How Heart Sensors in Fitness Trackers Work

Today being fit & trim is such a (good) craze that sales of fitness trackers are booming and several smartwatches incorporate fitness focused features like sleep tracking, sweat-proof construction and variety of sensors. Heartbeat sensor is one such feature found on almost all smartwatches and fitness trackers.

Hr Cp

While the name gives away its main function (tracking heart beats), we will today see why recording heartbeats is important and how the sensor works.

Why Tracking Your Heartbeat is Important?

Heart beats are a vital parameter of your heart which can reveal many things about its health. From diagnosing a simple chest pain to knowing the health of heart muscles, so much can be known by checking your heart beats. The test conducted for this is known as ECG(Electrocardiography). In this test, few electrodes are to attached to your chest and your pattern of heart beats is recorded in form of a graph. This is the same basic concept behind your heart rate sensor, which functions as a primitive ECG.

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Graphical Interpretation of a Heart Beat | QRS Complex

So medically, heart beats are very important, but for fitness buffs too, keeping an eye on your heart beat will help you workout out more efficiently. Let’s see how.

Heart Rate Target Zones

Your heartbeat is affected by many factors such as hormones, stress, arousal and activity level. During exercise your heart beats increase from normal to pump more blood which transports oxygen to hungry muscles. When you are at rest, your heart beats in range of 40-100 bpm (beats per minute). This is known as Resting Level.

Did you know that cyclist Miguel Indurain, who won Tour De France five times, had a resting heart rate of 28 beats per minute?

Yes, a lower resting level could indicate better heart health.

When you do some physical activity, this rate increases and the maximum it can reach depends upon your age. Your activity is classified into five HR zones, which are expressed as percentage of your max heartrate, as shown below.

Hr Zones
The Five Main Heart Rate Zones

And your max heart rate is determined through the VO2Max test, but it’s a specialised test and quite expensive. The easy and acceptable way to know your max heart rate is subtracting your age from 220.

So working out in correct zone will determine whether you’re burning fat or building endurance.

You can check out here what effect working out in each zone has on your body.

The Types: Electrical vs Optical

Heartrate sensors are classified based on how they measure your heart beats.  The first type measures the electrical signals generated due to contraction & relaxation of the heart ventricle muscle. A peak in the signal indicates a heart beat and the number of peaks occurring are counted to generate the bpm number.

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A HR Chest belt | Garmin HRM-Tri

Of course it’s a crude way of explaining it with the actual science being too complex to dumb it down to our level. All the chest based sensors are of this variety. They sort of work as a crude ECG with the conductive pads or conductive fabric acting as leads.

Coming to the second type, the optical sensors, which are found on fitness trackers and smartwatches use a technique known as PPG(Photoplethysmography).

Watch Measure Sensors
Optical Sensor in Apple Watch | Apple

In layman’s language, this method works by shining a Red or IR light onto the blood flow under your wrist’s skin and measuring the difference between scattering of the light, which changes between heartbeats. The change is counted to give the bpm value. Again its not that simple, with DSPs, algorithms and accelerometers working in tandem to process that number.

And those silvery bars you find on the treadmills in gyms? Well they also show your HR by acting as a electrode to measure electric activity, much like the chest sensors.

Cool Tip: You can even manually count your heart beats. Place your thumb or index finger on your left wrist, pressing lightly. You should feel your pulse. Now count that for one minute which is your BPM.

Which Is the Better One?

Choosing between the two comes down to accuracy vs convenience. The current crop of optical sensors present in all wearables do not offer maximum accuracy and the cheap ones are even affected by strong movements and external light. That’s why optical HR sensors require an accelerometer to detect sudden movement and a signal processor to clean up and process the data.

Bpm Difference
The Differences | Wareable

And all this adds to latency issue, with your current heartbeat shown with a delay of seconds. While everybody is not training for next Olympics, this latency can be problem for serious athletes doing HIIT.

To reduce the effect of movement, the optical heart rate sensor is being added to earphones, as they remain steady during workouts. The Samsung Gear Icon X and Bragi Dash are the two examples.

Bragi Dash
At $299 they are expensive though

Comparatively, chest strap sensors are more accurate(being in proximity to heart) with almost nil latency issue. And they are cheap. You can get a decent chest sensor for as low as $30 while a optical sensor is always embedded in some kind of wearable, inflating the price. Where they fall short is in ease of use. Wearing a strap during exercise may not be likened by everyone and a companion device, either your smartphone or a watch, is required to view and store the readings. That said, if you are only after giving your workouts a little scientific advantage at minimal cost, a chest strap HR sensor is a your option.

Keeping It Healthy

So now you know how heart rate affects your performance. But don’t just start working out at your max HR in hope of burning that flab away. No, it doesn’t work like that. It should be done slowly, with increasing intensity over time. One method, as described by folks over at Polar, is shown below.

Hr Zone Training Plan

And as a general, our “don’t say we didn’t tell you”advice would be to consult your physician if you have any pre-conditions before embarking upon any strenuous fitness plans. Do share your thoughts & views through comments.

Last updated on 03 February, 2022

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