When Xiao Ke, 18, came into the detention center in Wuyi, Zhejiang Province a year ago on charges of murder, her thin face was a picture of rebellion.

"She was unruly, she cursed, and did not follow rules here," said Tang Junhui, a guard at the center. "Now she's a different person."

Tang, 47, had learnt some yoga before she became a detention center guard in 2010. The idea of teaching the inmates yoga came to her mind one day in 2011 during a chat with some of them. The inmates had complained that they felt tired and sore because of a lack of exercise.

Tang found many of them were psychologically troubled and refused to communicate with others.

"Why not practice yoga?" she asked.

At first, only six people followed. Others laughed at them. "Are you mad? Doing yoga in jail?" they said.

After a while, some of the inmates who laughed at them also joined in. They changed their minds because they saw what changes the new exercise brought. People who practice it sleep better and have better appetites.

Approved by the head of the detention center, Tang bought every participant a yoga mat, and gave lessons twice a week. The inmates also practice yoga every night before they sleep.

Xiao Ke has changed a lot after picking up yoga. She became friendly to new inmates, and invited them to join her in practice.

"I haven't seen people outside for almost a year, but I'm happy now, because I can do something different," Xiao Ke said.

Another young woman, who was sent to the detention center in April 2012 for harboring a criminal, cried a lot during her sleep in the beginning. After practicing yoga with Tang, she became more optimistic and began talking more with other inmates as well as Tang.