How does the accelerometer in my device measure how far I run on the treadmill?

I'm exhausted, yet I must keep running. The Brunswick settlement needs to be warned about the approaching zombie horde. As the song I’m listening to ends, static fills the comms channel. Sam delivers the bad news: the zombies are closer than I am, and I need to increase my pace to reach the settlement in time. With lives on the line, there's no option but to push myself for a burst of speed.

Of course, all of this happens safely at home on my treadmill. The fatigue I feel is very real. The apocalyptic scenario, however, comes courtesy of the "Zombies, Run!" app, which transforms running and walking workouts by layering an intense survival storyline over them. I love immersing myself in these fictional challenges while running, but in reality, I’m training for a half marathon. I need to track my progress and meet my running goals to be ready for the event. It’s crucial to know my distance and pace.

There are plenty of running apps available for iOS and Android that can track your time and distance. Almost all of them feature built-in GPS tracking, widely considered the most accurate way to monitor your stats. However, sometimes circumstances prevent you from running outside, and in those cases, your app uses your device's accelerometer to track your pace and distance. But how does it work, and are these measurements reliable?

Can an Accelerometer Determine Its Own Boundaries?

Accelerometer motion sensors detect changes in direction and velocity — or acceleration. If you carry a mobile device in your pocket while running, the accelerometer continuously tracks your movement and cadence, calculating your speed as you accelerate into each stride and decelerate after your foot strikes the ground.

The Apple App Store and Android Marketplace are flooded with running apps, yet many of them don't provide indoor distance tracking. Why is this? While using GPS to track your distance when running outdoors is fairly straightforward, determining the distance you cover indoors on a treadmill is more complex.

Alex Macmillan, the lead developer of the "Zombies, Run!" app, points out one of the challenges of using an accelerometer to track a runner’s distance on a treadmill is the accelerometer itself. He notes that "Accelerometer data can be very 'noisy,' which makes it difficult to interpret. You need to be able to distinguish the runner's motion from the movement of the phone just bouncing around in their pocket."

Another obstacle highlighted by Macmillan is the immense amount of data required to compute distance statistics. As he explains, "You need to read new data (or 'sample') from the accelerometer very frequently in order to get good results. This means you have to write code fast enough to perform all of your calculations in the space between one sample and the next."

When you factor in the challenge that you’re not actually moving anywhere on a treadmill, it becomes evident that programming your phone to measure indoor distance is a considerable challenge. So, how do developers handle these hurdles?

You've Just Completed a 5K ... Or Have You?

One relatively simple way to calculate distance using an accelerometer is by programming it to count steps like a pedometer, then multiplying the step count by the runner's stride length to determine the total distance. Voila! Distance achieved. Well, kind of. While many apps use this method for treadmill running, it comes with the risk of a large margin of error.

The process requires input from the runner. For example, you’ll need to provide your height and weight so the app can estimate your stride length — but what if your legs are proportionally longer than someone else’s with a similar height? Some apps also ask you to run a known distance, like a mile (1.6 kilometers), while using the app, then tell the app how far you ran, so it can adjust the estimates to fit you more accurately. The "Nike+ GPS" app for iPhone, for instance, allows you to update your settings by using its "calibrate run" option, which recalculates your stride length for future runs based on your input.

What if you're using run/walk intervals in your training? Your walking stride is likely to differ from your running stride. What if your calibration run was either faster or slower than your usual pace? Or what if your gait is a bit inconsistent, with each stride being slightly different in length? As Alex Macmillan pointed out, "If the stride length measurement is wrong by even a small amount, the total estimated distance can be wrong by a long way." If you've used a running app while on a treadmill, you might have noticed a gap between the distance the app reports and the distance the treadmill shows — a sign of this issue. This is far from ideal when you're preparing for long-distance runs like a half marathon.

Macmillan’s app, "Zombies, Run!" doesn’t yet support accelerometer-based distance tracking, but his team is working on it. They’re using treadmill research along with a new approach to calculating distance. According to Macmillan, "By only needing to measure the relative change in a person's speed, rather than trying to precisely estimate the runner's actual speed, we believe we can create a model that works for a wide range of runners. In this way, we hope the runner's stride length, height, and weight won’t matter when we make our calculations." Will this method work? Only time will tell.

Why not just let your treadmill track the distance? After all, most treadmills come with digital displays showing how far you’ve run based on the belt’s movement beneath your feet. However, a quick search for "treadmill distance accuracy" will reveal that many people believe the numbers on your display might not be entirely accurate. Even a minor error in belt length can lead to a significant miscalculation over the thousands of revolutions it makes during a 5K training run.

So, as a runner, what can you do to track your treadmill time as precisely as possible?

Tailor Your Setup for Optimal Results

In the end, if you plan to run indoors and get precise data, you'll need to put in some effort. The first step is to ensure that your app includes calibration inputs, since not all apps have this feature. Once you find an app with this functionality, here’s a way to test and fine-tune its calibration for maximum accuracy:

Choose a location to run where you're absolutely sure of the distance.

If there’s a standard track available nearby, that’s ideal. If not, consider running outdoors with a GPS device to measure a known distance, such as 1 mile or 1 kilometer, and be sure to note the precise start and end points.

Configure your app for an indoor run. Whether you're indoors or outdoors, this step is crucial, as it tells your phone to rely on the accelerometer instead of GPS tracking.

Head out for a run! There are a few key factors to consider here. Be sure to start and stop at the exact points required for accurate distance measurement. If you're training with run/walk intervals, use your current running-to-walking ratio during this run. The longer the distance of your calibration run, the more accurate the results will be. More data provides better calculations.

Enter your distance data after completing the run. Your app will divide the total distance by the number of steps recorded by your accelerometer to calculate your average stride length. This is why it’s crucial to include your current interval timing during the calibration run. If you only run during the sample data, it won’t account for stride changes when walking.

Test it out. Go for another run on your known-distance course with your app set to indoor tracking mode. Check if the app calculates the correct distance. If it's slightly off, you can try repeating all the steps at a longer distance to provide a better data sample for calibration.

If your running habits change, or if your weight fluctuates by several pounds, you’ll need to recalibrate. Losing weight typically makes you faster, and if you shift your interval plan to include more running and less walking, it will also affect your stride averages.

Any app that tracks indoor distance relies on calculated estimates based on either the data you input or a predefined average. If tracking your indoor distance accurately is important to you, testing your calibration for precision is essential. The key term here is 'estimate' – achieving 100% accuracy with the common method of multiplying steps by average stride length is highly unlikely, but you can still effectively monitor your progress as you run longer distances and increase your speed.