I can't remember now if I asked you this question before, but do you experience a sort of vibration when you tense a muscle? For example, if you extend your arm in front of you, palm down, and press your fingers against your palm, do your fingers and wrist vibrate, as in, being unable to keep a steady contraction? The same thing that usually happens to anyone after absolute exertion to the limit, but all the time and with just light/moderate contraction of the muscles? And if so, did this symptom appear at the same time than your exercise intolerance?