or evening runs in summer can reduce drift magnitude by 30-50%
4. Build an aerobic base: Long-term aerobic training increases plasma volume (at rest), improves stroke volume, and lowers the metabolic basis for drift

Practical Recommendations

1. Control long-run intensity by pace rather than heart rate: A rising heart rate in the latter half of a long run is normal. Maintain a steady pace and accept the natural cardiac drift
2. Record your drift rate: Once a month, perform a 90-minute run at a fixed pace and compare heart rate between the early and late stages as an indicator of aerobic progress
3. Cooling strategies: Soak your wrists in cool water before a long run; in summer, bring a small towel dampened with water to wipe your neck—these can effectively delay drift caused by rising body temperature
4. Different heart rate targets for summer and winter: Note the ambient temperature in your training log to make heart rate data more comparable

Conclusion

Cardiac drift is a normal physiological phenomenon in long-distance running—not a warning sign of declining fitness, but the result of the combined effects of thermoregulation, fluid metabolism, and the energy system. Understanding it allows you to avoid being misled by the numbers and to interpret the heart rate data from every workout more intelligently. Managing it requires active hydration, choosing appropriate environments, and consistently building your aerobic base. The heart rate monitor doesn’t lie—as long as you also learn its language.

Related Reading

• The Science of Cardiac Drift: Why Heart Rate Keeps Rising at the Same Pace
• Monitoring Cardiac Drift: A Barometer of Aerobic Efficiency in Long Runs
• The Aerobic Drift Phenomenon: A Scientific Analysis of the Heart Rate-Pace Relationship
• Cardiac Drift Analysis: The Fitness Diagnostic Significance of Heart Rate Drift in Long-Distance Training