The Insider.com

Mark Wahlberg rose to prominence thanks to his six-pack abs and underwear ready body -- and while he's still in fine form at 40, Mark's traded his Calvins for Jockeys.

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PICS - Mark Gets Buff -- and Shirtless

Snapped on his balcony while filming Michael Bay's Pain & Gain (a movie that's poised to be the most flesh-flaunting of 2012 ... much to Magic Mike's chagrin) in a pair of black bower-briefs, with the waist-band clearly sporting a new name.

RELATED - 5 Things You Don't Know About Mark

The film, co-starring Dwayne Johnson, is based on a 1999 Miami New Times article about two steroid-abusing bodybuilders who unwittingly get involved in an extortion and kidnapping plot that goes horribly awry.