At our weekly Thursday tempo run, someone asked me about my experiment with the two heart rate (HR) watches, why were both of them so out of sync? To be fair, watch manufacturers make great pieces of technology, and it’s usually a user problem, right? It’s Yes and No, in this case. Here’s what happened

The science behind HR sensors

First, let’s understand how the HR sensor works. On the underside of the watch, a LED sensor measures how blood flowing under the skin scatters. When blood flow speeds up because of a heartbeat, or changes in blood volume, your arteries move and expand, creating subtle light intensity shifts of less than 2 percent. Detecting this is a process called photo plethysmography (PPG)

So there lies the problem, physical movements like swinging your arm when running or shaking your wrist are relatively huge movements that causes changes to the light. It’s not explicitly mentioned, but Suunto’s a leading GPS watch maker’s website said that arm movements, crossfit style training, and vibrations will cause issues. Wear your HR watch comfortably loose and the HR reading goes up due to the surrounding light interfering with the sensor, which was what happened in my experiment.

PPG has limitations

PPG Signal losses can also happen if you have a tattoo on your wrist, or during a swim because water flow affects the sensors ability to detect light. Also, high levels of motion can cause a total loss of PPG signals. Not good, especially when you want to know your HR during bouts of more intense exercise like interval or hill training. In fact, it was so bad that Fitbit has faced class action lawsuits because of this inaccuracy.


However, if you wear your watch so tight that it affects blood flow, that will also reduce the sensor’s ability to monitor HR. Also, when it gets too cold, blood flow shifts to the body core and the PPG signal gets too weak.

According to Joseph Chua, a athlete and Ironman triathlete ’’Having completed more than 16 Ironman races, I found no heart rate watch that works well in water, so when swimming I learned to regulate effort by feel – breathing and exertion were my tools.’’

Chest straps are more accurate

A better alternative is to wear a more accurate HR chest strap sensor, but these are uncomfortable against your skin and can cause chaffing and itchiness. Personally, I have a hard time keeping a chest strap in place when I run as it keeps slipping. Try using one and you will soon realize the elastic band has a lifespan of an overweight goldfish. It’s expensive to keep using and replacing this option.

So if you still plan to train with some fairly inaccurate data, take heart (no pun intended) unusually high HR during and after training can sometimes be a false alarm

HR readings are laggy

Besides being inaccurate, HR readings are also laggy – run up a hill and your increased HR doesn’t register until it’s over. Slow down to keep within the zone? It’s too late. Drives you nuts doesn’t it? The good news is GPS enabled watches are relatively accurate at tracking other important data – time and position. The watch uses up to three geo-static satellites, and depending on the mode you set, it will interact with the satellites once every few seconds. This means it is accurate to about 3 meters, and consistently so, even after 100s of miles. At least some things are reliable.

When do athletes use HR?

Occasionally, my athletes track their resting HRs first thing in the morning for two reasons. First, to ensure they are well rested and ready to start a new day. They do this in the morning after an intense training session. If the athlete has a Resting HR of 7 to 10 beats higher than normal before they get out of bed, then that day is a rest or easy cross training day. My Resting HR is a low 43 beats per minute (bpm), so if my HR reads 50 bmp or more, I would switch to an easy spinning or swim session for that day. Using Resting HR takes away the subjectivity of feeling blah, or grumpy in the morning (who doesn’t feel that way sometimes, right?). This data-driven approach to ensures you don’t overtrain and it is not based on motivation or mood.

A better way to use HR training zones

Another reason why Resting HR is useful – it is used to calculate your heart rate zones using the Karvonen method. This calculation takes into account both your fitness level and your age. I mentioned in my previous posts, it does not make sense to simply take a number and deduct your age to get your heart rate zones. Two persons of the same age are not likely to have the same fitness level, so their training intensities should be different. Setting the wrong zones could mean you will train too easy and your progress will be very slow or worse, too hard and suffer. Take your Resting HR every few months, use this formula to reset your training zones as you get fitter and train better. Better yet, join our training session and let me help you plan your training intensities.

I will be posting a third and final part of this 3-post series to explore the relationship between the Karvonen method, HR and RPE and how you can apply it for training and racing.

The HR watch – the good part?

Besides the HR function, the GPS enabled sports watch is a great tool for tracking the important data like time, and position, hence able to calculate distance, speed, and pace. More advanced models can track your running motion as well – from vertical oscillation (your up and down movement as your run forwards), to ground contact time, all these are useful inputs which we use at during gait analysis to help our athletes become more efficient runners.

Train with the right tools

As they say, garbage in, garbage out. If you still plan to train based on the reading from your HR watch, you have to at least ensure it’s accurate. Wear a chest strap for a start, or else choose what the mainstream of runners do – train with time and distance, pace and speed, use your RPE to help understand your intensity, compare your race and time trial results to plan your training targets. Least of all, if I may add, find a good coach that understands how to structure a training plan that is customized for you.


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