Determine your Fitbit stride length using a GPS watch

I have been carrying my Fitbit One for a little over two years with me and it keeps tracking my daily steps. It also tracks my distance covered by multiplying those steps using the stride length which you can either provide explicitly or implicitly setting your heights. In the winter of 2012 I bought my first Garmin Forerunner 410 (replaced by a Garmin Forerunner 920XT) GPS watch to help me track my running (and other outdoor) activities. Since then I have worn it at every activity. Additionally before every running session I started a Fitbit activity to measure my step count. Over the last 18 month I have recorded 69 runs using both, the Fitbit and the watch. I put the data in a csv file so I could load it into R. Then I performed a linear regression to determine my stride length.

<img src="https://i0.wp.com/blog.tafkas.net/wp-content/uploads/FitbitStrideLength.png?resize=600%2C400" alt="Fitbit Stride Length Regression" class="alignnone size-full wp-image-215" srcset="https://i0.wp.com/blog.tafkas.net/wp-content/uploads/FitbitStrideLength.png?w=1200 1200w, https://i0.wp.com/blog.tafkas.net/wp-content/uploads/FitbitStrideLength.png?resize=300%2C200 300w, https://i0.wp.com/blog le viagra de l.tafkas.net/wp-content/uploads/FitbitStrideLength.png?resize=1024%2C683 1024w, https://i0.wp.com/blog.tafkas.net/wp-content/uploads/FitbitStrideLength.png?resize=624%2C416 624w” sizes=”(max-width: 600px) 100vw, 600px” data-recalc-dims=”1″ />

In an ideal world, where the GPS watch would perform with a 100% accuracy and the Fitbit would recognize every single step taken, the regression fit would meet the origin. Unfortunately neither of them is that accurate. Our model gives us an intercept of 398.46m with a slope of 1.00421m and an adjusted R^2 of 0.97505 Based on that I have covered a whole lap of a competitive running track before I made a single step. 😉

I have noticed that the placement of the Fitbit has a huge impact on the amount of steps recorded. Attached to the pocket of my running shorts it seems to miss certain steps. I get much better results attaching it directly at my waist. Overall I am quite satisfied with the accuracy provided by the Fitbit. If you do a lot of interval training or vary your stride from session to session your result will be far more off.

The code for the regression is on Github:
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Downloading Fitbit Data using Google Spreadsheets

One of the most important features in quantified self is the ability to export your data in an open format. Fitbit lets you download your personal data if you subscribe to a premium membership. Alternatively they provide an API at dev.fitbit.com/ that allows developers to interact with Fitbit data in their own applications, products and services.

In a blog post at quantifiedself.com Mark Levitt shows a way how to export your Fitbit data into Google Spreadsheets. I explored to API myself adding and removing some of the fields to get more insights to the data.

In a future post I will delve into the data in order to understand some of my own physical activity patterns.

Update 14 November 2014: I removed the Active Score since it has been dropped by the Fitbit API