I'm not sure you realise how fucking hateful you sound when you dismiss mental illness as "just in the mind".
The brain controls the muscles that constitute your forearm. It’s not impossible to imagine that some sort of central nervous system dysfunction could make your arm go stiff. Limb rigidity is a classic symptom of Parkinson’s disease, for example, and its cause is several steps removed from motor cortex. Even the wrinkling of your fingers when they’re wet involves the nervous system: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2464382/
Obviously, issues with your arm could also be caused by problems with your arm, but you can’t trivially rule out the brain either.
Also, while I have you, could you please stop creating accounts for every comment or two you post? This is in the site guidelines: https://news.ycombinator.com/newsguidelines.html.
HN is a community. Users needn't use their real name, but should have some identity for others to relate to. Otherwise we may as well have no usernames and no community, and that would be a different kind of forum. There are legit uses for throwaways, just not routinely.
It's fucking stupid and it makes you look fucking stupid.