http://revach.net/article.php?id=1337

Section: Halacha   Category: Tshuvos
V'Darashta V'Chakarta: Raising Your Hands To Heaven By Shmoneh Esrei
Traditionally Tefilla is with hands raised to Shamayim.  We find this explicitly by Moshe in Mitzrayim and by Milchemes Amaleik.  Also by Yitzchok, Shlomo HaMelech, as well as by ordinary people.  Rav Aharon Yehuda Halevi Grossman in his sefer V'Darashta V'Chakarta was asked if we should assume this position during Tefilla today and if not why not?  He quotes a Teshuvos Be'er Sheva that says we should not since the goyim pray this way we should not daven to Hashem in a similar style. 

He also brings the Rambam (Tefilla 4:34) that says your left hand should be on your heart and your right hand covering the left.  The Mishna Brura says (95:3:5) that one should not make any strange motions while davening B'Tzibbur and nowadays raising hands is among these strange motions.

The Mishna Brura also says in the name of Rav Moshe Cordevero that one should hide his thumbs inside the palms of hand by Shemoneh Esrei.  Rav Grossman says that he heard from Mekubalim that each finger except for the thumb has three joints for a total of 12 on each hand representing the 12 Shevatim.  The thumb has two joints Kineged Eisav and Yishmael.  We therefore hide it by davening as well as when looking at our fingers during Havdala.

Disclaimer: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.