You start your run at a comfortable pace and maintain it consistently, but after a few kilometers, you notice that your heart rate begins to rise. The effort feels harder, even though you haven’t increased your pace.
This is quite common during prolonged exercise and can happen for several reasons. One of the main ones is cardiovascular drift, a phenomenon in which heart rate gradually increases during exercise even when the external intensity remains relatively stable.
What Is Cardiovascular Drift?
Cardiovascular drift occurs when your heart rate progressively increases during exercise while your pace, power, or workload remains relatively constant.
In other words, you keep running at the same pace, but your heart needs to work harder to maintain the effort.
This phenomenon is especially common during prolonged exercise, in hot environments, and when hydration is inadequate.
Why Does It Happen?
During exercise, particularly in hot conditions, the body needs to send blood to the working muscles while also increasing blood flow to the skin to help dissipate heat.
As exercise continues, fluid loss through sweat can also reduce plasma volume. With less blood available per heartbeat, the heart may increase its rate to maintain the cardiac output required by the body.
The result is simple: your pace stays the same, but your heart rate increases.
Heat Can Make It Worse
Environmental temperature has a significant influence on cardiovascular drift.
In hot conditions, more blood flow is directed toward the skin to help regulate body temperature. The more difficult it is for the body to dissipate heat, the greater the cardiovascular response may be.
That’s why it’s not unusual to see a higher heart rate during a run in hot weather, even when maintaining exactly the same pace as a previous run in cooler conditions.
What About Dehydration?
Fluid loss through sweating can also contribute to an increase in heart rate.
As dehydration progresses, plasma volume decreases, and the cardiovascular system needs to compensate to continue delivering blood and oxygen to the tissues.
This doesn’t mean that every increase in heart rate is caused by dehydration. The response depends on exercise duration and intensity, temperature, sweat rate, and individual characteristics.
What Does This Mean in Practice?
Imagine you start a run at 5:30 min/km.
During the first few kilometers, your heart rate is 150 bpm. After some time, maintaining the same pace requires 155, 160, or even more beats per minute.
You are not necessarily becoming less fit at that moment. Your body is dealing with accumulated heat, fluid loss, exercise duration, and the physiological stress of prolonged activity.
That’s why interpreting a single metric in isolation can lead to incorrect conclusions.
How Can You Reduce Cardiovascular Drift?
Some strategies can help manage this increase:
Hydration: maintain fluid intake according to your individual needs and sweat rate.
Heat management: when possible, choose more favorable training times and environments and use cooling strategies.
Appropriate intensity: starting exercise at an intensity that matches the planned duration can help prevent excessive effort later in the session.
Training: improving aerobic fitness increases the body’s ability to sustain prolonged exercise more efficiently.
Electrolyte replacement: during prolonged exercise with high sweat losses, especially in hot environments, sodium replacement may be part of a hydration strategy, depending on the athlete’s individual needs.
Does a Higher Heart Rate Mean You Should Slow Down?
Not necessarily.
Heart rate is influenced by several factors, including temperature, hydration, sleep, stress, caffeine, accumulated fatigue, and even the time of day.
Therefore, it should be interpreted alongside other indicators, such as perceived exertion, pace, power, environmental conditions, and training history.
A small increase throughout prolonged exercise can be an expected physiological response. However, a much larger-than-usual change, especially when accompanied by symptoms or a significant drop in performance, deserves attention.
Conclusion
If your heart rate increases even though your pace remains the same, it doesn’t automatically mean that you are running harder or that your fitness has declined.
During prolonged exercise, cardiovascular drift is a common physiological response that can be influenced mainly by heat, fluid loss, exercise duration, and the body’s ability to regulate temperature.
That’s why understanding the relationship between pace, heart rate, environment, and perceived exertion is much more useful than looking at a single number on your watch.
