I can’t do that with my thumb. Mine goes straight even if I press to force to bend it backwards. It’s the only finger joint that doesn’t overextend, all my other fingers curve backwards nicely and even more so if I apply pressure.
Bending the thumb forwards (opposite as in your photo) is a different story. The joint can bend past the 90⁰, easily 80⁰ or more if I press it against something.




Most realistic illustration I’ve ever seen in a book