The Anabathmoi are a series of hymns sung after the hypokoe during Orthros. They find their origin in the ascension psalms developed in Jewish worship from the ascension to Jerusalem after captivity and, later, sung at each step leading up to the Jewish temple in Jerusalem.
Source: Divine Prayers and Services of the Catholic Orthodox Church of Christ. arr. the late Reverend Seraphim Nassar. Antiochian Orthodox Christian Archdiocese of North America. 3rd ed. 1979.