Yes, weakness and muscle stamina issues have become a bigger problem for me than the delayed muscle relaxation. I also have it mainly in my upper body, but ....I get a few different types of weakness. I've written a lot of posts on the matter. Unfortunately I haven't really found anything that helps it other than resting those muscles, though the full body weakness that I sometimes wake up with, usually when it's hot, I find I can sometimes walk off.
The weakness I get in my arms and hands gets worse with use. Some studies have documented what is called a de-polarization conduction block that was associated with weakness in some people with Becker MC. They also saw an abnormal muscle fiber activation pattern. What I assume this means is that when they applied an electrical signal to the muscles, some muscle fibers eventually stopped responding...they became paralyzed. And in what seemed to be an effort to compensate for this, some muscle fibers started activating when, in individuals with normal muscles, they wouldn't have.