Smart watch on wrist

How Wearable Fitness Trackers Work: Smart Watches to Fitness Rings

Fitness trackers were first invented in 1965 by Dr. Yoshiro Hatano, while he was researching ways to combat obesity. He proposed that walking 10,000 steps a day would provide the correct balance of caloric intake and activity-based calorie expenditure to maintain a healthy bodyweight. Fitness trackers still tend to use 10,000 steps as a benchmark today. So, let’s get into the nitty gritty of how fitness trackers actually work and how accurate they are.

How are steps measured?

A fitness tracker senses the movement of the body on a 3-axis accelerometer which tracks movement in every direction. Some trackers also come with a gyroscope to measure orientation and rotation, which is helpful in accounting for more complex motions.

The altimeter measures altitude, which isn’t just handy for hiking or mountain climbing, but for measuring the number of stairs climbed throughout the day.

All of the data is stored for processing, creating a personalised algorithm which detects the type of movement you are performing, such as walking or running. The movement is categorised into different activities which generates more information.

How is sleep measured?

Most trackers come with a ‘Sleep Mode’ function, which allows for the device to accurately measure sleep. Sleep is measured using a process called actigraphy, which allows the three-axis accelerometer to record movements made. Though most trackers that use actigraphy have a high margin of error, for example if you are just laying still in bed, it may be recorded as sleep.

Inside the tracker…

The more sensors your tracker has, the more accurate the reading. The Fitbit Charge 2, for example, uses optical sensors which shine a light on your skin to measure your pulse. The light illuminates your capillaries, and a sensor then measures the rate at which your blood is being pumped. These types of sensors are less effective than bioimpedance sensors for measuring your overall health, as bioimpedance sensors work by monitoring the rate at which a low-level electric current travels through the body.

How accurate are they?

Measurement of steps tends to be pretty accurate. However, for the best results its recommended to wear a tracker on your hip, or have your smartphone in your pants pocket, as wrist worn devices are likely to record small hand movements as a ‘step.’

In terms of monitoring heart rate, studies have shown that fitness trackers are apt at measuring the heart rate whilst resting, but not as effective at measuring the heart rate whilst working out as you get sweaty.

Calories burned differ for each individual and are difficult to estimate. You’ll be asked to input information like your height, gender, and age into your tracker upon set up, which will help to inform the data. However, the tracker won’t know certain information about your body which is necessary to accurately calculate the number of calories burned, such as the proportion of muscle and resting energy expenditure. A meta-analysis of 60 published studies found that fitness trackers are not very accurate at measuring energy expenditure, particularly for lower intensity activities such as walking or housework.

What’s next…

Wearable trackers have evolved from fitness watches to fitness rings. The most notable being the Oura ring. The ring is packed with two infrared LED sensors, two negative thermal coefficient body temperature sensors, a 3D accelerometer, and a gyroscope. Rather than taking measurements from the capillaries on the surface of your wrist, the Oura measures from the arteries within your fingers. It’s one of the only wearables that measures your temperature directly from your skin and needs to be on your finger for at least two weeks to understand all your baseline metrics.

Due the nature of the device, you’re unable to swipe through your stats like you would do on a smart watch, all your data is available via an app. The accompanying phone app enables you to see data on your activity goal progress, your readiness, and your sleep score. Readiness is a metric that factors in resting heart rate, activity levels and sleep, to let you know how ‘ready’ you are for the day ahead. However, where this differs to other wearable trackers is that rather than comparing your data against the general population, it uses deviations from your own baseline.

In terms of accuracy, the Oura ring is one of the most accurate wearable devices on the market. It’s resting heart rate function has a 99.9% reliability compared to medical-grade electrocardiogram (ECG) and was validated against the sleep lab golden standard.

The ring has 7 days of battery life, is waterproof and lighter than a ring you’d wear to accessorise. However, it’ll cost you £217, so if you’re only intending to track your step count it may be worth looking at some of the more basic functioning fitness trackers.


If you enjoyed this, check out our blog on the best running safety gear.


Back to blog