Consistent pace doesn’t always equal consistent heart rate.
In two previous articles, I’ve discussed the importance of breaking free from your GPS dependency and three reasons not to train with a heart-rate monitor. Reading both articles may make it seem like I am harsh critic of all running technology, but in reality I am a proponent of any device that helps runners train smarter.
It’s important for all runners, beginners and veterans alike, to take the time to understand training concepts and not blindly follow numbers and data derived from their technological gadgets. With a better understanding of training theory, runners can take full advantage of their chosen piece of technology and better optimize it for improving performance, rather than running to hit subjective numbers.
That brings me to this latest article on heart rate training—the influence of cardiac drift on heart rate during long runs. By understanding the concept of cardiac drift as it relates to effort and heart rate, you can train more effectively and maximize your potential as an athlete.
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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.
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.
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The Importance Of Understanding Cardiac Drift
If you use heart rate to measure your effort, especially during easy and long runs, you need to understand what effect cardiac drift has on your heart-rate readings or you’ll constantly be under-performing in workouts.
Let’s assume you’re targeting a marathon-paced run within your aerobic threshold training zone (80-85 percent of your maximum heart rate). For ease of math, let’s assume your maximum heart rate is 200 beats per minute, which puts your aerobic threshold training zone range at 160-170 beats per minute.
For the first 20-30 minutes of your run, a pace of 8 minutes per mile might put you within that goal range, which will probably also be your goal marathon pace.
After 30 minutes of running, cardiac drift may cause your heart rate to increase so that you have to slow down to maintain that heart-rate window of 160-170 bpm. However, this decrease in pace does not correspond to your effort or fatigue levels.
Now, you’re running at 8:15 per mile pace and spending less time training at your marathon potential and thereby not getting the most out of your workouts or your training time.
Calculating Calories To Lose Weight
Another potential issue to be aware of in regard to cardiac drift is the calculation of calories burned during a run. Many online calculators and fitness machines use heart rate to measure effort and thereby calculate calorie expenditure.
As we’ve learned by looking at cardiac drift, an increase in heart rate doesn’t necessarily correlate with an increase in effort, oxygen uptake, or calorie expenditure. So, using your heart rate as a way to measure calories burned, which is a built-in feature on many heart-rate monitors, could lead to false data and eating too much if you’re trying to lose weight.
What Can You Do About Cardiac Drift?
Don’t be a slave to your heart-rate monitor numbers. Rather, use your heart rate as a tool to help you learn your efforts and paces and as a guide to help get you started on the right track.
One way to accomplish this is to learn to use breathing rhythm as a way to intuitively assess your intensity. You won’t be perfect with this method at first, so wear the heart-rate monitor and use it the first 10-15 minutes of your run to get on track. Then, cover it up and practice running by feel. Look at the data only after you’re done running and compare your breathing rhythm and the effort you felt to the heart-rate numbers.
By understanding the underlying concepts behind heart-rate monitors and other training gadgets, you can learn to better listen to your body and apply these tools to help you train smarter in the long-term.