OPTICAL HRMs : HOW DO THEY WORK?

Heart rate is one of the most reliable indicators to assess your physical fitness and the optical technology is the latest in heart rate monitoring tools.

Find out how optical HRMs work and, as a bonus, our answers to your 5 questions about this technology!

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HEART RATE IN THE SKIN!

Optical HRMs are based on one or several LEDs that emit light and on a sensor that receives the light signal: the key principle of optical HRMs is to shine a light through the skin.

You should know that our skin comprises three separate layers: hypodermis, dermis and epidermis.

The dermis is vascularised and contains many capillaries filled with blood. When the heart pumps blood, it slightly changes the amount of blood in the dermis.

The principle is therefore to send green light through these capillaries and, depending on the level of light that is reflected, the sensor will be able to assess the blood flow, and consequently the heart rate.

Indeed, the light was specifically selected to be absorbed by blood. The blood sent into the capillaries will modify this green light that will then be reflected by the skin and detected by the HRM watch.

This type of measurement is called photoplethysmography (PPG).

The signal is then processed by algorithms that separate the noise from the heart rate measurement. Depending on your type of activity, the results will be more or less accurate.

5 FREQUENTLY ASKED QUESTIONS ABOUT OPTICAL HRMs

Is an optical HRM suitable for all sports?

Embedded algorithms used to analyse the data provided by the light sensor are optimised for activities with repeated movements, such as walking or running.

 

Most product will not ensure an accurate heart rate measurement for activities such as body building and fitness. Depending on the product, the heart rate error can reach up to 20 or 30 bpm compared with a chest strap.

Are optical HRMs less accurate than HR chest straps?

The technology featured in HR chest straps measures the electrical activity of the heart. It is therefore very accurate. Conversely, the optical technology does not involve direct measurement, so it is slightly less precise. In addition, it is greatly affected by movements and depends on the algorithms used.

 

Consequently, not all optical HRMs offer the same performance: the best ones are however accurate to 2 bpm on average, compared with chest straps, for running. And they are very popular amongst runners because they eliminate the constraints related to chest straps (comfort, weight, etc.).

Is an optical HRM effective with all types of skin?

For most of the products available, the sensor performance will not be affected by the skin colour or hairiness. However, the amount of light reflected is reduced on darker skins. Most optical HRMs then adapt by sending more light or by adjusting the sensitivity of the light sensor.

 

But the sensor may experience difficulties on tattooed skins: the ink is injected in the dermis and may affect the performance of your optical HRM. If this is your case, you can wear your watch on the other wrist or opt for a chest strap!

How to wear an optical HRM?

In order to obtain an optimal measurement quality, the optical HRM should be positioned at the wrist, right after to the protruding bone.

 

 The sensor should be in contact with the skin, so the strap must be somewhat tightened, without discomfort. The performance of your optical HRM will be decreased if your watch is too loose.

 

The wrist strap material of your optical HRM has been specifically selected to ensure a secure and comfortable fit during your workout, taking into account the fact that your wrist will swell slightly during exercise.

Is it dangerous to wear an optical HRM?

Aside from any skin allergy to the material of the inner side of the watch, wearing an optical HRM is completely safe.

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TO EXPLAIN FURTHER...

Most optical HRM watches also provide features such as interval training to perfect and vary your sessions!

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