What is cardiac drift?
Cardiac (or cardiovascular) drift refers to the natural increase in heart rate that occurs when running with little or no change in pace. Many runners mistakenly assume that if they keep their runs at a consistent pace, their heart rate will remain relatively constant as well.
However, exercise research has shown that it is common to see heart rate “drift” upward during an easy run or threshold run, even with no increase in pace or effort — sometimes by as much as 10-20 beats per minute over a 30-minute period.
The photo above is an illustration of how cardiac drift looks during a 20-mile long run. This is an actual Garmin report with the athlete’s training pace overlaid with their heart-rate data. As you can see, the pace remains relatively constant (blue) while the heart rate (red) continually increases.
It is important to emphasize that cardiovascular drift results in an increased heart rate without a corresponding rise in effort, breathing rate, or calories burned. In the example of the long run pictured in above, the athlete reported no changes in breathing rate or effort.
What causes cardiac drift?
Cardiovascular drift is mostly caused by the natural increase in core body temperature when running. This increase in core body temperature elevates heart rate the same way running in hot conditions does. Correspondingly, the stroke volume of the heart decreases so that cardiac output and oxygen uptake remain the same, keeping your breathing and effort similar while heart rate rises.