[source: Catholic Customs and Traditions, p. 

PROSTRATION WAS COMMON IN ANCIENT PRAYER, JESUS PRAYED THIS WAY
Another prayer position is prostration. It is an intense, total, and dramatic 
expression of adoration, penance, or supplication. It was common among the 
ancient peoples and from them passed into Christian tradition. Jesus himself 
prayed in this way in the Garden of Gethesemane the night before he died: "He 
advanced a little and fell prostrate in prayer" (Matthew 26:39). A full 
prostration by the priests began each Mass until after the early Middle Ages. It 
is still retained at the beginning of the Good Friday liturgy.
[source: Catholic Customs and Traditions, p. 177]