One of the most common conversations we have with the athletes we coach is around heart rate and how to interpret heart rate data coming from a smart watch. It makes sense - training by “zones” is simple, and heart rate data is readily available at any time.
Unfortunately, to get the most out of your training, it’s not that straightforward!
Heart rate zones and data has been preached by fitness magazines and watch companies as an absolute; however, many factors affect heart rate beyond exercise intensity. Heat, humidity, fatigue, caffeine, stress, previous work within a training session, etc all substantially impact heart rate. Think about it - there is no question that caffeine alters heart rate, so should your heart rate data really be looked at the same on a day when you’ve had a shot of espresso versus a day with decaf? No! Heart rate is often used as a proxy for intensity, but it shouldn’t be.
Another big issue with basing your workouts around heart rate is accuracy. Most people get their heart rate data from a wrist-based smart watch which is a notoriously inaccurate measurement. (Pro tip: a chest strap will read much better!)
The best way to use heart rate data is to take it with a grain of salt, and use it to assess training along with pace and RPE (rating of perceived exertion).
Let’s take a peek at one of my latest runs and see how I interpreted my heart rate, pace, and RPE data.
In this workout, I ran at an easy pace for the first 6km, ran hard for 2km (km 7-8), then cooled down at an easy pace for my final 2km.
I typically aim to keep my heart rate below 150 bpm for easy running (my “Zone 2”); however, I know that heart rate often climbs in the later parts of an easy run due to the phenomenon of cardiac drift, so I’m not worried about creeping above Zone 2 after a solid 5km of running. Had I continued jogging, I would expect my heart rate to continue creeping upward. Because this pace feels very easy to me and I’m able happily chat with friends (so I pass the the talk test), I’m not worried about heart rate creep.
After a brief break to do some drills, I ran fast for km 7-8, throwing down an effort somewhere between 10km and tempo pace. My heart rate doesn’t reflect this effort at all! My take: It takes several minutes for heart rate to rise after starting exercise, so I’d expect my heart rate to be lower in my first km than my second, which happened. Aside from that, I figured that my watch must not be reading properly because my heart rate was much lower than my pace and effort reflected. No problem - I trust that I am running fast and hard just as I aim to.
Finally, my heart rate was sky high (170-183 bpm) for my cool down jog- the highest recorded heart rate AND slowest running I did that day! As I jogged, my heart rate continued to increase, not lower, even though I was going slow and at a very easy effort. That’s OK- I assumed that I was tired from the fast running and/or my watch was still malfunctioning. I didn’t care what the watch read because I knew I was running very easy, as is appropriate for cool downs.
Take home message: Heart rate is interesting and useful, but only when integrated with pace and effort , and only when you know it’s accurate!Otherwise, you risk inadvertently shortchanging your workouts or suffering unneeded stress when your heart rate doesn’t seem quite right. As with other training variables, use heart rate as a guideline, not an absolute.
By Jessica O’Connell MSc Exercise Physiology, CSEP-CEP, OLY, endurance coach at Grit Coaching