Siman 113
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א אֵלּוּ בְּרָכוֹת שֶׁשּׁוֹחִין בָּהֶם: בְּאָבוֹת תְּחִלָּה וָסוֹף, וּבְהוֹדָאָה תְּחִלָּה וָסוֹף; וְאִם בָּא לִשְׁחוֹת בְּסוֹף כָּל בְּרָכָה, אוֹ בִּתְחִלָּתָהּ, מְלַמְּדִין אוֹתוֹ שֶׁלֹּא יִשְׁחֶה; אֲבָל בְּאֶמְצָעִיתָן, יָכוֹל לִשְׁחוֹת.
1 The Laws of Bowing in the Eighteen Blessings [i.e. Amidah]. Containing 9 S'ifim These are the blessings in which we bow: in Avot [the first blessing], [at the] beginning and end; in Hoda-a [the second-to-last blessing], [at the] beginning and end. And if one comes to bow at the end of every blessing or at its beginning, we teach [that person] that one does not bow, but in their [i.e. the blessings'] middles, one may bow.
ב הַנּוֹהֲגִים לִשְׁחוֹת בְּרֹאשׁ הַשָּׁנָה וְיוֹם כִּפּוּר כְּשֶׁאוֹמְרִים זָכְרֵנוּ וּמִי כָמוֹךָ, צְרִיכִים לִזְקֹף כְּשֶׁמַּגִּיעִים לְסוֹף הַבְּרָכָה. הַגָּה: וְאַף עַל גַּב דִּבְאָבוֹת כּוֹרֵעַ בְּסוֹף הַבְּרָכָה, מִכָּל מָקוֹם צָרִיךְ לִזְקֹף מְעַט בְּסוֹף זָכְרֵנוּ, כְּדֵי שֶׁיְּהֵא נִרְאֶה שֶׁחוֹזֵר וְכוֹרֵעַ מִשּׁוּם חִיּוּב (דִּבְרֵי עַצְמוֹ לפי' הַטּוּר).
2 Those who have the custom to bow on Rosh Hashana and Yom Kippur when they say "Zokhreinu" ("Remember us") and "Mi Kamokha" ("Who is like You") [the insertions into the first blessing of the Amidah] need to straighten [themselves] up when they reach the end of the blessing. Rema: And even though in [the blessing of] "Avot", one bows at the end of the blessing, nevertheless, one needs to straighten a little at the end of "Zokhreinu" so that it should be apparent that one is going back and bowing [again] because of the obligation [to bow at the end of the blessing of "Avot"] (His own opinion based on the Tur)
ג הַכּוֹרֵעַ בִּוְכָל קוֹמָה לְפָנֶיךָ תִּשְׁתַּחֲוֶה, אוֹ בְּוּלְךָ אֲנַחְנוּ מוֹדִים, אוֹ בְּהוֹדָאָה דְּהַלֵּל וּבִרְכַּת הַמָּזוֹן, הֲרֵי זֶה מְגֻנֶּה. (פי' שֶׁאֵין לִכְרֹעַ אֶלָּא בְּמָקוֹם שֶׁתִּקְנוּ חֲכָמִים).
3 One who bows [when saying] "U'vechol Koma Lefanecha Tishtachaveh" ["and every upright one shall prostrate oneself before You"] or "U'lecha Anachnu Modim" ["and to You [alone] we give thanks"] [both from the "Nishmat Kol Chai" prayer], or [when saying] "Hoda'a" [Thanksgiving] in Hallel or Birkat Hamazon [The Blessings after a Meal], behold this is improper (meaning that one doesn't bow other than in a place that the Sages established).
ד הַמִּתְפַּלֵּל, צָרִיךְ שֶׁיִּכְרַע עַד שֶׁיִּתְפַּקְּקוּ כָּל חֻלִיּוֹת שֶׁבַּשִּׁדְרָה; וְלֹא יִכְרַע בְּאֶמְצַע מָתְנָיו וְרֹאשׁוֹ יִשָּׁאֵר זָקוּף, אֶלָּא גַּם רֹאשׁוֹ יָכֹף כְּאֶגְמוֹן.
4 One who is praying needs to bend until all the vertebrae in one's spine stick out. One should not bow from one's hips with one's head remaining straight, rather one should also bow one's head like a reed.
ה וְלֹא יִשְׁחֶה כָּל כָּךְ עַד שֶׁיִּהְיֶה פִּיו כְּנֶגֶד חֲגוֹר שֶׁל מִכְנָסַיִם; וְאִם הוּא זָקֵן אוֹ חוֹלֶה וְאֵינוֹ יָכוֹל לִשְׁחוֹת עַד שֶׁיִּתְפַּקְּקוּ, כֵּיוָן שֶׁהִרְכִּין (פי' שֶׁהִשְׁפִּיל) רֹאשׁוֹ, דַּיּוֹ, מֵאַחַר שֶׁנִּכָּר שֶׁהוּא חָפֵץ לִכְרֹעַ אֶלָּא שֶׁמְּצַעֵר עַצְמוֹ.
5 One should not bow so much that one's mouth would be opposite the belt of one's pants. If one is old or sick and cannot bow until [all the vertebrae in one's spine] stick out, since one bends (i.e. lowers) one's head, it is sufficient since it can be recognized that one wished to bow, but rather that [the lack of bowing] is on account of one's pain.
ו כְּשֶׁהוּא כּוֹרֵעַ, יִכְרַע בִּמְהִירוּת בְּפַעַם אַחַת, וּכְשֶׁהוּא זוֹקֵף, זוֹקֵף בְּנַחַת רֹאשׁוֹ תְּחִלָּה, וְאַחַר כָּךְ גּוּפוֹ, שֶׁלֹּא תְּהֵא עָלָיו כְּמַשּׂאוֹי.
6 When one bows, one should bow quickly and all at once. When one straightens up, one straightens gently, [with] one's head [up] first and then afterwards, one's body, so that it not be burdensome for oneself.
ז כְּשֶׁכּוֹרֵעַ, כּוֹרֵעַ בְּבָרוּךְ, וּכְשֶׁזּוֹקֵף, זוֹקֵף בְּשֵׁם.
7 When one bows, one bows at [the word] "barukh" and when one straightens up, one straightens at the [Divine] Name.
ח הַמִּתְפַּלֵּל וּבָא כְּנֶגְדּוֹ עַכּוּ"ם, וְיֵשׁ לוֹ שְׁתֵּי וָעֵרֶב בְּיָדוֹ, וְהִגִּיעַ לְמָקוֹם שֶׁשּׁוֹחִין בּוֹ, לֹא יִשְׂחֶה, אַף עַל פִּי שֶׁלִּבּוֹ לַשָּׁמַיִם.
8 One who is praying, and an idol worshiper came in front of one with a [cross] in hand and [the person praying] arrived at the point at which where one bows, one should not bow, even though one's heart is [directed] toward heaven [i.e worshiping only God].
ט אֵין לְהוֹסִיף עַל תָּאֳרָיו שֶׁל הקב"ה, יוֹתֵר מֵהָאֵל הַגָּדוֹל הַגִּבּוֹר וְהַנּוֹרָא; וְדַוְקָא בַּתְּפִלָּה, מִפְּנֵי שֶׁאֵין לְשַׁנּוֹת מִמַּטְבֵּעַ שֶׁטָּבְעוּ חֲכָמִים, אֲבָל בְּתַחֲנוּנִים אוֹ בַּקָּשׁוֹת וּשְׁבָחִים שֶׁאָדָם אוֹמֵר מֵעַצְמוֹ, לֵית לָן בָּהּ; וּמִכָּל מָקוֹם נָכוֹן לְמִי שֶׁיִּרְצֶה לְהַאֲרִיךְ בְּשִׁבְחֵי הַמָּקוֹם, שֶׁיֹּאמַר אוֹתוֹ בִּפְסוּקִים.
9 One may not add to the descriptions of the Holy One Who Is Blessed more than "The Great and the Mighty and the Awesome God". And this is specifically in the Prayer [i.e. Amidah], since one may not change the formulation that the Sages formulated. But in the supplications, pleas and praises that a person says oneself, there is no [problem] with it. Nevertheless, it is proper that one who wants to lengthen the praises of the Omnipresent should say it using [biblical] verses.