fall on knees

fall on (one's) knees

To kneel down, usually in a show of respect, fealty, awe, subjection, or reverence. The members of the congregation all fell on their knees to begin their morning prayer. I fell on my knees at the sight of the devastation.
See also: fall, knee, on

fall on one's knees

 and fall to one's knees
to kneel down, usually in respect. The people fell on their knees and prayed in gratitude for their salvation from the flood. They fell to their knees in awe.
See also: fall, knee, on