I have the same bladder, and also detrusor sphincter, dyssenergia. I fully understand the spasms and pain, and for years had to bear down and strain really hard just to pee a little for those darn pains to stop.
After a number of sphincterotomies to my external sphincter that part is now open. My internal sphincter though still has a mind of its own. Sometimes it will open and I'll pee without even thinking about it (yea no pain or spasms). Other times I will get a bladder spasm and it stays clamped down forcing me to stain to pee again.
I too have also noticed that sitting down and lying down in certain positions will also nearly guarantee my bladder will fill up. I believe this is simply because the urethra is being compressed enough to effectively clamp it shut. The bladder fills up regardless of the sphincters, and there you go.
Leaning to the side, rolling forward, getting up, etc all relieve that pressure on the urethra and you pee. Gaining weight will also add more fat in that area and increase the chance of the urethra clamping off too.
I also know that when you stand up the blood pressure in you body will naturally redistribute with gravity. Circulation in the legs picks up, which then dumps more urea on to the kidneys. This in turn fills the bladder just a little faster, and makes you need to pee. This process takes a moment though. I've noticed the need to pee when standing doesn't happen right away, but a few seconds to a minute after.
At least those are my educated guesses and theories any ways.
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