Hi Younes, “Specimen” in this context is a word for thinking about this person’s body in a very physical way, like it is an object. The person is saying that this weightlifter’s body would have been in such good physical condition that it was like an ideal example of a human body. You can think of it like “so good it should be studied in a laboratory”. I can understand Marcela when she says she wouldn’t use the term, because it is old-fashioned to speak this way. “You must have been something else!” would be a more common expression nowadays to say how impressed you are with the weightlifter’s body.

In this example: 'specimen' = 'in good physical shape' = 'quite fit' 'physical falling-aparts' = 'physical/health problems' or 'physical injuries' 'physical falling-aparts' sounds a bit British to me. A U.S. English speaker would not use that construction. He might say '..., what about all the health problems?' or '..., what about all the physical injuries?"

I wouldn't really use "specimen" in this sentence. I would instead say "You must have been something else!". But, what "specimen" is intended to convey is that he was impressive, "some specimen!", really out of the ordinary, exceptional, etc. Also, I wouldn't say "what about the physical failling-aparts?" It would be a separate sentence, and would be phrased something like" What about the physical deterioration?" or "what about the negative physical consequences?", or something like that...