to fall to one's knees; to go down on one's knees (e.g. to express respect)
膝をついて謝罪した。
I got on my knees and apologized.
激しい痛みで彼は膝をついた。
He fell to his knees from the intense pain.
選手が疲れ果てて膝をついている。
The athlete is down on his knees, exhausted.