Looks shopped. Just look at the un flexed arm. It's smaller than the other dude's arm.
What you really need to look at is the outside of his forearm/tricep. You can see some of the background is moved a bit and there is a slight blur which leads me to believe the wannabe Photoshopper probably used the Smudge and blur tool instead of Liquify, which would have been much more convincing.
Anyone who works in digital art (Photoshop, etc) can pretty much tell. I know I'm not the only one here, but his arm is still gonna be huge.
If I were to do it, I would duplicate the shot, pull the forearm, work on it in a neutral background and then just slide it back over the top of the original, but hey, that's just me.
