Hyper-flexibility in the thumb

A few of us noticed the extremely excellent "thumb-under technique" that David Lee displayed in his playing in this week's Musicale. I even kidded that maybe he had had surgery to dislocate his thumb! Just how different can our thumbs all be, and does that determine our ability to play scale passages smoothly? I decided to do a bit of research on thumb flexibility. It's frequently a topic in my household - my husband does not have flexibility at all, and bemoans the fact, as it hinders his guitar playing.

So on Youtube, I discovered a demonstration of the tests for diagnosing Ehlers-Danlos syndrome, a connective tissue disorder where hypermobility in various joints occurs.

https://www.youtube.com/watch?v=HkEHAXgEjaA

One can have hypermobility in various joints without having this syndrome, so don't worry if you can do some of the flexibility tests!

Also interesting were the images displayed when I Googled "Steinberg sign thumb". People who are positive for this sign are able to fold their thumb completely under the rest of their fingers to the point that it protrudes beyond their hand - wow! I kinda think that would be an advantage in playing scales smoothly....

Further Googling images, I tried looking for what MY thumb looks like, so I Googled "meaty thumb". I did find a diagram of the hand that identifies this part as the "Thenor". The opposite side of the hand - for me, also meaty - is the "Hypothenor". But most of the images had something to do with testing the doneness of a steak....so, before I really go down a rabbit hole, I'll end this here - although I'd love to hear your thoughts and observations.