Although I already knew that the base joint of the thumb is actually adjacent to the wrist, and not the joint where the thumb and first finger meet, until a couple of hours ago I really hadn’t understood what this means for my guitar playing. All of my current thumb movement comes from my second joint. Because this joint is almost as strong (if not stronger) than the first joints of my fingers, this has felt fine. However making thumb movements from the first joint makes me aware of the following

• this is new feeling for me - the muscles are not familiar with this motion

• there is a lot of leverage involved - a small muscular movement provides a large and powerful movement in the thumb

• there is a feeling that a great deal more sublety of control is available - because control is coming from earlier in the chain of movement

I need to add this awareness to my practice working on thumb fundamentals (perhaps with a thumb pick as well