It was suggested to me that Parkening did have focal dystonia, but it's unconfirmed and I don't believe Parkening himself ever said he had it. I don't want to start rumors but this late in his career it hardly matters. There are many videos of him in later periods (late 1980s-90s) where he seems to be re-fingering things in the right hand and his m-a seem curled into the hand.
When I visited David Leisner for a few lessons in the early 1990s, I was also referred to see a lutenist/guitarist named Patrick O'brien. O'brien was one of the few people at that time who was supposedly an expert on focal dystonia. He whispered to me that Parkening had come to see him about his dystonia. I assume he was being truthful, I don't know why he would make something like that up, but he was acting hush about it. http://www.juilliard.edu/journal/1409/o ... ick-obrien
All gossip aside: Both of these teachers were giving/selling advice that I now believe to be largely nonsense.
Leisner was all about swinging from the elbow and supposedly using the large muscle groups--not necessary a bad idea for some things, but no one can play fast free strokes with his method. It's kind of like saying eating Kale is good for you, sure, you can't deny that, but a person can't just eat kale, it doesn't encompass all of the food groups. O'brien was insistent that the only way to play again was to completely relax the tip joints so they passively bent back -- 100% of the time and for all strokes. He demonstrated it for me, but I was unimpressed with his own implementation of what he was teaching me. (btw, even though the advice of both seemed odd at the time, I independently gave both methods around 3 months of serious practice until abandoning them. It's just another reminder of how much bad information is out there and following the advice of any one teacher religiously and without question is not wise.)
While my playing is not back to where it once was, the things I've learned here from studying videos has helped me get back to playing again. I've also gotten personal emails from other guitarists who have said privately it has also helped them. Nothing I have tried has improved my playing as much as the realization that the MJ (yes PIP) is the main producer of the sound and main KJ (MP joint) is not following through into the palm after the note is sounded. I'm not arrogant enough to say it's some kind of cure for hand injuries, the videos can stand on their own as evidence for what is and isn't the correct way to move the fingers.
An eyewitness will often only see what he already believes to be true.