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These two may be the most fit celebrity couple out there.

"Twilight" hunk Kellan Lutz and his "Step Up 3D" star girlfriend Sharni Vinson hit the beach in Miami yesterday -- where they showed off their extremely fit physiques.

We challenge you to try and find an ounce of fat on Sharni's body ... we don't think it exists.

And Kellan, well, he's always been one very buff guy -- and can usually be seen running and doing vigorous workouts around Venice Beach.

Lutz will next be seen at the Comic-Con panel for "The Twilight Saga: Breaking Dawn -- Part 2" tomorrow in San Diego ... clothed, presumably. Come back tomorrow for photos from the event!

Check out more celebrity beach bods in the gallery above!

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