Just How Accurate Is Your Wrist-Based Heart Rate Monitor?

The availability and features of wearable technology, activity trackers, and apps for running has increased significantly over the past decade. What was once a market that nearly exclusively consisted of Timex sports watches and other basic wrist watches, has now exploded to include GPS-enabled running watches, heart rate monitors, cadence or stride rate sensors, and even power meters.

Although some runners are more traditional or “purest” in their approach to running and prefer to run sans technology or with just a basic wristwatch to keep track of overall workout duration, many runners take the “the more the better“ approach to capturing running data and biometrics through wearable technology.

These runners will likely be outfitted with the latest and greatest running watches and heart rate monitors from the now thousands of brands available, including Garmin, Polar, and Fitbit, hoping to gain more insight into their training and their body’s response to each run. 

And, running technology like heart rate monitors are now much more comfortable to wear, particularly with the advent of optical heart rate monitors in wrist-worn smartwatches, making it all the more accessible and seamless to add this valuable training tool to your running gear.

But are these wearable fitness trackers and devices accurate? Specifically, just how accurate is your heart rate monitor watch? Are wrist-based heart rate monitors less accurate than chest straps? 

In this guide, we will explore the accuracy of wrist-based heart rate monitors for running and the implications for training with a wrist-based heart rate monitor.

We will look at: 

• Downsides of Chest Strap Heart Rate Monitors

• How Do Wrist-Based Heart Rate Monitors Work?

• Benefits of Wrist-Based Heart Rate Monitors

• Just How Accurate Is Your Wrist-Based Heart Rate Monitor?

Let’s get started!

Downsides of Chest Strap Heart Rate Monitors

Chest strap heart rate monitors used to be the only option available to runners who wanted to measure their heart rate while running. These devices are quite accurate and are often considered the gold standard to measure heart rate, but several drawbacks exist.

#1: Chest Strap HR Monitors Can Be Uncomfortable 

Heart rate monitors that use a chest strap can be quite uncomfortable when you are running or working out. The chest strap has to be tight enough to keep the electrodes stationary against your skin, which can restrict breathing, or at least create the sensation of restricted breathing.

#2: Chest Strap HR Monitors Can Be Hard to Position Properly for Heavier Runners

The accuracy of heart rate data from a chest strap HRM is also highly dependent on proper positioning. The electrodes should ideally sit just below the nipple line. 

However, this can be a difficult placement for runners with more body fat, and unfortunately, the farther away the HR monitor gets from your heart, the weaker the electrical signal, which can lead to a loss of connection and an absence of your heart rate data.

#3: Chest Strap HR Monitors Can Interfere With Sports Bra Bands

Women who wear a sports bra while running often complain that chest strap heart rate monitors interfere with the positioning of the sports bra, as the chest strap and the lower elastic band or underwires of the sports bra often compete for the same “real estate” or skin area of the chest. 

Although there are some sports bras for runners specifically designed with a sleeve in the band for a heart rate monitor, they are expensive and may otherwise not meet the needs or likings of the runner.

#4: Chest Strap HR Monitors Can Slip Down

Heart rate monitors that rely on a chest strap are prone to sliding down or twisting around laterally from their initial placement while running or doing any cardio, especially if it’s high intensity. 

This is not only a frustrating nuisance, as you have to keep readjusting the strap or shimmying it back into position, but it also interferes with the consistency and overall percentage of heart rate data captured during the run.

In other words, if the heart rate monitor falls down or slips out of place, the electrodes will lose contact with your skin in their proper placement positions and won’t be able to capture your heart rate data accurately.

#5: Chest Strap HR Monitors Require a Wrist-Worn Device to See Your Data

Another drawback of heart rate monitors with a chest strap is that you have to wear a wrist unit of some sort simultaneously if you want to be able to see your heart rate metrics in real-time through ANT or Bluetooth. 

Therefore, you are encumbered with yet another device, and if you have a different running watch, you’ll have to wear units on each wrist.

#6: Chest Strap HR Monitors Corrode

Finally, the snaps that hold the electrodes in place on chest heart rate monitors are prone to corrosion from sweat, which starts to render them faulty, as the corrosion interferes with the accurate and consistent electrical conduction that detects your heart rate.

Corrosion of the snaps frequently leads to the premature breakdown of the HR monitor.

How Do Wrist-Based Heart Rate Monitors Work?

The technology used to assess your heart rate is different between chest strap heart rate monitors and wrist HR monitors.

Chest strap heart rate monitors use electrodes to record the electrical conductivity signals of your heart as it contracts (beats) to pump blood. 

Much like the electrodes used on an electrocardiogram (ECG), the electrodes of chest HR monitors are sensing the spike on the ECG (called the QRS complex) when the left ventricle contracts and pumps blood out of the heart through the aorta into circulation. 

Your heart rate is reported as the number of times this electrical signal is detected per minute because this event occurs once per cardiac cycle, or complete heart beat.

In contrast, wrist-based heart rate monitors use a process known as photoplethysmography (PPG) to detect your heart rate. 

Wrist HR monitors are also known as optical heart rate sensors because photoplethysmography uses light (usually green LED lights) and special light-sensitive diodes to measure your heart rate. 

The lights flicker hundreds of times per second to determine how many times your heart is beating per minute.

The LED light penetrates your skin and reflects off your blood. The degree to which your blood absorbs or reflects light varies according to whether your heart is contracting (beating) or relaxing in between beats.

When your heart contracts, it pumps blood out to your body, increasing blood flow in your wrist. The greater blood flow in your wrist causes more light to be absorbed, rather than reflected, from the LED light.

When the heart relaxes in between beats, blood is flowing from the body (and wrist back into the chambers of the heart.

As the blood volume in the wrist drops during the relaxation of the heart, the skin absorbs less light from the flickering LED.

The light-sending diode registers these two variations in light absorption, integrates the data in the device with an accelerometer (which measures movement), and feeds the data through an algorithm to yield a heart rate value based on the number of times these fluctuations occur per minute. 

Benefits of Wrist-Based Heart Rate Monitors

Most runners find wrist-based heart rate monitors to be less cumbersome than wearing a chest strap, especially because these days, many GPS-running watches integrate the HR and sports watch functions into one smartwatch. 

They can be worn all day, so you can get heart rate variability, resting heart rate, and sleep tracking, which can give insight into how well your body is recovering after workouts.

Just How Accurate Is Your Wrist-Based Heart Rate Monitor?

The main drawback of wrist-based heart rate monitors is that they are typically less accurate than chest strap HR monitors.

Research has shown that wrist HR monitors are fairly accurate, or accurate “enough,” but the acceptability of any errors may depend on your goals with wearing one and how accurate you need your data to be.

There are several factors that can reduce the accuracy of wrist-based HR monitors, including the following:

#1: Skin Tone

There is some evidence to suggest that optical HR monitoring is less accurate on darker skin tones because it’s harder for the LEDs to penetrate darker skin tones to reflect off the blood.

As a result, if you have a darker complexion, a wrist HR monitor may underestimate your heart beat because the light-sensing diodes can miss beats if the light absorption and reflection is poor or the difference between absorption levels is minimal.

#2: Wrist Size

If your wrist is small and bony or your watch face is large, the watch can slide around, which will interfere with the accuracy of data collection.

#3: Band Tightness

If you tighten the watch too much, it compresses the skin. This is problematic because PPG is calibrated for the density of human skin in its resting state, not compressed state, so this again can skew the accuracy of the HR measurement. 

The wristband should be tight enough to prevent movement, but not so tight that it chokes your skin or leaves an indentation.

#4: Workout Intensity

The heart rate monitor needs to stay in contact with your skin at all times in order for the optical sensing to measure your heart rate. 

Studies have shown that the accuracy of wrist heart rate monitors decreases with high intensity or increased running speed because there is more movement or “noise” of the device against the skin.

Another study found that wrist heart rate monitors consistently underestimate heart rate during exercises, particularly at higher intensities. 

Essentially, the faster you run, the less accurate the HR data becomes. At rest, walking, and slower running, the HR monitor stays in position.

#5: Number and Color of LED Lights

Most optical heart rate monitors use one, two, or three green LED lights. The more lights, the better chance they have at shining light effectively enough to absorb it by the blood.

There is also a general consensus that tri-color or red lights in addition to or instead of just green lights increases accuracy because red light penetrates skin better.

#6: Moisture 

Sweat or water while swimming can reduce the accuracy of wrist heart rate monitors by introducing noise and interfering with the light reflection.

#7: Cold Hands

Your wrist-based heart rate monitor is often less accurate in the winter if you have poor circulation in your hands and wrists. If blood flow is reduced, there is less volume coming in and out with each heartbeat, making it harder to detect a change in light reflection.

#8: Activity

Unfortunately, wrist-based HR monitors seem to be less accurate running than cycling.

While there are factors that can reduce the accuracy of wrist HR monitors, they are generally considered to be accurate enough for most healthy runners. Errors are usually arounds 5% or 3 beats per minute.

There are plenty of options, including Apple watches, Garmin, Polar, Fitbit, and even Samsung, all of which will track your heart rate and come with various features.

If you are looking to purchase a heart rate monitor, you can check out the best options, here!