I think one reason for muscle weakness can be magnesium or calcium deficiency.
If you google muscle weakness and magnesium or calcium, you will get a lot of hits.
When I first started taking lots of magnesium oil, my legs had a lot of bounce. I was running up the stairs like I couldn't do for years.
I was taking too much magnesium oil, however, and created some other problems.
There are probably other reasons as well, but I'm sure that cells being low in magnesium and calcium may be one reason for muscle weakness