This is the 3532nd most frequent Korean word.
무릎을
"무릎을" means "knee" in English (objective form of "무릎" meaning knee).
The word "무릎을" is the direct object in the sentence, referring to the knees, indicating the action performed involving them (kneeling).
그는 무릎을 꿇고 기도했다.
He knelt down and prayed.
Here, "무릎을" is also the direct object, showing the knees as the recipient of the massage action.
운동 후에 나는 무릎을 마사지했다.
After exercise, I massaged my knees.
In this sentence, "무릎을" is used to describe the knees, emphasizing the action of checking them after falling.