I looking for help because try many standig position and many sort of piques level (sorry my english is not so good) and i get always pain when I try to play in thumb position...
I looking for help because try many standig position and many sort of piques level (sorry my english is not so good) and i get always pain when I try to play in thumb position...
It is very important that while playing in thumb position there is no difference in your body structure than when you play out of thumb position. The bass just simply rests on your shoulder instead. Make sure that your back is straight, but not to extreme, just comfortably straight. Also, be sure not to rest on the instrument (a problem I am actively combating), and make sure that your shoulders are not up. Relax your shoulders and your elbow. There should be no extra effort to go into thumb position, just gravity naturally pulling your hand position down the fingerboard.
If all else fails you can always try sitting. Good luck.
Take a step back from the bass and bend slightly at the waist. When you leave the bass vertical and get all hunched up and tense trying to reach over the bass you are asking for discomfort.
Rest the neck on your shoulder and stand up straight until you aboslutely have to bend at the waist (to get to the end of the fingerboard or the harmonics beyond it, for example).
You will have freedom of movement then.
Someone in the Rabbath cult will inevitably tell you to get a bent endpin.
That could be something to try if you play a lot of thumb pos.
thumb position give back pain 
