Position Of The Arms For Men In Rukoo

With regards to the Ruku of Rasoolullah Sallallahu Alayhi Wa Sallam, the following narrations have been recorded in the books of Ahaadith:

“When the Messenger of Allah Sallallahu Alayhi Wa Sallam would perform Ruku, it was such that if a cup of water was placed upon his back, it would not spill.”
(Musnad Ahmad)

…then the Messenger of Allah Sallallahu Alayhi Wa Sallam performed Ruku, so he placed his hands upon his knees as if
he was grasping them, and he straightened his arms and moved them away from his body. In Al-Nihaayah, it is stated under the commentary of this narration that the Messenger of Allah
straightened his arms like the string of a bow…”
(Awnul Ma’bood Sharh Abi Dawood Dar Nashr Al-Risaalah V.1 P.267)

Under the commentary of the same narration, Allamah Aynee Rahimahullah explains that the Messenger of Allah Sallallahu Alayhi Wa Sallam placed his hands on his knees while stretching out his arms.
(Sharh Sunan Abi Dawood Lil Aynee V.3 P.326)

Allamah Benori  Rahimahullah states that the meaning of this narration is that the Messenger of Allah made his arms straight like the string of a bow, therefore making his arms resemble the string, and his bent body resemble the shape of a bow. He then says that this position is amongst the Sunnats of Salaat according to us and according to all others as well, and that it is a means of easily straightening the back, which is also amongst those things that have come in the Sunnat.
(Ma’aarifus Sunan Dar Al-Tasneef V.3 P.8)

After analyzing the texts of the books of Fiqh, one will come to know that the fundamental matters of Ruku are: the straightening of the back, aligning the head with the buttocks, to lean on the knees whilst grasping them, and to keep the fingers spread apart. When the Messenger of Allah would keep his arms straight, it made his back accordingly straight as well. Therefore, if one can keep his arms and back straight simultaneously, then he will achieve both Sunnats.

If by protruding out you mean that the elbows be jutting out towards the persons on your left and right, this is incorrect. The shape of the bow, from what we have explained above, is that the tight straight hands gripped on the knees resemble the string of the bow and the bent back down to the hip then finally to the thighs resemble the bow when viewed from the side of the person in ruku.