Question: If one who cannot bend one's knees is also unable to sit on the floor, how should one perform salat sitting on a chair, in an armchair, or on a bed?
ANSWER
One who cannot bend one's knees is allowed to perform salat by gestures (ishaarah) sitting on the floor or in one's bed with one's legs toward the qibla direction. To perform salat by gestures, one should bow one's head a little for ruku' and more than it for sajda. If one prefers to perform salat in an armchair or on a chair, then one should put one's legs on a coffee table or another armchair (or a chair) placed before one. If one is unable to perform salat in this way too, then one should perform salat lying by gestures.

A paralyzed person confined to a wheelchair is allowed to perform salat on his/her wheelchair if there is no one to put his/her legs on a, for example, coffee table placed before him/her. It is not permissible for him/her to perform salat on his/her wheelchair hanging his/her legs down if there is a person who will help him/her put his/her feet on a coffee table. Similarly, a sick person who can bend his/her knees is not allowed to stretch his/her legs toward qibla when performing salat sitting. He/she can sit according to his/her health state. For example, he/she may sit in the way he/she sits in salat or, if it is not possible, may sit cross-legged.