They mean you feel a distinct inability, oddly and suddenly, to do what your mind tells your muscles it wants them to do. You might reach up or out okay, but your grip is weak, or you simply cannot lift what you know you should be able to manipulate. Or, you are holding something, say a glass of water, not exactly a 'light' glass, and suddenly your arm can't keep it suspended any longer. Your arm droops, spilling the water, or you lose your grip altogether.

Not pain! You have something else going on. My inexpert opinion, of course.