If someone has knee and ankle issues, can they pray seated in a chair, and is their prayer considered valid? Additionally, how should they perform Sujood while sitting in the chair?
It depends in what he can do and he can not do. He is responsible to do everything which he can in his Salah, like standing (Qiyaam). If he is unable to do Rukoo’ or Sojood, then he sit in a chair and do whatever he can. While sitting in a chair, Sojood is on a table in front of him, or he can take the Turbah (Mohr) in his hand and put his forehead on it.
Wassalam.