One of the most common thing you can see if you look at the people around you. It is easy to find someone with disproportionately smaller arms than the rest of the body.I call that the thootpick arms syndrome.
It is not just atrophy caused by being sedimentary, if you are sedimentary the entire body is going to atrophy not just one body part.
Some people with this issue are going to have problems even with training, gaining muscle and strength on their arms becomes almost an impossible task. This generally gets blame on genetics, but that makes no sense.
Why are just your arms genetically weak and not the rest of your body.
When you look at someone with this condition, you are going to see two things, one you are going to see a reversed curve in the neck, or the cervical spine curving forward rather than backwards, and second you are going to see disproportionally smaller weaker arms than the rest of the body.
It is a nerve impingement in the neck, that cuts the signal of the nerve going to the arms, which is what is going to cause your arms to atrophy.
Think of it like this, if you are going to break an arm and put it in a cast for a month, do to not using that arm it is going to atrophy and get smaller than the other arm.
Your body is a very efficient machine it won't allocate more resources than it is necessary to any part of the body based on how much it is being used including your arms.
The way it knows how much you use a certain body part and how much resources to allocate to it, is through the nervous system.
There is a constant communication between your brain and all your body parts through your nerves. The more you use a body part the more signals go back to the brain and let your brain know that the body part is something that is important and needs to allocate more resources to it.
If you are going to have a nerve impingement that reduces those signals going back to the brain, your brain is going to bealive that you are not using your arms and not allocate enough resources to them, regardless the fact that you are using them. It is like your brain bealives that your arms are in a cast and not being used.
This is actually easy to fix, all what you have to do is remove the nerve impingement, and restore the propper communication between your arms and the brain.
Although it affects the arms, it is an actually neck problem. The nerves that supply your arms start from the cervical spine. So it is just a matter of restoring a propper curve in the cervical spine which restores propper nerve signals to the arms.