I may be wrong, I often am, but I think muscle weakness is fairly common in systemic scleroderma. If you search our main site, you'll see that "exercise intolerance" can be caused by many things associated with scleroderma, including fatigue, fibromyalgia, heart, thyroid, and lung problems as well as scleromyositis (when scleroderma overlaps with polymyositis).
I think it is normal for many of us to struggle with weakness issues. There can be such a fine line between doing what we can, and doing too much, and it can be almost impossible to figure out where that line is, especially as I suspect the line itself moves around from hour to hour, from day to day, and from week to week. Or should I say, weak to too weak?
Although it's definitely not "in our head" when we have weakness, we can still use our head to try to work with it, or around it. I've found that just because I can't do something one day, doesn't necessarily mean that I can't do it the next, because my strength and endurance levels vary so much.
It took me over six months to build up to doing 10 toe touches a day. That was a very worthwhile endeavor, because now I can get an idea each day of how much I can get away with, by how impossible, painful, difficult, or easy it is to do them. If its easy, hooray, I can try doing a bit more activity than usual.
Scleroderma is the worst form of arthritis. So if we can find an activity that we can use as a bellweather for the day, like the toe touches work for me, then we have more information to work with in planning -- or revising -- our activities for the day.