(A) Ibn Abbaas (RA) related that once a man asked the Prophet (peace and blessings of Allaah be on him) about [the acts of Salaah; the Prophet (peace and blessing of Allaah be on him) answered: "When you do rukuu hold your knees with your hands till you feel a sense of tranquility and when you do sujuud, tough the ground firmly."[Ahmad]
(b) It was also related that the Prophet (peace and blessings of Allaah be on him) used to make his back straight when he does Rukuu that even if water were to be poured on it, it would settle. Accordingly, a praying Muslim must not hang his head loose in rukuu but instead level it with his back for it was related that the Prophet (peace and blessings of Allaah be on him) used neither to hang down his head nor lift it [high up above his back level] in rukuu". And in rukuu all ones joints should settle in their positions in order to achieve calmness.
(c) the Prophet (peace and blessings of Allaah be on him) said to the companion who performed his salaah improperly: "A Muslims Salaah is not complete and correct unless he performs perfect wuduu as was ordered by Allaah (SWT) then recites Takbeer...then does Rukuu till all his joints are settled and relaxed in their positions then raises himself from Rukuu reciting "Sami allaahu liman Hamidah". And stands completely till his back is fully erect .... [Abu Dawud)
To conclude, in order to achieve calmness in bowing [rukuu] in Salaah, one has to do the following:
1- Holding the knees with ones hands [in rukuu]
2- Spreading fingers.
3- Straightening ones back.
4- Staying in the same posture as long as it takes till all the joints settle in their positions.
All of these acts are affirmed in many [authentic] narrations. To be concluded next edition insha Allaah