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Can Fitbits Actually Read Your Heart Rate?

by | August 30, 2018

Technology is always evolving. What couldn’t have been imagined 100 years ago is now in control of our lives. Internet, mobile phones, Tinder are all things we would struggle to live without.

In the fitness world, this works similarly. Exercise used to be done through manual labour and daily living (including moving 2.5-tonne limestone if you’re an Egyptian). Now the only ‘real’ way to exercise is in a gym with barbells and treadmills. And without uploading the workout to Instagram, does it actually count?

Picture of a smart phone.

Another evolvement of technology is to assess how effective a workout has been. In the past, this was done by asking ourselves ‘how do you feel?’. But this was obviously not objective enough, so there are now trackers worn around the wrist which monitor heart rates. These trackers are able to calculate whether we’ve worked hard or not, in theory anyway. They can typically tell the time too, which in this unpunctual world should be appreciated!

Fitness trackers are incredibly popular today. Many different choices are available, with the most famous being Fitbit, Apple Watch and Samsung gear, to name but a few. Back in 2015, it was said 25 million people worldwide have some form of wearable fitness tracker, with this number expected to grow.

However, how confident are we that the trackers are actually accurate? Not all new technology has such an impact as the internet or mobile phones. In fact, very few ideas ever truly take off, remember the Knife-Wrench?

Portable heart rate monitors have been around since the 1980’s, however, have always needed a chest strap as well as a wrist receiver to calculate heart rate. In the last 10 years though, fitness trackers have used what is know as photoplethysmography (PPG) to asses heart rate, meaning only a wrist strap is needed.

Picture of a heart rate monitor.

PPG is a technique which uses a light (usually LED) on the back of the wrist strap. The light shines onto the skin which calculates changes in blood flow. Greater volumes of blood in the arteries cause changes in the direction of the light shining through. Heart rate is then assessed through the rate of changes in light. This non-invasive technique has become popular due to the relative ease it is able to detect heart rate.

There have been a couple of studies which have compared gold standard devices (chest and wrists strap) to PPG techniques such as Fitbit and Apple watches. One study tested people on a treadmill where they gradually increased their speed. The results were similar in both the gold standard and PPG techniques.

Another study had people cycle and use a treadmill, also gradually increasing the challenge of the exercise. Although this study too concluded similar results between the techniques, on closer inspection, there was almost a 10 beat difference between the chest-worn monitor and the Fitbit device. That’s quite a difference! Another issue with this study was that it was funded by Coca-Cola, who likely have a bias towards activity trackers working, to take notice away from unhealthy nutrition being the cause of poor health.

Some mixed results there. However, in total, there were only 72 people who took part in both studies. This isn’t a vast amount of subjects to make definitive conclusions from. There has actually been a third study which also found differences between the 2 techniques. However there were only 11 people in that study, so barely worth calling that science (I’m kidding, sort of).

So what conclusions can be made? When a wrist tracker is worn during a relatively steady state exercise session, heart rate appears to be reasonably accurate. For example, going to the gym and using a treadmill/cross trainer at a moderate intensity.

However, when intensity is increased quickly, large variances of heart rate occur. These types of changes in intensity are a regular occurrence throughout the day in most of us. Something as common as standing from a chair to grab the phone or chasing a bus can spike our heart rate. Recovery to normal levels should happen relatively quickly, but over the course of a day, these occurrences can cause misleading numbers on the trackers.

Picture of a fit-bit

Therefore we should be cautious with how we interpret the readings we get from our fitness tracker. There are indeed benefits to them, especially when performing moderate aerobic exercise. Also, the trackers can be great for assessing how much we move by using the step count. But when doing higher intensities such as weightlifting of interval training, then a chest strap monitor would be a better choice.

Alternatively, using a vomit scale may be another option (1=no vomit, 10=all vomit, only vomit, oh god please stop*).

*not currently backed by science…yet

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