Fitness trackers: Accurate? Or costly accessory?

First, we must understand energy expenditure

Fundamentally, fitness tracking devices use sensors on our skin to collect data. They then convert that data into measurable constructs such as steps, heart rate variability (HRV), sleep quality, and of course, caloric (or energy) expenditure.

Understanding how to calculate the ‘calories out’ portion of the energy balance equation requires the input of many different factors. These factors include:

Are fitness trackers capable of measuring our energy expenditure accurately?

In a recent meta-analysis, O’Driscoll and colleagues (2020) reviewed 60 studies that included a total of 1946 participants. The included studies compared the accuracy of 40 different fitness trackers in measuring total energy expenditure (TEE).

The findings showed that the accuracy of the devices varied depending on the activity it was measuring such as cycling or stair climbing. They also found that research-grade devices were superior in estimating TEE compared to the commercial-grade devices (i.e. Fitbit, Apple watch), which demonstrated a significant underestimation of the participants’ TEE.

The effect size – a statistical quantification of the size of a study difference - was -0.68, indicating a strong difference. Practically speaking, this means that the number of calories burned on your device is more than likely going to be inaccurate due to the large underestimation.

Similarly, a single study conducted by Hajj-Boutros and colleagues (2022) found that the Apple Watch 6, Polar Vantage V2, and the Fitbit Sense all showed poor accuracy in assessing energy expenditure (EE) over 5 different physical activities.

In fact, the R-value, which measures the strength of the correlation between true EE accuracy and the estimations according to the devices were all bearers of bad news. The Apple watch (r=0.05), Polar Vantage V (r=0.17) and Fitbit Sense (r=0.55) all had an r-value that signified ‘impractical’, and ‘very poor’, respectively. Although the Fitbit Sense came out on top, this once again showed that using data from commercial-grade fitness tracking devices to measure EE is likely to carry a large degree of error.

Of somewhat concern, one study that measured the Oura ring’s accuracy for measuring EE actually found that it overestimated people’s step count by an average of 1416 steps per day and even overestimated their daily sedentary time (Niela-Vilen et al., 2022).

The ‘ideal line’ (also known as the ‘line of best fit’ for those interested) is a line on a graph that expresses a relationship between two variables. In this instance, the relationship was between the accuracy of the Oura ring compared to an ActiGraph (a research-grade activity tracking device) in relation to both step count and sedentary time. The results showed a large difference between the two, with the Oura ring generating significant overestimates.

In other words, using the Oura ring to count your daily steps or to track other physical activity metrics is a very imprecise method to do so.

Fitness watches can still be beneficial

Before you throw your Apple Watch in the bin, understand that commercial-grade fitness tracking devices – despite the inaccuracies - are unlikely to hinder your results. In fact, a study conducted by Brickwood and colleagues (2019) found that people who wore a fitness tracking device increased their physical activity throughout the day including their step count, levels of moderate and vigorous activity, and TEE, compared to people who didn’t wear one. The effect sizes were small, yet statistically significant, and highlighted the psychological component of health-seeking behaviours.

Further, Ryan and colleagues (2019) conducted an online survey with 237 respondents, measuring their positive and negative emotions related to their fitness tracker. The results demonstrated that people experience more positive emotions whilst wearing their device and have 'little risk of negative psychological consequences'.

Overall, the psychological and behavioural implications of fitness trackers seem to be strongly positive.

In summary, although the numbers on your display might not be accurate, fitness tracking devices come with little inconvenience, while providing users with the capacity to monitor energy expenditure in relative terms, while encouraging health seeking behaviours and eliciting positive emotional affect.

Keep in mind, while fitness trackers do carry a high degree of error, there are few readily accessible alternatives. Until we’ve all got metabolic chambers at our disposal, using a commercial-grade fitness tracker is the next best thing.