No one expected it, not even the model herself.

When Women’s Running magazine asked Erica Schenk, an 18-year-old plus-size model from Palm Springs, Calif., to pose for its August issue, everyone involved thought the photos were just going to run alongside an article about workout apparel for different body types. At the last minute, editor-in-chief Jessie Sebor decided, “Why not put her on the cover?”

Schenk didn’t know about the decision until a reporter asked her to comment. “I told them, ‘I don’t think I’m on the cover,'” she recalled with a laugh.

But she is. Clad in black spandex tights and a bright sports tank, the Wilhelmina model is shown running through Brooklyn’s Prospect Park on a sunny day. (“It was 100 percent humidity and 95 degrees,” Schenk groaned.)

After the issue hit newsstands last week, social media users reacted quickly and passionately, with many expressing elation at seeing a larger body type featured on the cover. According to Sebor, it has incited the biggest response the magazine has ever had.

“It’s really important for media to begin to realize that you don’t need someone with a six-pack and our culture’s version of a ‘perfect body’ to sell magazines,” Sebor said. “Readers now really want to see themselves portrayed on covers, especially women with curvier frames who have only seen themselves depicted in ‘before’ pictures.”

The cover appears as other body positivity movements are gaining traction online and in the fitness community. In recent years, hashtags such as #curvyyoga, #rockthecroptop and #curvy (on which the Instagram ban was lifted just yesterday) have encouraged women to celebrate their bodies regardless of how much they weigh. These campaigns offer a counterpoint to #thinspiration blogs, which have been accused of condoning eating disorders in the name of fitness.

Earlier this month, another fitness-related message went viral for very different reasons. Army veteran and personal trainer John Burk uploaded a video to his Facebook page that criticizes overweight people for their “sheer f–king laziness.”

“No, I will not accept you for who you are,” he shouts at the camera. “This isn’t me trying to fat shame. This is me saying you need to change your life before a doctor comes in and says, ‘Look, I’m sorry, there’s nothing else we can do, we gotta take the leg.’ ”

Although the body positivity movement moved swiftly to denounce Burk, a large portion of the nearly 10,000 comments on the video have been supportive. Markola Yanga of Sacramento writes, “I’m over weight and I totally agree with this, but I’m already making changes. Good tip!” Michael Hunter of Sierra Vista, Ariz., comments simply, “I need to change.” Burk has responded to many of these with words of encouragement and more fitness tips.

Rob Williamson, a customer service representative at an online clothing store that has partnered with Burk, said Burk inspired him to start exercising after they met two weeks ago. “I was eating a doughnut, and he tackled me,” recounted Williamson, who is overweight. “I’ve lost two or three pounds since.”

But Jessamyn Stanley, a yoga instructor and “fat femme” based in Durham, N.C., said people like Burk are speaking from a place of ignorance. “You are shaming people, but you’re saying that you care,” Stanley said. “There are a lot of people who live a ‘healthy’ lifestyle who actually have eating disorders. This kind of thing contributes to bullying, cyberbullying and sometimes suicide.”

For plus-size fitness gurus like Stanley, who helped popularize #curvyyoga, the movement demonstrates that people are moving beyond the attitude that overweight people are incapable of doing exercise or feeling beautiful. Yoga in particular, Stanley said, has long been considered off-limits for larger people because it involves tight clothing and a good deal of upper-body strength.

“A lot of people who are larger-bodied are coming into yoga with a gentle approach — like you can only go this far because you’ve got a belly or large breasts,” Stanley said. “I’m coming at it from the perspective of, ‘I can do anything a smaller-body person can do.’ “