The Biur Halacha notes that although it is forbidden to bow during
davening except at the points designated by Chazal, there is a custom to bow when reciting and responding to
Barechu. A precedent for this custom can be found in an incident in Tanach, when Dovid Hamelech urged everyone to bless Hashem (
barechu) and the people fell to the floor and bowed. The Biur Halacha concludes, "
minhag Yisrael Torah-Jewish customs are [authentic] Torah." Although the bowing mentioned in Tanach probably predates the prohibition against bowing at other points in
davening, it is unlikely that Chazal would have forbidden something that is clearly permitted by the
pasuk.
(
ביה"ל ד"ה הכורע; ביאורים ומוספים דרשו, 14)