You are running at the same pace, but you notice that your heart rate keeps getting higher. This may seem strange. After all, if your speed has not changed, why would your heart be working harder?
The answer lies in the fact that the relationship between pace and heart rate is not fixed. Throughout exercise, especially during prolonged efforts, the body may need to increase cardiovascular effort to maintain the same speed.
The Body May Be Working Harder to Do the Same Thing
During continuous exercise, body temperature tends to increase, and changes in plasma volume can occur as fluid is lost through sweat. As a result, the amount of blood pumped by the heart with each beat may decrease.
To continue delivering blood and oxygen to the muscles, the body may compensate by increasing heart rate. This phenomenon is known as cardiovascular drift.
That is why it is possible to maintain exactly the same pace while heart rate progressively increases.
A Practical Example
Imagine a runner doing a 60-minute workout at 5:00 min/km.
During the first 20 minutes, their heart rate is 150 bpm. The pace remains stable, but after 40 minutes, their heart rate reaches 158 bpm and, near the end, 163 bpm.
The athlete did not speed up. Even so, the heart is working harder to maintain the same pace.
This can happen due to factors such as increased body temperature, dehydration, exercise duration, and the relative intensity of the effort.
Does This Mean the Athlete Is Getting Worse?
Not necessarily.
A gradual increase in heart rate during prolonged exercise can be a normal physiological response. Context is essential when interpreting what is happening.
If heart rate rises significantly while pace remains the same and perceived exertion also increases, it may indicate that the intensity is becoming progressively more demanding for the body.
On the other hand, environmental conditions, hydration, sleep, stress, and even caffeine can influence heart rate and should also be considered.
Why Does This Matter for Endurance?
In long races, understanding this relationship helps athletes interpret effort more accurately. A pace that feels comfortable at the beginning may represent a much greater physiological demand after several hours.
Therefore, pacing strategies should not consider speed alone. Heart rate, perceived exertion, temperature, and exercise duration can also help indicate how hard the body is working.
Pace shows what you are doing. Heart rate helps show how hard your body is working to keep doing it.
