The Quran gives us the three positions to be observed during the Salat. These are Standing, Bowing and Prostrating. The Salat starts in the standing position:
(2.238) You shall maintain the Salawaat and the Salat Al-Wusta, and stand devoutly before God.
(3.39) So the angels called him while he was standing praying in the sanctuary: “God gives you news of John; confirming a word from God, a master, moral, and a righteous prophet.
(4.102) And if you are among them and you lead the Salat for them, let a group of them stand with you and let them hold on to their weapons. Then once they have prostrated let them be positioned behind you and let another group, who have not yet observed the Salat, observe the Salat with you while remaining cautious and while holding on to their weapons.
All the above three verses indicate the standing position. Verse 4.102 in particular indicates that the Salat starts in the standing position.
The words: “if you are among them and you lead the Salat for them, let a group of them stand with you and let them hold on to their weapons” indicate the beginning of the salat, that is because it is not feasible to say that the group of believers would join the salat in its middle or near its end! Consequently the words “let a group of them stand with you” indicate that the salat starts in the standing position.
Submission to God should be declared physically and symbolically by first bowing down and prostrating:
(22.26) We appointed Abraham to establish the Shrine: “You shall not idolize any other god beside Me, and purify My shrine for those who visit it, those who live near it, and those who bow and prostrate
(48.29) Muhammad-the messenger of God, and those with him are harsh and stern against the disbelievers, but kind and compassionate amongst themselves. You see them bowing and prostrating, as they seek God’s blessings and approval
The greater majority of Quranic verses speak of bowing and prostrating in that order. This seems the logical progression from the standing position, i.e. standing then bowing then prostrating.
The Salat ends with prostration:
(4.102) ” …… Then once they have prostrated let them be positioned behind you and let another group, who have not yet observed the Salat, observe the Salat with you while remaining cautious and while holding on to their weapons.”
The words “once they have prostrated” indicate clearly that prostration marks the end of the Salat, after which a new group comes to observe their Salat in the place of the first group. There is no mention in 4.102, or anywhere else in the Quran, that the believers should follow prostration by standing once again and repeating the cycle.
These three Quranic physical movements (standing, bowing and prostrating) are not required at the times of emergencies, fear, and unusual circumstances (2.239).
(2.239) If you are in a state of fear, then while walking or riding, then once you are safe, you shall commemorate God as He taught you what you previously did not know.