How fit is your Fitbit?

How reliable are fitness trackers?


(Note: this is the final blog post from trainer Jordan Sahlberg, who has moved on to exciting new professional challenges. Good luck, Jordan!)


Wearable fitness trackers have seen a massive increase in popularity with the everyday person interested in more accurately measuring and quantifying their fitness level. Companies like Fitbit, Jawbone, Nike, Apple, Garmin, MOOV and more offer many different devices aimed at doing just that. But before you go out and buy a device it is worth asking yourself a few questions. First, do these devices actually measure what they say they are measuring? And if so, how reliable are their measurements?

A review article from 2015 published in the International Journal of Behavioral Nutrition and Physical Activity looked to summarize the evidence for the validity and reliability of the most popular consumer-wearable activity trackers, more specifically those devices made by Fitbit and Jawbone [1]. For those unfamiliar with research speak, validity refers to the ability of something to measure what is says it is measuring. So, in the case of the Fitbit Classic, when it says it accurately measures your step count for the day, is it actually accurately measuring your step count? Something with high validity measures what it says it’s measuring while something with low validity does not do a good job of measuring what it says it is measuring.


Reliability, on the other hand, refers to the repeatability of a measurement and is the degree to which an assessment tool produces stable and consistent results. To better understand reliability consider the following example. Let’s say we have you walk for 10 minutes on a treadmill while wearing a Fitbit wrist device. You come back into the gym the next day and we do the test again, keeping everything the same (duration, speed of the treadmill, ect) and we do this every day for a week. A reliable device would measure the same amount of steps on each day for all seven days (or, at least, it would be very close). With these devices we want something that is valid AND reliable. So, what did the researchers find?


Well, they looked at five different measurements; step count, distance traveled, physical activity level, energy expenditure and sleep. And in order to create a clear picture I think it is best to break down each measurement below.


Step count:
There was a high amount of validity and reliability for step count across devices (Fitbit and Jawbone). So, when it comes to assessing the number of steps you take in a given day, Fitbit and Jawbone both deliver. However, it is worth noting that hip worn trackers generally outperformed wrist-worn trackers for step accuracy.


Only 1 study assessed the validity and reliability of distance walked and they found that distance was over-estimated at slower speeds and underestimated at faster speeds [6].
Physical Activity:
In general, none of the devices were able to accurately measure physical activity. They tended to overestimate the number of minutes per day spent exercising at a moderate-to-vigorous level.


Energy expenditure:
Energy expenditure refers to how many calories you are burning due to exercise, work, daily activities, etc. It seems that energy expenditure was underestimated. Fitbit and Jawbone showed poor validity.


In general, the devices tended to overestimate total sleep time and sleep efficiency and underestimate wake after sleep onset, which is to say that they do not accurately measure sleep parameters.


So, this is all well and great, but what is the takeaway? I think the authors summarized their findings well when they said, “No single specific tracker had a complete assessment across the five measures” and “the review indicated higher validity for steps, fewer studies on distance and physical activity, and lower validity for energy expenditure and sleep”. So, most devices made by Fitbit and Jawbone do a great job of accurately counting your steps. But, they are much less dependable when it comes to determining physical activity level, energy expenditure, distance traveled and time spent sleeping. This seems to be the general consensus at this time from other research [2, 3, 4]. However, it is worth mentioning that there is a lot of variability between different devices and manufacturers [1].


But wait! Does this mean that these devices have no benefits to helping people achieve better health and fitness? Of course not! In fact, using these types of monitors has been shown to help people with goal setting, self-monitoring and other behavior changing techniques associated with achieving a healthy lifestyle [5]. In fact, an article published in the Journal, Psychology of well-being, surmised that, “Through incorporation in daily life, they [wearable activity trackers] offer new social experiences, new ways of boosting self-esteem and getting closer to our ideal selves. It is not only about health and loss of weight. The experiential perspective reveals how activity trackers become pivotal for establishing practices of health, which will increase individual wellbeing” [7].


In conclusion, most activity trackers (specifically Fitbit and Jawbone devices) do a very accurate job of measuring step count and hip-worn trackers outperform wrist-worn trackers. However, when it comes to other measurements such as distance traveled, physical activity level, energy expenditure and sleep they vary widely from device to device and are not particularly accurate with their measurements. Finally, beyond the raw data and metrics there is high value in such a device in helping people set goals, track changes and boost self-esteem, all of which help to increase individual well-being.



1. Evenson, K.R., Goto, M.M & Furberg, R.D. (2015). Systematic review of the validity and reliability of consumer-wearable activity trackers. The International Journal of Behavioral Nutrition and Physical Activity, 12(154), 1-22. DOI 10.1186/s12966-015-0314-1

2. Kooiman, J.M., Dontje, M.L., Sprenger, S.R., Krijnen, W.P., Van der Schans, C.P. & de Groot, M. (2015). Reliability and validity of ten consumer activity trackers. BMC Sports science, Medicine and Rehabilitation, 7(24), 1-11. DOI 10.1186/s13102-015-0018-5

3. Brook, S., An, H.S., Kang, S.K., Noble, J., Berg, K. & Lee, J.M., (2016). Concurrent Validity of Wearable Activity Trackers 1 in Free-living Conditions. Journal of Strength and Conditioning Research, [print ahead of publication].

4. Ferguson, T., Rowlands, A.V., Olds, T. & Maher, C. (2015). The validity of consumer-level, activity monitors in healthy adults worn in free-living conditions: a cross-sectional study. The international journal of behavioral nutrition and physical activity, 12(42), 1-9. DOI 1186/s12966-015-0201-9.

5. Lyons, E.J., Lewis, Z.H., Mayrsohn, B.G. & Rowland, J.L. (2014). Behavior Change Techniques Implemented in Electronic Lifestyle Activity Monitors: A Systematic Content Analysis. Journal of medical internet research, 16(8).

6. Takacs, J., Pollock, C.L., Guenther, J.R., Bahar, M., Napier, C. & Hunt, M.A. (2014). Validation of the Fitbit One activity monitor device during treadmill walking. Journal of Science and medicine in Sport, 17(5), 496-500.

7. Krapanos, E., Gouveia, R., Hassenzahl, M. & Forlizzi, J. (2016). Wellbeing in the Making: Peoples’ Experiences with Wearable Activity Trackers. Psychology of well-being, 6(4), 1-17.