If you don't exercise all your joints just rust up/go rusty. Also the muscles and ligaments go rusty and you seize up. If you do not continue to study or use your knowledge you go rusty. Figuratively speaking..."Rusty" is more used for skills.
For instance, I finished my last French class 6 years ago and haven't used it, so I could say "My French [as in the skill or ability to speak French) is a bit rusty."
For exercising, you'd generally hear, "I out of shape."