Should a mother who suffers from knee osteoarthritis pray standing, then sit after the bowing (rukūʿ) and nod her head for prostration (sujūd) in all the cycles (rakʿāt), or only in the first one and then sit for the rest of the cycles?

If the mother is unable to sit for the Tashahhud and prostrate, then she prays standing and motions for prostration. If she is able to bow and then sit and motion for prostration, she is obligated to do so, and she must do this in all rak'ahs if she is able. If she is unable to do so except in one rak'ah, she should do it; because standing is a pillar [of prayer]. Prayer must be performed in its original form except for the one who is unable, due to the saying of the Prophet, peace be upon him, to Imran ibn Husayn: "Pray standing; if you are unable, then sitting; and if you are unable, then on your side," and due to his saying: "Make your prostration lower than your bowing."