We all have that Facebook friend — or 10 — who regularly posts photos of his or her fitness pursuits: on the elliptical at the gym, hiking through the wilderness, crossing a 10K finish line.

The goal of those posters might be to brag, to inspire or simply to share a part of their lives with friends. But what they may also be doing, for better or worse, is influencing the health of their social media pals, according to a study by researchers at Texas State University and the University of Arizona.

Researchers found that the more exercise-related posts a person sees on social media, the more concerned they feel about their own weight, which could result in unhealthy body image.

"When people received more posts about exercise, it made them more concerned about their weight — more self-conscious — and that's not a good thing," said Stephen Rains, a UA communication professor and co-author of the study, which appears in the journal Health Communication.

People are especially likely to feel concerned about their weight when they perceive their friends who post about physical activity as being very similar to themselves, said Rains, who co-authored the study with lead author and UA alumna Tricia Burke, a professor of communication studies at Texas State University.

"We thought about this from the perspective of social comparison theory, and the idea that we use others as benchmarks to figure out where we stand," said Rains, also an associate professor of psychology at the UA. "Similarity heightens social comparison, so if the person posting about exercise is someone who's in your age group, has a similar build or a similar background, you might think that's a pretty good reference, and that might spark in you even greater weight concern."

The news isn't all bad, though. For certain people, friends' exercise posts seem to have a motivating effect when it comes to attitudes about exercise. Researchers found this to be true for people who are more likely to engage in "upward social comparisons," or look at themselves in the light of people whom they aspire to be like.

"With upward social comparisons, you tend to compare yourself to those you perceive as superior to you," said Burke, who holds three UA degrees: a bachelor's in communication and psychology, and a master's and doctorate in communication. "So, for example, if you're in a classroom, you'd compare yourself to the smartest kid in class. In terms of exercise, if a person is posting a lot about exercise, they must be really fit, so you're using that as a motivator."

In contrast, those who engage in "downward social comparisons" use as their benchmark people who they perceive as doing less well than they are. Tendency to engage in downward comparisons did not have an impact on weight concern or exercise attitudes in the study.

The researchers asked 232 study participants to log into their favorite social media site and look at the past 30 days' worth of posts from their friends. The participants counted how many of the posts — photos or text-only — depicted their friends engaging in exercise.

For the study, exercise was broadly defined as any physical activity for the purpose of maintaining fitness and health, which could include anything from hiking to taking a walk to going to the gym.

Participants then chose the three of their social media connections who made the most exercise-related posts and rated their perceived similarity to each of those individuals.

They also completed questionnaires that measured their level of concern about their weight, their general attitudes about exercise, and their tendency to make either upward or downward social comparisons.

"Our results were mixed. Good can come out of this, in the sense that it can make some people more interested in exercising and feel better about exercising, but it might make other people feel worse about themselves if they're more concerned with their weight," said Rains, who recently was identified by the journal Communication Education as the second most productive communications scholar in the country, having published 10 articles in the field's top journals between 2012 and 2016.

Social networking sites are interesting, researchers say, because users are exposed to a constant stream of information about specific health aspects of their friends' lives — such as exercise — that they might not have known anything about otherwise.

"We wondered what happens when those exercise behaviors pop up, because the people on our feeds are not just random strangers — they are people we know and have some connection with, and we wondered if that matters," Rains said.

The mixed findings suggest that social media's impact on health is indeed real, if nuanced, and deserves additional attention.

"We're still trying to figure out the effects of these technologies, which have barely been used over 10 years," Rains said. "This is all still pretty new ground, and we're trying to make sense of what it means, and if and why it matters."

Burke also wants to learn more about what motivates people to post about their exercise behaviors online in the first place — a topic she plans to explore further in future research.

"I think people post for a variety of reasons — to try to motivate the others, to hold themselves accountable, or just because they want to share part of their lives," she said. "We want to know more about why people are posting about this and how they make decisions about what to post."