I am a 20-year-old woman in good health. I've noticed the last few weeks that my right middle finger moves from side to side when I rest my hand, or if I sit in a chair, and I rest my hand on the arm of the chair with my hand dangling over edge. It happens when I wake up, which is why I write now 04:28. I held my hand up and my finger is still moving from side to side like a pendulum. Now that I've written this message, it stopped moving. Should I see a doctor? Should I be worried?
That is actually a phenomenon which is seen quite commonly by hand surgeons. It has to do with the way the intrinsic muscles of the hand are attached to the base of the fingers.
If you look at your hand, when you spread your fingers widely apart, the middle finger really doesn't move much. The other fingers move away from it. If you think about it, you can move the middle finger side to side. (And make the "Vulcan Live Long and Prosper" sign, from Star Trek.)
So, the middle finger does have the muscle attachments, but they aren't used much, since it is the center focus of the hand. When at rest, the middle finger is a little schizophrenic, it doesn't know which way it wants to go, so in some patients it will wriggle back and forth slightly.
But, usually, it does not affect movement. Once the hand is in motion the wriggles goes away. And if a little tension is applied to the hand, the wriggling stops.
Mine will do it if I hold my hand with the palm up and fingers straight out, but not if the palm is down. Go figure. And it has never bothered me when using the hands, even for very fine movements.
However, if you are concerned about it, or it is seeming to get worse, have someone take a look at it. Good luck.