The phrase "fell on one's knees" implies dropping down to the ground suddenly, often in a worshipful or pleading manner. Antonyms for this phrase may include standing or remaining upright, keeping one's balance, or maintaining composure. Alternatively, synonyms for "fell on one's knees" might include kneeling, prostrating oneself, or bowing down. The context in which the phrase is used may determine the most appropriate antonym or synonym - for instance, in a religious setting, "fell on one's knees" may have positive connotations, whereas it may connote weakness, desperation, or defeat in other contexts.