fall to one's knees
drop to one's knees
sink to one's knees

I fell to my knees next to him and took his hand.

She fell to her knees and would not look at him.

It might be better if you fell to your knees.

At the moment of death, the serving women fall to their knees.

But she couldn't stand, and fell to her knees at the edge of the design.

He fell to his knees, and I didn't go to help.

After a couple of steps I fell to my knees.

He fell on to his knees, still holding her down.

I fell to my knees, and didn't even feel the impact.

The second shot took him a little higher up the back because he was falling to his knees.