I have been doing a little research into catabolism, looking to see if any studies have been done showing where exactly your body will choose it's fuel sources and what factors determine this choice. A few personal trainers have told me that, in a situation where your body is getting fuel from muscle mass to support resistance training, it will take fuel from muscles not in use. I wanted to verify that this information is accurate and was hoping to learn more, but I just can?t find the information I need. I would assume there are many factors that contribute to how catabolism affects ones body, but the only information I can find simply states it causes breakdown of tissue and doesn?t delve much further.
I'd recommend calling a local college bookstore and asking if they carry a physiology text about the learning more, but for your immediate question...
It's not that your body won't catabolize the muscles you use, persay, more that there's an anabolic stimulus to build them back up a bit. It's not a linear process though, but each is continuously happening, so as some proteins are broken down, others are built back in the stressed muscles. The other muscles simply have no stimulus for rebuilding.
I was thinking that might be the case. I'm sure the trainers were just oversimplifying the process. It's definitely an interesting subject matter. I'm surprised there is not more detailed information readily available.