Bending at the Waist until the Knees (בריכה) Hebrew breikhah, Arabic ruku
The source of the Barchu prayer is found in three places, before the Shema, and before Zimun. In all these places it is an invitation of a group of people to prayer and in most cases it is a blessing that precedes the study of Torah.
The Rabbinic custom in the Barchu before the Shema and Torah Readings is to bow from the waist. The Karaites custom is bending of the upper part of the body until the hands touches the knees. The Karaites call it בריכה, breikha, which is linguistically related to the word Barchu. The Islamic custom and the Karaite custom are identical and it is reasonable to assume that this is the original Talmudic custom as well. The hands on the knees relating to the word for “knee” berekh as well as the word for Blessing. The Islamic term for this position is ruku, which is also linguistically related to the word breikha.