Muscles?!
Yes, my Sylvari has perfect abs. He swims a lot.
Lore answer: The form of the sylvari is intended to mimic the human form. the caveat to this is that the Plae Tree only saw the outside of the human form so the inner workings aren’t a 1:1 relica. We also don’t know if it was a conscious descision from the Pale Tree to do this. So while a Sartorius muscle isn’t noticible from the outside, it may have just been a logical or intuitive necessity.
RL answer: A-nets artists are probably familiar with the musculature of the human body so it was incorporated. :-P
Male Sylvari were created by female ANET designers to be as sexy as possible!
Even I have a man-crush on their sexy British accent!
if you think about it we do glow so its not hard to believe our surface layer isn’t more transparent then humans and with the luminating effect we give off our muscles would show through a bit.