Britney Spears gets in shape

"We're very excited to see Britney using our clubs so often and enjoying her workouts," Bally's chief marketing officer John Wildman tells OK!. "She's taking advantage of our personal-training and nutritional programs and getting great results. She's working extremely hard on both programs."

Now Brit is working with Bally's trainer Mark Corlis, an insider tells OK!. "She's doing really well with him," the source says. "Mark doesn't cut Britney any slack. He doesn't baby her, and that's why she likes him."

Britney's exercise program is called Active Recovery. That's when you work each muscle group intently for a short period of time until you exhaust it, moving — without resting — from muscle group to muscle group. The mother of two is doing it a couple of times each day, followed by late-night cardio at home.

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