Handbook of ROCOR Liturgical Practice:

When do we kneel, rather than make a prostration?

Kneeling (i.e. standing on one’s knees) is not a traditional posture of prayer in Orthodoxy. The only occasion on which we kneel, rather than prostrate ourselves, is during the ‘Kneeling prayers’ on the Vespers of Holy Pentecost. At all other times we are either standing, or we make a full prostration to the ground.