“Kneeled upon his knees three times a day, and prayed, and gave thanks before his God, as he had done previously”

It is appropriate, though, for a High Priest to bow at the end of each and every blessing; and for a king to bow at the beginning of each and every blessing and at the end of each and every blessing. This is because the more lofty one’s status, the more important it is to demonstrate his subservience to God. Rabbi Yitzḥak bar Naḥmani said: It was explained to me directly from Rabbi Yehoshua ben Levi himself differently: An ordinary person, conducts himself as we said; a High Priest bows at the beginning of each and every blessing; the king, once he has bowed at the beginning of the first blessing, does not rise until he concludes the entire prayer, as it is stated: “And it was that when Solomon finished praying all of his prayer to the Lord, he rose from before the altar of the Lord, from kneeling upon his knees with his hands spread forth toward the heavens” (I Kings 8:54). Having mentioned Solomon bowing, the Gemara distinguishes between various types of bowing. The Sages taught in a baraita: The term kidda means bowing upon one’s face, with his face toward the ground, as it is stated: “Then Bathsheba bowed [vatikod] with her face to the ground” (I Kings 1:31). Keria means bowing upon one’s knees, as regarding Solomon it is stated: He finished praying and “he rose from before the altar of the Lord, from kneeling [mikkeroa] upon his knees.” Finally, hishtaḥava’a, that is bowing with one’s hands and legs spread in total submission, as it is stated in Jacob’s question to Joseph in response to his dream: “Shall we, I and your mother and your brothers, come and bow down [lehishtaḥavot] to you to the ground?” (Genesis 37:10). On the topic of bowing, Rav Ḥiyya, son of Rav Huna, said: I saw Abaye and Rava, who would lean their heads and not actually prostrate themselves on the ground. The Gemara asks: One baraita taught: One who bows in the blessing of thanksgiving, it is praiseworthy. And it was taught in another baraita: One who bows in the blessing of thanksgiving, it is reprehensible. These baraitot are contradictory. The Gemara reconciles these two baraitot: This is not difficult; this baraita, which praises one who bows in the blessing of thanksgiving, refers to one who bows at the beginning of the blessing. This baraita, which condemns one who bows in the blessing of thanksgiving, refers to one who bows at the end of the blessing. Rava bowed in the blessing of thanksgiving, both beginning and end. The Sages said to him: Why does our master do this? He said to them: I saw Rav Naḥman who bowed in the blessing of thanksgiving, and I saw Rav Sheshet who did so as well. But wasn’t it taught in a baraita that one who bows in thanksgiving, it is reprehensible? Rava explained: That baraita refers to one who bows in the thanksgiving that is in hallel, when one recites: Give thanks to the Lord. Then, bowing is inappropriate. The Sages continue to question Rava’s conduct: But wasn’t it taught explicitly in a baraita: One who bows in thanksgiving or in thanksgiving of hallel, it is reprehensible? The term thanksgiving unqualified does not refer to thanksgiving of hallel; it obviously refers to the blessing of thanksgiving recited in the Amida prayer. One who bows in either, it is reprehensible. The Gemara rejects this challenge as well: When that baraita was taught, it was in reference to the blessing of thanksgiving, the second blessing recited in Grace after Meals: We thank You.

The Sages taught: The term kidda indicates falling upon one’s face, with one’s face toward the ground, as it is stated: “Then Bathsheva bowed [vatikod] with her face to the ground” (I Kings 1:31). Keria means bowing upon one’s knees, as it is stated with regard to Solomon: He finished praying and “he rose from before the altar of God, from kneeling [mikkeroa] upon his knees” (I Kings 8:54). Finally, hishtaḥava’a, that is bowing with one’s arms and legs spread in total submission, as it is stated that Jacob asked, in response to Joseph’s dream: “Shall I and your mother and your brothers indeed come to bow down [lehishtaḥavot] to you to the ground?” (Genesis 37:10).

(13) Prostration. After he has raised his head, subsequent to the fifth genuflection, the worshiper seats himself on the floor and "falls upon his face" and offers all the supplications he pleases. Genuflection, wherever mentioned, always refers to bending the knees. Kidah means "falling on the face" [with knees bent]. Hishtahvayah means stretching out the hands and feet, so that one is lying prone with face to the ground.

One should put the (entire length) of his foot next to the other in such a so that they will seem as much as possible as one leg, like the angels. (unlike those who just put their heels together). Meaning, their legs look like one leg.

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And those who are exacting have practiced to shake at the time that we read the Torah as an illustration of the Torah that was given with trembling, and so too at the time that we pray, because of "all my bones will say to you, Lord who is like You?" (Abudarham)

R. Yehiel Michal Epstein (d. 1908)

And during the Amidah there are some who sway and some who don't and it depends on the person's nature. If by swaying, his kavanah improves, then he should sway; and a person whose kavanah is clearer when he stands perfectly still should not sway - and [either option] should be done for the sake of heaven...

Prayer is union with the Shechina. Just as two people will move their bodies back and forth as they begin the act of love, so must a person accompany the beginning of prayer with the rhythmic swaying of the body. But as one reaches the heights of union with the Shechina, the movement of the body ceases.