Students unfurling their mats before a recent class at Studio 34 Yoga in West Philadelphia paused for a minute, puzzling over a bowl filled with glossy yellow cards.

"I think you're supposed to take one if you want a hands-on adjustment?" someone suggested. A few people tentatively selected cards, as if awaiting a magic trick.

Then the teacher arrived, with an explanation: They're "consent cards." Place one by your mat if you're open to physical guidance from the yoga instructor. Or don't.

The topic of consent has, perhaps inevitably, made its way into a growing number of yoga studios. They're deploying consent cards - and variations such as permission stones, yes-no cards, and a trademarked product called the Yoga flipchip - to empower students to accept or decline hands-on adjustments, a teaching tool in many studios.

Some instructors link the trend to greater awareness of yoga injuries (and how overzealous instructors can cause them), thanks in part to a 2012 William Broad book excerpted in the New York Times under the memorable headline, "How yoga can wreck your body."

Others mentioned a need for boundaries in the wake of sexual harassment and assault claims against guru Bikram Choudhury, who was this year ordered to pay more than $7 million to a former staff member.

For still others, it has more to do with the conversations happening across the internet and college campuses in recent years: about affirmative consent, the "yes means yes" rules for intimate contact among students on college campuses; and about trigger warnings, giving to students advance notice, and a chance to opt out, of potentially re-traumatizing materials.

"The way people hold trauma is in their bodies or disconnecting from their bodies," said Gwen Soffer, cofounder of Enso, a yoga studio in Media. "Yoga is a place where you're supposed to feel safe in your own body, and if people are coming up and touching you without your permission, we feel like there is something unethical there."

She has been teaching yoga for 15 years, but after she began studying for a master's in social work, she began to consider how widespread trauma is.

According to government studies, about 10 percent of women and 4 percent of men suffer post-traumatic stress disorder at some point in their lives. One in five women is a victim of rape, and a third of rape victims develop PTSD.

Soffer wanted to avoid triggering those memories through touch, and to create a system where the default would not be consent. Her solution was to set out a bowl of "permission stones" students could place by their mats.

"I've had students that I have adjusted for years before I started doing this that don't put stones out, which tells me for years I was not respecting their boundaries," she said. "I also have some students who come to classes who didn't used to come, because they know that is something that we enforce."

Soffer and Melissa Lucchesi, who runs a nonprofit, Voices Inc., that works with trauma survivors, have been offering workshops on trauma-informed yoga instruction.

"It's really challenging for many trauma survivors to go into a yoga class to begin with," Lucchesi said, "and then the idea that somebody might come up and touch them, it can send them reeling."

Mellow Massage & Yoga in East Falls instituted the permission-stone system in October, after yoga director Vanessa Hazzard attended one of their trainings. So far, Hazzard said, no student has declined permission. "But they like that they have the power to say yes or no," she said.

After all, there are many reasons students might opt out, from a nagging injury to cultural norms to mere personal preference.

For Nina Jackson, a physical therapist in Charlottesville, Va., the number of patients with yoga injuries was the catalyst.

"It was typically people who already had minor injuries, and then were heavily adjusted in a posture," said Jackson, who also teaches yoga. "I said, 'Why didn't you say something to the teacher?' And they said they didn't feel comfortable. They said, 'I just never went back to that class.' "

In 2012, she began marketing the Yoga flipchip, a pastel-color disk, made of canvas or bamboo, that says "assist" on one side and "no hands-on assist" on the other. She now sells packs of 20, 30, or 60 flipchips to studios around the country.

"Initially, teachers weren't that responsive," she said. "I brought it to a yoga studio in Washington, D.C., and they said, 'No. We assist everyone. If they don't like it, we don't want them coming here.' . . . Now, that's totally changed. We rarely get someone saying that anymore. People are saying, yes, injuries do happen. People have medical issues. We can't just go in a class with 30 or 40 people and teach them all the same."

Still, not everyone likes the idea. Heather Rice of Amrita Yoga & Wellness in Fishtown said that, as more people pick up yoga, often at the suggestion of a doctor or physical therapist, teachers have to be more attuned to each student's needs.

"As a teacher, it's important to connect with the student. It's a really personal experience," she said. Using cards could short-circuit that relationship: "Instead of making a connection, you put down a card."

Angie Norris, cofounder of Studio 34 in West Philadelphia, said that instituting consent cards was a way of formalizing a studiowide policy around something she'd long done informally.

In the past, she'd ask students to let her know at the start of class if they didn't want hands-on adjustments.

But sometimes she forgot to ask. After a student had a strong reaction upon receiving unwanted, hands-on assistance, Norris decided she needed a better system.

Norris, who also worked as a nurse-practitioner until recently, said the cards reflected an evolving understanding of trauma-informed practices. But she hopes students won't think of them in that context.

"I wanted it to feel like no big deal," she said.

Some students complained that the cards didn't allow for privacy, but, to Norris, that's the point.

"It takes any kind of the stigma around not wanting to be touched away," she said. "That boundary doesn't have to be a secret, private thing. We should all feel good about setting boundaries for ourselves."