skin heads is sth different
łysy refers to bald head (shaved one, true)
but 'kark' simply means that a guy goes to the gym, pumps some iron and that's why his back muscles od the shoulders are so visible or sth like that
Siunia, you know that 'abs' refers to abdominal parts of the body, so in this way we can assume that, a guy who works out at the gym all the time trains all his muscles to have a great 6-pack or 8-pack (karki in Polish are called also "absy", which makes it a perfect translation).
Actually, there's no good enough explanation to cover that subject, but
I thought it would be appropriate to inform you about it.