The Science of HRM – Why Strapless Wrist-Readers Can NEVER Be As accurate As Straps

We have commented recently about the rapid growth of fitness bands, and how increasing numbers of them are now coming with a wristband heart rate monitor. This feature is leading to much greater mass market appeal for the devices, as it adds a genuinely useful advancement – taking their data from nice to have to much more insightful, especially during workouts. However there may be a problem: they could be misreporting your pulse rate.
We were made aware of the story of Sharon. She found out the hard way that are pulse rate, as recorded by her fitness band through her wrist, was inaccurate. The hard way you ask? well yes – it was her cardiologist who had to tell her the bad news.
In her doctors office, whilst pounding away on the treadmill with an EKG machine hooked up she was informed that her heart rate was 146 beats per minute. This was measured by a medical device, in a controlled environment with a prescribed 99% accuracy rate. Her fitness band – at the same moment in time – read just 93 beats per minute.
And immediately this throws up a major flaw with the latest round of fitness band growth. You see, the introduction of the HRM functionality into bands has widened their market, and has led to traditional chest-strap based manufacturers – think Polar and Garmin – rushing to release a strapless band version of their usual tech.
This is great, because suddenly the old annoying strap based readers, useful only for workouts can be consigned to the scrap heap and in return we can use the activity tracker band al day – gaining much more ‘around the clock’ data which informs out health and fitness both before, during and after exercise. However, if these devices are much less accurate than their older and slightly more annoying cousins, then how much use are they for athletes and serious trainers who are trying to use the data to improve?
Well if they’re out by 63%, as indicated by Sharon’s data, then the answer has to be ‘not very useful at all’. A year ago, fitness bands were only really designed for monitoring the most basic of activity – hence their original positioning as ‘activity trackers’. It was all about how far you walked, and how well you slept and other basic data points. Nice to know, but quite separate from the serious business of training. Many gym goers and athletes – the data driven obsessives amongst us – the markets early adopters – liked to use the bands to help inform us around the rest of our day. Others who followed us into the market used them to encourage greater activity and to gain better accountability. Which was great, and many overweight folks have seen great benefits in using these devices.
strapless HRM
However today they’re being repositioned as ‘fitness bands’ and armed with pulse readers – and overnight they go from supplementary [to our real HRM] to our fitness and become a much more integral part of our fitness tracking and measurement. They’re suddenly integral because we’re trusting them to measure our workouts, and for serious athletes, the heart rate behaviour mid-workout is one of the best markers or improvement and performance.

The science: How do wristbands measure your pulse compared to straps?

Before we can dive deeper into this, we need to outline and identify the very different ways that these technologies work. This is where we get (ever so slightly) scientific on you.
The strap based method – or the traditional way that you would hook yourself up with a Polar or Garmin HRM – actually mimics the way that doctors and EKG machines read your pulse. They measure electrical pulse rates, and this is made both possible and easy by positioning the sensor near your heart. By picking up on the electrical signals coming from your heart, they’re using both a fairly – in the scheme of things – easy and accurate way to get a good reading and understanding of what your heart is doing.
This method does not work for strapless heart rate monitoring – this method is entirely different. So wristband monitors instead use special LED lights which light up the capillaries (small blood vessels) in your wrist. They then have a special sensor which sits next to the LED, and this uses the illuminated view of your blood vessels in order to form an opinion about the frequency that your blood pumps through.
Blood flow is not constant, it instead ‘pulses’ in line with the rhythm of your heart. The faster your heart is beating, the more of a stream – or hose like – manner in which your blood flows. The slower your heart beat, the more ‘pulse-like’ the flow. So the sensor seeks to count each blood surge, and that number equates to your pulse rate. Or at least it should.
The flaw is that the optical sensor reader requires perfect lab-like conditions in which to work. There can be no sweating. No moving. No tensing. No talking. No anything, really. All of this is a problem if you’re trying to measure heart rate output whilst exercising.
Sadly, that’s the least of the devices problems, because actually the bigger issue lies in the fact that reading a pulse rate at your wrist will never – scientifically – be as accurate as doing so from your chest. Why? Because by the time the blood arrives at your wrist, it has already been on a long journey.
Think about a river. Its source is in the mountains and its mouth is where it enters the sea. Between the source and the mouth, the water travels significant distances. Its similar for the human body. Your respiration system begins at your heart – and this is the source of the river. The heart pumps, and the blood begins its journey through the bodies blood vessels or river system. By the time if reaches your wrists it has already travelled a significant distance. What started out with a 100 beats a minute of intensity has lost some of its energy, and the energy waves – or pulses – have become less defined and thus less accurately readable. So the reading taken from your wrist does not accurately reflect your real heart rate.
(As an interesting scientific aside – the reading from the arteries in your finger tips is accurate.)

So there we have it – the science has debunked the idea that a wrist based sensor can ever be as accurate as a chest strap.

How big is this problem?

If wrist based pulse readers are being marketed as being as accurate as chest straps, then the market has a real problem – here is why

I also feel that the markets expectation is rising for these devices. We were very forgiving of the limitations of the generation one devices. They were made by companies like Fitbit – who have now built a flourishing and hugely successful business as indicated here – who were start-up companies. They had a great idea, and they did a lot of stuff right, but a lot of generation 1 products were quite clearly not the finished article. We bought in because we liked the direction, and now the growth in the market is indicative of higher expectations. The big boys are in the space now – Samsung, Apple, Asus, and the traditional fit-tech companies like Polar and Garmin. These guys don’t charge $29.99 for a plastic bangle with a funky companion app – they’re charging real money and are shifting product lines around (less heart rate monitors, more fitness bands) to reflect what we perceive as their belief in these products.
What we’re learning is, from a heart rate POV at least, we may be better sticking our finger to our wrist, or heck, sticking it in thin air rather than basing our stats off of a wrist worn tracking device. Alternatively – serious athletes may have to accept that a simple wristband based method for reading our pulse mid-flow is too good to be true – and if we want data that we can [get closer to] betting the house on, then we’re going to have to strap up.
Wrist based pulse readers will always have the stringent constraint of biology against them. Biology puts wrist measurement on the back foot. That’s because of the method of heart rate measurement used, as we have have discussed.
So we’re now looking at what amounts to a significant problem. The fitness band market is commercially booming but technically failing. How long will the market – the consumers – allow this to happen for? The bottom could literally fall out of this market virtually overnight as this data comes out.

Should I buy a chest strap monitor or a fitness band that reads my pulse from my wrist?

If accuracy is what you’re seeking, then having read this far then this is a daft question. However, before we over-react…
I believe there is a time and a place for the wrist based heart rate monitors – the fitness bands and smart watches – because they should be serving a different market.
Lets go back to our example of Sharon at the start of this feature. Sharon was pounding away at 146 beats per minute. Thats intense exercise. You’re unlikely to get to those sorts of BPS lifting weights, or jogging, or doing yoga, or riding the stationary bike. My point? this is specialised high intensity training (or fitness testing, in her example). This is not what fitness bands are for. If your gig is HIT exercise, or your an athlete, or otherwise a competitor, then of course you’re better with a strap.
However, in this non black and white world that we live in, you are better with a fitness band or smart watch if you’re simply looking to track more ‘everyday workouts’ and to measure the affect of walking an extra stop from the bus station. In this situation, we’re analysing trends not peak performance, and a fitness band is infinitely easier to use, less intensive, continuous in its measurement and overly practical verses getting strapped up.
So the bottom line point; chest straps are more accurate then wrist-readers. However wrist based reading open sup HRM to a much wider audience, and accuracy issues aside, the ability to measure trends here is a good thing. So lets not kill the HRMing fitness band just yet.
Whats your view? feel free to share in the comments below.