Can a woman pray while sitting if prostrating affects her knees?

One is not permitted to abandon standing in obligatory prayers except for a legitimate Shar'i excuse such as illness, fear, or weakness. A dark knee is not an excuse. Whoever prays sitting without an excuse, their prayer is invalid. As for supererogatory prayers, it is permissible to perform them sitting, but standing is better, and the reward for the one who prays standing is double the reward of the one who prays sitting.