How a Heart Rate Monitor Works

Using a Heart Rate Monitor

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A heart rate monitor (HRM) is a device you wear to measure and display your heart rate continuously. Electrode sensors in a chest strap detect each heartbeat and transmit the data to a receiver such as a watch, fitness wearable, or phone app. The data is displayed as the number of beats per minute.

Heart rate monitors using chest straps with electrode sensors are considered to be as accurate as electrocardiograms (EKG). This sets them apart from arm- or wrist-based LED heart rate sensors such as are found in many activity monitors and smartwatches.

While that technology is improving, it is less accurate. A study of the Fitbit Charge 2 found it consistently underestimated heart rate by 5.9 beats per minute, and the accuracy of Fitbit's LED heart rate detection is the subject of a class-action lawsuit. If you want to use a wrist-based device, it may be wise to check your heart rate with an electrode strap heart rate monitor and compare it to the results you get from your strapless monitor.

Personal heart rate monitors measure exercise intensity so you know whether you are achieving the level of effort you targeting.

Benefits of Heart Rate Monitors

Heart rate monitors can continuously track and record heart rate during exercise, as compared to an on-demand pulse monitor which only shows a value at a single point. This frees you from having to stop and take your pulse to determine how intensely you are exercising.

Most heart rate monitor models have indicators showing whether you exercising in your chosen heart rate zone and give audible or visual alerts when you are above or below that zone. This allows you to adjust your workout to stay at your target heart rate by slowing down, speeding up, or changing incline or resistance.

Many heart rate monitors save and display workout heart rate on a graph along with time, speed, elevation and other aspects measured during the workout. The time in different heart rate zones may be shown at the end of the workout. Simpler heart rate monitors may only show the average heart rate for the session.

How Heart Rate Monitors Work

Seppo Säynäjäkangas, the founder of Polar, invented the wireless personal heart rate monitor in 1977 in Finland. His company was instrumental in popularizing it for athletes worldwide through the 1980s. As a result, these devices are often called Polar monitors, even though they are now produced by many manufacturers.

The chest strap on a wireless EKG-accurate heart rate monitor has electrode sensors that can detect the electrical activity of the heart as it beats. The chest strap can be made of plastic, elastic, or fabric. The sensors can be embedded in it or attached.

Sensors can also be embedded in the fabric of a sports bra or shirt rather than a strap. The original sensors needed to have moist contact with the skin to get accurate readings. To get a good contact, you might wet the contacts with saliva or a medical-grade gel such as used for ultrasound scans. This is no longer necessary.

Heart Monitor Data Display

Originally, the signal from the sensors was sent by radio waves to the receiver. Technology has developed to send it through ANT or Bluetooth. This enables devices such as cell phones and other mobile devices and wearable devices to use apps to receive data from the heart rate monitor sensors.

In choosing a heart rate monitor, look for one that has the type of display you can most easily use and understand during your workout. Some have pre-programmed exercise zones based on your age, while others allow you to set your zones to your individual preference.

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  1. Benedetto S, Caldato C, Bazzan E, Greenwood DC, Pensabene V, Actis P. Assessment of the Fitbit Charge 2 for monitoring heart rate. PLoS ONE. 2018;13(2):e0192691. doi:10.1371/journal.pone.0192691

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