
I haven't actively tried to "gain" muscle. I'm a tall/thin/lanky female. So bulking up wasn't ever an interest (I already feel like a beast as it is due to my height & a distorted perception of my figure, haha). If I'm active, I get fairly lean looking, which looks more toned (bicycling for travel, push/situps were the only exercise I'd do). When I've been less active for extended periods, my size doesn't change/I don't gain weight, but I do become a little softer, will lose some definition I had. When active, I don't think I have trouble
strengthening what muscle I do have, though. That's all that's important for me, really. Protecting joints, etc.
A couple of my exes were similarly built (ecto), & those who did actively try to gain muscle had trouble compared to their meso/endo friends. One tried to supplement working out with high protein shakes.. they were vile & didn't seem to help, much. /shrug