This will be a short post about one thing all on its own because it’s a common mistake and I’ve seen it in several books I’ve read recently. Repeat after me:
The word biceps IS singular. I cannot make it singular by taking off the “s” because then I make it a muscle that doesn’t exist. It is a biceps because it’s a muscle with two heads. Biceps. I have one biceps in each arm and I WILL leave the “s” where it belongs. The plural of biceps is biceps. Therefore I never have to worry about changing the word. It will always be biceps.
But because I am not one to say “because I said so” I’ll provide you with this from the Online Etymology Dictionary:
1634, from L. biceps “having two parts,” lit. “two-headed,” from bis “double” + -ceps comb. form of caput “head” (see head). Despite the -s it is singular, and there is no such word as bicep.