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The Dark Side of Fitness Trackers

Almost a third of people in the UK own a fitness tracker (Statista, 2021), with Fitbit making up 41% of wearable fitness devices, followed by Apple watches at 31% (Statista, 2020). It’s easy to see why fitness trackers are so popular - one of the most common reasons for wearing them is the accountability they provide to help you stick to your exercise goals, whether that’s through sending you alerts to break up your sitting time, daily step targets or weekly intensity minutes to reach. And it’s clearly working - a review of 19 studies found that behavioural self-monitoring using pedometers or smart watches can significantly reduce sedentary behaviour (Compernolle et al., 2019). However, before you rush off to buy yourself the latest fitness wearable (I’ve heard that smart rings are set to be the new thing in 2021…), here are some of the potential problems with fitness trackers that you might not have considered before.

Their accuracy is questionable

The first problem with fitness trackers is their accuracy. Research with 60 adults aged 21-64 showed that although heart rate data for seven popular wrist-based fitness trackers, including the Apple Watch and Fitbit Surge, was accurate to within 5%, energy expenditure was off by between 27% and 93% (Shcherbina et al., 2017). This finding was confirmed by another more recent study, which concluded that none of the four fitness trackers it studied (Apple Watch Series 4, Polar Vantage V, Garmin Fenix 5, and Fitbit Versa) should be used to monitor energy expenditure for sitting, walking or running as they were all so inaccurate (Duking et al., 2020).

So if you’re using your fitness tracker for heart rate data, carry on, as you’re probably getting fairly accurate readings. But if you’re using it to calculate your daily calorie burn, you might as well not bother, as it’s probably not hitting the mark. The reason for this is because fitness trackers are based on machine algorithms, and humans are not machines. To illustrate just how inaccurate fitness trackers can be for energy expenditure, I ran 5km on a treadmill and my smart watch said I had burned 165 calories. Meanwhile, the treadmill said I’d burned 453. In reality, both were way off - I probably burned more like 300 calories. So don’t let the fitness trackers fool you!

They can cause addictive behaviour

My second concern regarding fitness trackers is that they can lead to addictive behaviour. Martin Lewis, the founder of tells the Guardian of his so-called ‘healthy obsession’ with step-counting, saying that he’s taken no less than 10,000 steps per day in the past three years. However, he says he’s never happy if he ‘just’ does 10,000 steps, and he aims to get more like 25,000, even if it involves walking around the sofa in the evening to reach his goal. Lewis also admits that his walking was at one point controlling his life too much and dictating where he went and what he did each day (Tapper, 2019). The DSM-5 (Diagnostic and Statistical Manual for Mental Disorders) classifies exercise addiction as a behavioural addiction, and defines it as when ‘a person's behaviour becomes obsessive, compulsive, and/or causes dysfunction in a person's life’ (Degner, 2017). To me, the behaviour described above fits this definition as it appears to be obsessive and compulsive.

Obsessive and compulsive are two words that also come to mind when I look at the findings from a survey of 200 female Fitbit users, of whom 59% said their daily routines were controlled by their Fitbits and 79% felt under pressure to reach their daily movement targets. Moreover, 45% of respondents felt ‘naked’ without their Fitbits on and 43% felt that any activity they did without them was ‘wasted’ (Duus & Cooray, 2015), suggesting that people are becoming quite literally addicted to exercising with their fitness trackers.

It’s estimated that 3% of the general population are addicted to exercise, although this number is much higher in athletes (Szabo et al., 2015). Exercise addiction can lead to injury, illness and loss of menstruation in women, and it’s also associated with anxiety, depression and eating disorders. Research with almost 500 students found a link between using fitness trackers and eating disorder symptoms (Simpson & Mazzeo, 2017), which suggests that tracking our exercise may not always be good for our health, despite what the marketers of these devices may have us believe.

Fitness trackers can also feed into exercise addiction by promoting social comparison through data-sharing. Sport psychologist Martin Turner says this makes it easy to feel inadequate, because there’s always going to be someone who’s training more than you or running faster than you. This can be particularly problematic if we base our self-worth on our athletic achievements, and it’s important to recognise and challenge these damaging beliefs to recover from an exercise addiction (Kelly, 2019).

Perhaps one problematic assumption related to fitness trackers is the idea that we need to do 10,000 steps per day. This number came from a Japanese company marketing a pedometer called a ‘Manpo-kei’, which in Japanese means ‘10,000 steps meter’. However, this number was not based on any science. Since then, research from Harvard Medical School with 20,000 older women has looked at the importance of 10,000 steps for health, and although they found mortality rates gradually decreased when women took up to 7,500 steps per day, it levelled off after 7,500 (Lee et al., 2019). The authors also found that taking just 4,400 steps per day was associated with significantly lower mortality rates than taking 2,700 steps. This suggests that although some daily physical activity is important for preventing mortality, there’s no need to stress over hitting 10k a day – it’s not a magic number. Even if you only get half this amount on some days it’s still beneficial for health.

Are we forgetting how to trust our bodies?

My final and perhaps biggest gripe with fitness trackers is that I worry we’re losing the ability to trust our own bodies. Fitness tracking is an example of the ‘quantified self’ movement, i.e. ‘self-knowledge through numbers’. The premise of this is that we need to use technology to tell us what’s going on inside our body and how we should behave. When it comes to exercise, movement is reduced to data and there’s no consideration of how we feel. This is completely at odds with the idea of ‘intuitive movement’, the practice of listening to our body to figure out how it feels and what type of movement it needs that day, and honouring that. Rather than relying on data, intuitive movement involves paying attention to how we feel, avoiding rigid structure and prioritising joyful movement (Haggerty, 2019).

I fear that this obsession with quantifying everything we do can ultimately impair our ability to enjoy exercise. My opinion seems to be supported by research, as one study found that students who chose to wear a pedometer to track their steps for a day did not enjoy their walking as much as those who didn’t track their steps (Etkin, 2015). The authors of this study suggested that this was because tracking steps made the students feel less intrinsically motivated (i.e. motivated by internal factors). As a result, walking felt like more work and they enjoyed it less.

What’s the solution?

I’m definitely not saying that you should throw away your fitness tracker, but I think most of us would benefit from being more intentional with how we use them. We should just track the metrics that matter to us, and not stress about the rest, particularly the energy expenditure which is unlikely to be accurate. I find it useful to know my heart rate and pace when I run, because this data can help me to improve. However, I don’t use my fitness tracker when I lift weights – I monitor how I feel and focus on my exercise technique, because no tracker can tell me how well I’m squatting.

I think it’s also important to take a break from our fitness trackers every now and again, and to instead focus on how we feel and our surroundings whilst we exercise. After all, a workout is one of few times in the day when we don’t have to be staring at a screen, so we should take advantage of that opportunity. It would probably do us all a lot of good.


Compernolle, S., DeSmet, A., Poppe, L. et al. (2019). Effectiveness of interventions using self-monitoring to reduce sedentary behavior in adults: a systematic review and meta-analysis. International Journal of Behavioural Nutrition and Physical Activity, 16(63).

Degner, D. (2017). Rapid response to ‘Addiction to exercise’. BMJ. Retrieved from:

Düking, P., Giessing, L., Frenkel, M.O., Koehler, K., Holmberg, H.C., & Sperlich, B. (2020). Wrist-worn wearables for monitoring heart rate and energy expenditure while sitting or performing light-to-vigorous physical activity: validation study. JMIR Mhealth and Uhealth, 8(5):e16716. doi: 10.2196/16716.

Duus, R. & Cooray, M. (2015, June 19). How we discovered the dark side of wearable fitness trackers. The Conversation. Retrieved from:

Etkin, J. (2016). The hidden cost of personal quantification. Journal of Consumer Research, 42(6), 967–984.

Haggerty, J. (2019). My 10 principles of intuitive movement [Blog post]. Retrieved from:

Kelly, N. (2019). ‘I was addicted to exercise’. BBC World Service. Retrieved from:

Lee I., Shiroma, E.J., Kamada, M., Bassett, D.R., Matthews, C.E., & Buring, J.E. (2019). Association of step volume and intensity with all-cause mortality in older women. JAMA International Medicine, 179(8), 1105–1112. doi:10.1001/jamainternmed.2019.0899

Shcherbina, A., Mattsson, C.M., Waggott, D., Salisbury, H., Christle, J.W., Hastie, T., Wheeler, M.T., & Ashley, E.A. (2017). Accuracy in wrist-worn, sensor-based measurements of heart rate and energy expenditure in a diverse cohort. Journal of Personalized Medicine, 7(2), 3.

Simpson, C.C., & Mazzeo, S.E. (2017). Calorie counting and fitness tracking technology: Associations with eating disorder symptomatology. Eating Behaviour, 26, 89-92. doi: 10.1016/j.eatbeh.2017.02.002.

Statista (2020). What brands are your personal smart watches / fitness trackers?. Retrieved from:

Statista (2021). Share of respondents who own a smart watch/health-tracker computing device in the United Kingdom in 2019 (UK), by generation. Retrieved from:

Szabo, A., Griffiths, M. D., de La Vega-Marcos, R., Mervó, B., & Demetrovics, Z. (2015). Methodological and conceptual limitations in exercise addiction research. The Yale Journal of Biology and Medicine, 88(3), 303–308.

Tapper, J. (2019, November 10). A step too far? How fitness trackers can take over our lives. The Guardian. Retrieved from:


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