She may be on vacation, but that hasn't stopped Rihanna from working on her fitness!

The 23-year-old "You Da One" singer has been soaking up the sun in Hawaii since Saturday, where she's been spotted paddle boarding and swimming with the sharks. The bikini-clad star hit the gym on Wednesday, sharing a photo of herself exercising on the elliptical via Twitter.

How does the musician -- who's been hooking up with her violent ex Chris Brown for nearly a year, multiple sources tell the new issue of Us Weekly-- maintain her amazing abs?

"We focus on the core muscles," trainer Ary Nunez recently told Us of the martial arts- and dance-based workouts she does with the 5-foot-8 star. "And we do a lot of repetitions."