By Coach Maria Simone

You may have heard me say before that wrist-based HR monitors, also known as optical sensors, don’t work. A look at any file from a wrist-based strap will show you pretty quickly their general lack of reliability, and tendency to provide erratic readings.

Below is a graph of a run that uses a wrist-based HR monitor. You can see the erratic ups and downs of the HR that doesn’t always follow along with the changes or variations in pace or terrain.

Below is a graph of a run using a chest-based HR monitor. While there are still variations, this recording is much smoother, and the HR changes map more directly onto either terrain (hills) or pace.

Tracking accurate and reliable heart rate data is important for many reasons. For example, heart rate gives us a sense of the energy cost of any given effort, recovery patterns, and potential misalignments between heart rate and power or pace. This is important for ensuring we are hitting the appropriate training intensity and making smart race day decisions for sustainable efforts.

Unfortunately, with an optical sensor, we lose the ability to engage in these high-level analyses with any confidence, as the graph above illustrates. But, you may be wondering why these devices don’t work. To answer this question, we’ll review the technology behind these monitors, and the reasons why we get inaccurate and unreliable readings. We’ll also talk about options to get a more reliable and accurate HR reading.

What is an Optical Sensor?

Optical sensors use a technology known as photoplethysmography (PPG), which shines an LED sensor through the skin to track changes in blood flow. Do you know that green light on the underside of your watch? Yup, that’s what is tracking blood flow and from that, making an estimate of heart rate.

As Stahl et al. explain, “HR is determined based on the theory that blood flow through the artery is inversely related to the amount of light refracted.” As such, this technology is not a direct measurement of HR, and this is where the limitations begin.

Limiters of the Optical Sensors

While our wrist-based optical sensors may be accurate at times, there are a host of reasons the research has found why they are not as reliable at any given moment. The differences in accuracy and reliability vary by device type, with some being more or less reliable than others for certain populations and certain types of movement. Across several studies, the Apple Watch was found to outperform other optical options.

Skin color

Probably one of the most challenging issues with optical sensors is that they have been found to be much more inaccurate for darker skin tones. Given the nature of the LED sensor, differences in skin color will impact its accuracy. It’s important to note that this inaccuracy is noted regardless of whether the research participants were at rest or in movement.

Age

Some studies have found that these devices are less reliable for older athletes versus younger athletes. The working hypothesis for why this may be so relates to differences in circulation and movement patterns.

Placement

The accuracy of optical monitors can be improved based on placement. Whether using a wrist or arm-based optical monitor, restricting the movement of the device as much as possible can improve readings. And the less arm movement, the more reliable the readings become. As you might imagine, this poses an issue for running or swimming.

So, if you wear a wrist monitor, it needs to be quite tight, which can pose problems – especially for longer events. Arm-based monitors tend to work better for this reason.

Movement

The accumulation of research shows that these sensors are most accurate at rest for lighter-skinned people. So, if you are using a wrist-based sensor to track HRV (heart rate variability) or resting heart rate, that data is much more reliable. However, the more we move, the less reliability we get from these devices. The darker our skin, the less reliable as well.

You may find that HR tracks more reliably for some movements, rather than others (depending on the particular device). For example, some of these devices can confuse cadence in running with heart rate, based on the velocity of the arm swing and the subsequent change in blood flow that comes from the arm swing. So, if your cadence is 180 spms, then these devices may erroneously report your HR in the same vicinity.

Intensity

There are mixed results on the accuracy of these devices at intensity, with some studies finding decreased accuracy at low intensity, while others find decreased accuracy at high intensity. While not directly mentioned, I have a hypothesis that this discrepancy is related to movement patterns and idiosyncrasies of the specific models tested. It may also be confounded by the age of the participants or skin color.

What Should I Use Instead?

With any HR-measuring device, it is important to keep it clean and charged. If either of these conditions are not met, you are more likely to get erratic or inaccurate readings – no matter how great the device has tested in practice.

Chest Straps

In research studies, the gold standard is the Polar chest strap. So, if you are looking for the most accurate option, this is the one to get. It’s important to note, that if you use this device, you should detach the monitor from the strap when not in use. This will increase battery life and accuracy. It may also fall prey to Bluetooth interference at times.

Beyond the Polar, most Garmin chest straps get the job done quite well. Some of these straps also include mechanics for running. We don’t recommend splurging for the swim HR data since this is more likely to be inaccurate underwater. Also, RPE is your best bet for learning swim intensity.

Arm Bands

We also recognize that chest straps can be uncomfortable. I have a permanent scar from mine! We have athletes who have experimented with the arm-based optical sensors, which have proven to be more reliable than the wrist-based ones. This is likely due to the restrictions in movement and the ability to tighten the strap without as much issue as the wrist.

Our primary recommendation for the arm-based strap is the Wahoo Tickr Fit. This band will not be as reliable as the chest strap, but it is more reliable than the wrist-based monitors. For optimal performance with the armband, it’s important to keep it charged and clean. I’ve also started wearing mine ABOVE the elbow which seems more reliable.

Keeping track of accurate heart rate is important for understanding the energy costs of any given output (power or pace). It can help you understand how your body responds in different conditions, such as temperature, humidity, or altitude. It also provides important clues regarding recovery and overall health. With this information, you can make better decisions about how to apply a sustainable effort no matter what race day brings.

Links to Studies Reviewed for this Article:

Wrist Based HR Monitors: Do They Work?