Hello silpheed_tandy,


I hope I can help provide you with some direction to your question.

"what IS my wrist, anyways?"

Your wrist is nothing to you without appropriate brain structures and feedback systems to feel it. These exist within an area of the brain called your somatosensory cortex. There, you will find your whole body is mapped out. This is also referred to as the somatosensory homunculus. Basically, all the muscles and joints in your body have little receptors inside them which are constantly feeding information back to your brain to make you feel as though you have a wrist, leg, head, neck etc....

"what is happening inside it when i have the urge to stretch it? what causes that to happen?"

The urge is really a perception that you are having which is again a product of your brain. We have urges to eat, sleep, use the toilet etc. All urges are the product of neurotransmitters being released in various areas of our brains which make us do certain things to satisfy those urges. Because you are feeling your wrist "needs to be stretched" it would be fair to say that the area that does the feeling for the wrist which exists in the somatosensory cortex in your parietal lobe is looking for some more input. Urges can be protective to promote reflexes which will stimulate blood flow to joints or inhibit pain. Do you ever wonder why you shake your hand if you bang it with a hammer? Movement information overrides pain information. By holding your wrist in a locked position you rob your brain of the sensory reporting which it needs to keep a joint feeling good. Without this good flow of muscle feedback the result is discomfort which can lead to pain. Just try keeping your hand or arm totally still for as long as you possibly can. This should cause you to want to move it or stretch it which, when you move or stretch the body part in question, activates fuel delivery reflexes and sends messages to your brain to tell it that everything is OK down there.

- what does stretching actually physically do, and why does it feel better after stretching it?

I think I covered that in the last bit. It can get really technical though.

- pianists on the forum talk about it bad using a "locked and immobile" wrist, and how it's not good to "accumulate tension" in the wrist. what does it mean for the wrist to be locked, and what exactly is tension, and what does it mean for it to accumulate?

I'm not a concert pianist and thus am not familiar with the technique side of things but imagine you would not be moving your wrist as you played if it were locked. Interesting that your teaches feel that it is important not to accumulate tension in the wrist and advise you to keep it unlocked and mobile. Unlocked and mobile will maximize the muscle and joint feed back to your brain and keep fuel being delivered maximally to your fingers and wrists as you play which will keep the muscles from getting tired. When muscles fatigue it places increased stress on ligaments and other joint tissues and can lead to some difficult pain syndromes to treat. You can check out the post by vioalblue if you are interested in the names of some of these conditions.

Hope that was helpful.