Usually, it comes from a repeated movement or forceful use of your finger or thumb. Tendons are fibrous cords that attach muscle to bone. Each individual tendon is surrounded by a protective sheath.  The doctor will want to look at the affected individual's hand, like testing the injured spot's energy, https://www.theorthopaedicandpainpractice.com/why-cant-i-straighten-the-tip-of-my-finger/