Your hands are shaking is a muscle spasm (that may not be exactly what it is, but it is the same type of thing), it sounds like in this case it was so strong you couldn't overcome it, probably just means the attack was really bad.
Thinking of getting a cramp, you can't move the affected mussels because they've already, on their own, moved as much as they can.

I would say under the circumstances an attack of greater than normal proportions Is to be expected.