Chances are, you have at least one runner friend who obnoxiously posts daily accounts of her workouts on social media. (Or maybe, like me, you are that obnoxious running friend.) There's a reason that runners can't seem to resist posting about their runs. A new study has found that running is contagious, and it's all thanks to social media.

For the study, which appeared in the journal Nature Communications, researchers at the Massachusetts Institute of Technology looked at five years of running patterns from 1.1 million runners from around the world using data they had uploaded to social media accounts. They wouldn't say which social media platform they used, but they did say that it was one in which users' data was automatically uploaded after each run — so users weren't selectively choosing which workouts to post and which to delete. In fact, you would be hard-pressed to find a runner these days who isn't using a tool like Strava, Garmin, FitBit and RunKeeper to share their mileage and speed.

According to the researchers, it's the competitive spirit that make other runners run farther and faster. With these kinds of experiments, it can be hard to tease out whether or not people do more of any one thing because they're influenced by those around them, or if they simply surround themselves with people who share their passion.

So the team also cross-checked the running data with weather maps to determine if participants were influenced by their peers to go for a run. Because let's face it, if it's cold and windy in your area, you're less likely to get outdoors for a workout. But if your circle of running friends from other locations got their runs in, you might be influenced to do the same — regardless of the weather.

The study found this is exactly what happens. Running is so contagious, in fact, that runners tend to run longer and faster to keep pace with their friends. When runners in the study ran 10 minutes longer, their social media friends tended to run at least three minutes longer than normal. And when they went faster, their friends did the same.

Interestingly, while runners were inspired by faster runners, they seemed to be most influenced by runners who were slightly less fit than they were. Runners keep track of what their elite friends are doing, but they keep an even closer eye to see who might be gaining on them from behind.

Gender influences also played a role in running behavior. According to the data, men were influenced by both men and women, while women were only influenced by their female peers.

Researchers found that there was definitely strength in numbers when it came to running frequency and performance. The more running friends, the more a runner tended to run. Overlapping social circles within running networks also gave runners extra incentive to stay with the pack.

The study is proof that runners can't help but post their workout updates to social media. Running is contagious, and runners' obnoxious sharing behavior is just a symptom of the disease.