Some internet sites say the words are related historically, because the large fleshy back of the human leg can be thought to look like the shape of a young cow or bull. I can't really see this, but maybe our ancestors who lived by cattle herding might have been more attuned to such things.
Of course, I had to experiment. Who wouldn't? So here's a photo of my own calf.
Nope. Doesn't make me think of baby cows. So I tried turning the photo...
Hmmm. I decided to check the dictionary again.
The Online Etymology Dictionary says:
"young cow," Old English cælf (Anglian cælf) "young cow," from Proto-Germanic *kalbam (cognates: Middle Dutch calf, Old Norse kalfr, German Kalb, Gothic kalbo, perhaps from PIE *gelb(h)-, from root *gel- "to swell," hence, "womb, fetus, young of an animal." Elliptical sense of "leather made from the skin of a calf" is from 1727. Used of icebergs that break off from glaciers from 1818.
The idea of a connection with the root meaning "to swell", might explain the connection, I suppose, since this muscle swells and shrinks as we move our legs.