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It takes quite a muscular man to make Chris Evans and Chris Hemsworth jealous—an Iron Man, actually.

Yes, let's give credit where credit is due: A photo of Robert Downey Jr.'s arm gave his Avengers: Age of Ultron co-stars some serious bicep envy!

It all came about innocently enough when MTV News' Josh Horowitz invited the superhero gang to play a guessing game to see how well they know their biceps. Now, 33-year-old Evans recognized his upper arm instantly. "I'd know that armpit anywhere," he explained.

Scarlett Johansson quickly identified her own "soft and supple" body part, but not until after Cobie Smulders wrongly guessed the arm belonged to Joss Whedon and Jeremy Renner guessed it was "Evans' before he was Captain America." Mark Ruffalo, to his credit, "was going to say" the lithe limb belong to Johansson.

It was Downey's bulging bicep, though, that had his fellow beefcakes in disbelief! Evans facetiously guessed it was his own (not flexed!), while a more serious Hemsworth, 31, asked, "Is that me?"

But, no, boys, just no: That buff bod belongs to 50-year-old RDJ—recognize and respect! Neither Evans nor Hemsworth could believe it.

See their hilarious reactions in the MTV News clip above. Plus, make sure to check out all the stars' hot bods when Avengers: Age of Ultron hits theaters Friday.