The middle finger and ring finger share tendons. The ring finger is also connected to the pinky. If you fold your middle finger with the tip in contact with the palm and have the other finger tips rest on a table you will not be able to move your ring finger very much or without pain. This is because the tendon that has to flex in order to move the ring finger is already stretched to its limit when the middle finger is bent. Apparently this is so well known by medicine that it was a known practice in the 19th century to surgically sever tendons in the hands of pianists to increase ring-finger independence and allow better playing.