what is a muscle man and what did theysdo with them?

"Muscle man" refers to those who were noticeably stronger (strong connotates with muscle) than the other Jews. If you happened to be less skinny, or looked very healthy, you were considered a muscle man. What happened to the muscle men, was that they were either sent to the crematoria to throw people into the fire, or they were sent to Buna to work on the construction site. Later on, anyone was sent to Buna, not just muscle men, so mainly they were just sent to work in the crematoria.