Thanks Just sove but, to be honest, I wasn't doing them too regularly before:-)
It's nothing serious, just something called Dupuytren's contracture, where a bit of tissue acts like elastic, pulling on the end and causing it curl inward.
By all accounts it's an easy fix - they just 22d the bit of tissue et voila! The finger or 'hand springs' back open!
Ha! It looks like I will be doing a 3a after all ๐