I have had this too in varying degrees of irritating and mildly weak hands to incredibly painful and useless hands. It was of course blown off or misdiagnosed as carpal tunnel as I have positive nerve conduction tests. My new and amazing physical therapist says it is because of the compression of nerves between the collarbone and the 1st rib. From what I understand it is a very small area for the nerves to pass through and of course with the head injury we have neck and upper body injury. When she works on me she can actually restore my grip for a few days. Anyway hope this helps a bit. It's incredulous how our symptoms are so often blown off. I'm sure if I hadn't had positive nerve tests I would have been diagnosed as anxious even though my hands could not make a fist!