Inclusion Body Myositis has a typical course & does not respond to IVIG. The only thing that helps is daily exercises.
In my experience, it is somewhat more severe than Polymositis. At the beginning of 2020 I was walking with ease, whereas
these days I need a cane & sometimes a walker. There is also an element of pain in the muscles, sometimes severe.
The chest muscles & arms are also sometimes involved. You quite quickly become disabled.