When was the bending of the knees in bowing introduced?

Question

The Gemara in Berachos 12a says that we bow at Baruch and we straighten at Hashem. Nowadays the familiar practice is to bend the knee at Baruch, to bow at Atah, and to straighten at Hashem, as stated in Magen Avraham 113:4. Why did the practice change, and when?

I think you are conflating or confusing bowing in Borachu and in the Amida. The TB in Berachos 12a is talking about Borachu. — Yehuda W, Jun 12 '19 at 15:07

score 1 · Answer 1 · edited Apr 13 '17 at 12:42

Can't say for sure when it was introduced, but as Double AA commented it's [already] mentioned in the Zohar in פרשת עקב on דף רע''א ע''ב:

בָּרוּךְ דִּצְלוֹתָא, בַּר נָשׁ כּוֹרֵעַ בֵּיהּ בְּבִרְכּוֹי, וְגָחִין רֵישָׁא בְּאַתָּה, בְּגִין דְּאַתָּה אִקְרֵי רֹאשׁ. וְעַל דָּא כֹּהֵן נָטִיל בָּרֹאשׁ, וְאִיהוּ רֹאשׁ תָּדִיר. וּבְּגִין כָּךְ כְּרִיעָה בְּבָרוּך. וְגָחִינוּ דְּרֵישָׁא בְּאַתָּה.‏

Which translates to:

בַּבָּרוּךְ שֶׁל הַתְּפִלָּה אָדָם כּוֹרֵעַ בְּבִרְכָּיו, וְגוֹחֵן רֹאשׁוֹ בְּאַתָּה, מִשּׁוּם שֶׁאַתָּה נִקְרָא רֹאשׁ. וְעַל זֶה כֹּהֵן לוֹקֵחַ בָּרֹאשׁ, וְהוּא תָּמִיד רֹאשׁ. וְלָכֵן כְּרִיעָה בְּבָרוּךְ, וּגְחִינַת הָרֹאשׁ בְּאַתָּה.‏

At Baruch one bends one's knees, and bows at Ata, etc.

When was the bending of the knees in bowing introduced?

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