Mari Winsor said she's been working with the 19-year-old for the past four months on an almost daily basis at her Studio City, Calif., gym, People magazine reported Friday.
"What we focus on with Miley is posture. That was my main concern when she first came to me," Winsor said. "I wanted to get her shoulders down and get her to relax her upper body. If you notice, her posture is a lot better.
"She works hard," she added. "When this girl comes in, she is on fire. She's focused."
Winsor said she thinks people who are worried the actress is losing too much weight "need to chill out."
"She's healthy and she feels good, that's the main thing. She's smart about her body. She wanted to sculpt her body, that's all."