It’s a common scenario for runners: you are cruising along on a training run, feeling like you finally learned how to actually run at an easy pace. But then, you look at your watch, which reports a heart rate of 185 bpm. You didn’t feel like you were running at a high heart rate! Maybe the problem wasn’t you running too hard – maybe the problem was using a wrist heart rate monitors for running.
Why Wrist Heart Rate Monitors Aren’t Reliable for Running
If you were running at conversation pace, but your watch says your heart rate was 185 the whole time, the problem isn’t that you weren’t running easy enough. The problem is that you were not using a reliable heart rate monitor!
You will know if your heart rate is spiking into the 180 to 200 beats per minute range. For most runners, this high of a heart rate would leave you gasping for air after a few minutes.
The error also occurs when your watch reports a heart rate of 115 bpm, no matter how hard you push. If exercise intensity increases, so does heart rate. In these cases, the watch is likely wrong too.
When assessing data measurement tools, we look at two factors: accuracy and reliability. Accuracy refers to if the data is correct compared to a gold standard. If it is too high or too low, the tool is considered inaccurate for measuring data. Reliability refers to how often the tool is accurate. If it always measures data the same (whether inaccurate or accurate), it is reliable. A measurement tool is unreliable if it is sometimes accurate, sometimes slightly inaccurate, and sometimes wildly inaccurate.
The science shows that wrist heart rate monitors can be both inaccurate and unreliable.
While the technology of optical heart rate monitors may be accurate for resting heart rate, research repeatedly indicates that wrist-based monitors are less accurate for running. When running, the heart rate monitor in your Garmin will be even less accurate on tempo runs and intervals compared to an easy run.
A 2023 study in the Journal of Strength and Condition Research found that wrist-based heart rate monitors were less accurate when running at 7.5 mph compared to 6.0 mph. Even at 6.0 mph, they were less accurate than at walking paces. In comparison, the tested chest strap was more accurate at various running speeds.
Similarly, another 2023 study found that wrist-based heart rate monitors underestimated heart rate at lower intensities. At higher intensities, the watches overestimated heart rate. Either way, the optical heart rate monitors did not provide the accurate and reliable measurements needed for heart rate training.
How Watch Heart Rate Sensors Work
Why doesn’t your Garmin’s heart rate monitor work as well as a chest strap? Optical sensors utilize bright lights that shine through the skin. Blood flow reflects light back, and your device estimates heart rate based on the frequency of the reflected light.
If the watch is jostled (such as in a running stride) or loses contact with the skin, your measurement will be inaccurate. Often, the monitor will start to pick up movement from your arm swing and report that as your pulse – leading to a number that is too high. (This is sometimes called “cadence locking”).
Other factors impacting optical heart rate monitor reliability:
- Bone structure
- Blood vessel location
- Skin tone (darker skin tones do not reflect light as well)
- Tattoos on the hands/wrists
- How far away from the wrist bone you wear your watch
- How tight you wear your watch
- Ambient temperature/if your hands are cold
- Sweat or dirt on the sensor surface
For some runners, wrist-based heart rate will almost always function as well as a random number generator. Some runners will fall in the middle: it will sometimes be accurate, sometimes not, and it becomes a guessing game of when. A few runners will have accurate measurements.
Don’t assume you are always an exception to the science – if the data has a chance of being inaccurate, take it with a grain of salt.
How to Measure Heart Rate For Running (Accurately)
Inaccurate data can be worse than no data at all. If you are adjusting your workouts based on incorrect heart measurements, you are more likely to overtrain or under-train. If your heart rate is always high on easy runs even if you are walking, the problem may be your watch – not your fitness.
You do not have to use heart rate data for your running. You can choose to ignore your watch’s heart rate numbers and focus on metrics such as rate of perceived exertion, the talk test, or pace instead.
If you do want to train with heart rate, you want to ensure you have accurate data. Your watch heart rate monitor simply is not accurate enough – or accurate consistently enough – to use for training zones. When five beats per minute can make a difference, you need an accurate measurement tool.
Chest straps are considered the gold standard for heart rate monitors for runners. Chest straps use sensors that measure signals from the heart to record beats. While they require dampness to work appropriately, the device is less prone to noise. It will produce accurate and reliable readings, unless there is user-error or a device issue (such as low battery). Many of the above-cited studies found that chest straps produced data closed to the ECG, even at higher intensities.
Newer research demonstrates potential with the upper arm optical heart rate monitors, such as the Polar Verity Sense or Coros Optical Band. While these sensors still use optical light technology, the positioning on the upper arm reduces interference from jostling and poor placement. A 2023 study in Scientific Reports using trail runners found the Polar Verity Sense to be both valid and reliable for measuring heart rate.
Whether you use a chest strap or upper arm band, you want to ensure you have calculated your individual heart rate zones. Do not rely on age-based formulas, which can be inaccurate! Remember, bad data can be worse than no data at all. You can perform simple field tests (such as this one) to calculate your heart rate zones.
Got more questions on heart rate training?
