The problem I am having is, whenever I work with my Art Verdi DVD and practice fast single strokes with using just my fingers (no wrist movement, as Art says this hampers finger speed) for a pretty long period of time, the joints at the base of my thumbs get sore....I am trying not to grip too tight with my thumbs and just let the fingers do the work, but invariably, the joints are a little sore afterward...I don't really think it's bad technique, since I am working with the video, and trying not to grip hard with the thumbs....Has anyone else had this problem, and any suggestions of what I can do about it?

i think everyone's clueless because you said your grip isn't too tight, which is the general cause of these problems, lol