It could be arthritis.

I forgot, but I'm pretty sure that its rheumatoid arthritis that starts really stiff, and with use it gets a little better.

RA is an autoimmune disease.

OA is not. (osteoarthritis, which affects the elderly)

And yes, PN can affect the muscles. The needle EMG would point that out.
The NCS is the part that says if you have nerve involvement.
(its a 2 fold test) *nerve and muscle*