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Fitness trackers make simple measurements using an accelerometer

Fitness trackers seem to work like magic, so how can a wristband measure things like your heart rate or even, in some cases, blood pressure? These devices have some tricks to figure out what's going on in and around your body.

Step counts are measured using an accelerometer, a sensor that can detect movement in all three directions – up and down, side-to-side and back and forward. The device uses a complex algorithm based on decades of testing to interpret certain patterns of movements as a single 'step'.

This method isn't foolproof – if the device is put in a different location than expected, or simply left lying around, then steps won't be accurately measured. Doctor James Sanders, a Senior Research Associate in Digital Health at Loughborough University's National Centre for Sports and Exercise Medicine, has an example of this. He mentions that when his wife was walking their son “it wouldn't pick up that she'd gone for a walk because her hands were always stuck on the pram.” No hand movement meant no steps counted.

On the most basic models, the distance travelled is a simple extrapolation of your steps. If your device knows how tall you are, it can work out your stride length and then multiply this by the number of steps you’ve taken. More accurate measurements can be achieved with devices that include a GPS.

Heart activity can be monitored by fitness trackers for more complex measurements

Measuring your heart rate through a device uses a clever bit of science. Instead of measuring your heartbeat directly, fitness trackers instead measure your blood flow. A blinking green light on the underside of the fitness tracker shoots into the capillaries just below the skin's surface. Since blood is red, it absorbs some of this green light whenever it pulses through, the rest of the green light is reflected back to the sensors on the underside of the watch. This data is then used to create heart rate.

Measuring your stress levels is a bit more complicated. The device measures your heart rate variability – the variation in time between beats of the heart – to see how well it adapts to increased stress. James says a higher amount of variability demonstrates your body is having the correct physiological reaction to stressful conditions.

The last measurement, blood pressure, seems incredible. How can a simple wristwatch detect what usually takes a blood cuff to determine? Some watches do constrict in the same way as a blood cuff to obtain your blood pressure. However, James says they're generally less accurate, as they're measuring from your wrist rather than further up your arm. Other devices measure something called pulse wave time, which is essentially changes in your blood pressure over time – but it won’t tell you what your baseline is.

Fitness trackers are generally reliable when taking bodily measurements

To get an idea of what these devices offer, Greg tried out a range of watches at different price points – alongside a research-grade heart rate strap on his chest, which is seen as the gold standard for measuring heart rate. Fitted out in fitness tech – five watches, an upper arm band and a chest strap Greg put the devices to work.

Active heart rate was by far the most accurate measurement across the board, with even the cheapest device registering a consistent reading.

Active heart rate was by far the most accurate measurement across the board, with even the cheapest device registering a consistent reading. Ironically, resting heart rate was more difficult. Both the cheapest and one of the most expensive devices measured a heart rate almost twice that of the chest strap. Step count was consistent between every device, except for one watch that added a massive 20% to the real step count.

By far the least consistent measurement was calories burned. Doctor James Sanders explains that figuring out how many calories we're burning is an incredibly complex calculation, given that fitness trackers “don't take into account all the other processes that are going on in your body”. In Greg's experiment, all five of the devices gave different results, lacking reliability.

It's worth remembering, of course, that Greg's tests aren't exactly “proper scientific research. It's one person, and only brief, short tests.” As such, none of these personal results should be relied upon as definitive evidence of any device’s ability. And it's also worth saying you wouldn't usually wear this many fitness trackers at once – so some of them might not have been in the best position on his arm to take accurate measurements.

Price doesn't always translate to quality with fitness trackers

For some of the measurements tested, the more expensive fitness trackers were actually less accurate than cheaper alternatives, for example with step count. But James notes that “we can take a dip in accuracy with these wearables as long as they're reliably inaccurate.”

For example, if your device always overestimates your step count by the same amount, then you can still compare your results to one another across many days. In that way, the cheaper device may still get the job done depending on what you’re tracking.

Ultimately, the success of a fitness tracker is all about finding the right device for you

Doctor James Sanders is of the opinion that price shouldn't be your main concern when buying a fitness tracker. It's far more important to find a device that has features that you'll use often, since good fitness is all about good habits.

The more expensive devices have an array of note-taking and organisational apps that encourage users to keep them on their wrists. For other users, a more purpose-built device might give them more confidence. On the whole, all fitness trackers are shown to provide a small-to-modest benefit in the first six months of their use, giving users the motivation to get back out and start exercising.

To hear more from Greg's investigations, and to find out what fitness trackers can tell you about your sleep quality, listen to the episode in full on 大象传媒 Sounds.

The information contained in this article was correct at the time of broadcast on 29 February, 2024.