The muscular arms are actually an arm in the creator, called, if memory serves me right, "burly." I know that it is on the third row, one before last (going from left to right).
However, the thing I always found weird of this arm is that the deltoid part is lacking, so I had to stick two of these together to get deltoids along with biceps/triceps, and shape and reshape and reshape, till I got them to look right. Positioning of these two limbs is also crucial, since at the beginning, though it looked buff, the arm just didn't look right at all. Something amiss about it. So you have to play with the sizes (both the socket/ball and the in-between socket/ball). By this I mean that the arm increases in size when you play with the joints at the ends of each limb attachment, as well as when you point your mouse cursor in-between joints and increase the muscle definition there.
In short, each arm is actually made up of TWO burly arms. The deltoid part is made of half a burly limb, then a complete burly limb is added at the end of this one to make the bicep/tricep part as well as the forearm (though now that I think about it, I don't remember if it was two half burlys and a half something else, or a half burly and one complete burly, I forget).
But the point is that to make big muscular deltoids/shoulders WITH big biceps/triceps, you absolutely need to link two half burlys together.
The burlys were also used to make Hulk's chest, as well as his back, just put sideways and rotated to crazy degrees. It was a lot of trial and error.
Last but not least, the coloring scheme affects how much definition you see.