When I lived in Austin a dear friend of mine was a very respected Scholar. We discussed many of these issue at length. Because of my legs often I can not even sit for prayer and need to remain standing. He was the one who told me that if for any valid reason I can not do the physical motions my thoughts would be the same as if I was actually doing them physically.
I find that all scholars I have spoken with or read agree with that for example:
SOME QUESTIONS & ANSWERS RELEVANT TO SALAAT
Answer to the Question asked obtained from AyatullahAl-Uzama Sayyid Ali Al-Husaini Al-Seestani Dama Dhilluhu
Paper # 4 RE DISABLED PERSON
Q) What is the ruling for a disabled person who offers prayers while remaining seated on the chair. Is it correct that such a person joining Namaz must in the beginning stand till Ruku of the 1st Rakaat & then perform all the actions while seated up to the end bowing wherever possible. Is it right for this person to lift the sajdagah & place it in the forehead while reciting the Dhikr of sujood or just bows more without sajdagah touching the forehead ? ( Sajdagah is on the small table infront of the Musalli all the time )
A) Your question is general which requires a detailed reply as it would apply individually according to his incapacitated position.
In short if he is unable to stand while praying, he should sit down, & if he is unable to sit, he should lie down. As long as a person is able to offer prayers standing, he should not sit down. For example, if the body of a person shakes, or moves when he stands, or he is obliged to lean on something, or to incline his body a bit, he should continue to offer prayers standing in whatever manner he can. But, if he cannot st& at all, he should sit upright, & offer prayers in that position. As long as he can sit, he should not offer prayers in a lying posture, & if he cannot sit straight, he should sit in any manner he can. & if he cannot sit at all, he should lie on his right side. If he also cannot lie on the left side then he should lie on his back with his feet facing Qibla.
For example, as your question implies, a person is praying seated on a chair with a table in front of him on which is laid Mor (Sijdagah). As long as he is able to stand up he should do so & recite Takbiratul Ihram in a standing position & if he can continue standing up to recite the Qir’at he should continue to do so & perform Ruku & then sit down to perform Sajdah. If he cannot continue standing up he should sit down after reciting Takbiratul Ihram & perform Ruku in a sitting position. If he cannot stand up & recite Takbiratul Ihram then only he should recite Takbiratul Ihram in a sitting position. If he is offering prayers in a sitting position, & after reciting Hamd & Surah, he is able to stand up & perform Ruku, he should first stand up, & then perform Ruku. But if he cannot do so, he should perform Ruku while sitting.
If a person cannot bow down for Ruku properly, he should lean on something & perform Ruku. & if he cannot perform Ruku even after he has leaned, he should bow down to the maximum extent he can, so that it could be customarily recognized as a Ruku. & if he cannot bend at all, he should make a sign for Ruku with his head.
If a person is supposed to make a sign with his head for Ruku is unable to do so, he should close his eyes with the niyyat of Ruku, & then recite Zikr. & for rising from Ruku, he should open his eyes. & if he is unable to do even that, he should, as a precaution, make a niyyat of Ruku in his mind
, & then make a sign of Ruku with his hands & recite Zikr.
If a person cannot perform Ruku while standing, but can bend for it while sitting, he should offer prayers standing & should make a sign with his head for Ruku. & the recommended precaution is that he should offer another prayer in which he would sit down at the time of Ruku, & bow down for it.
If a person cannot find something high on which he may place the mohr, or any other allowable thing, & if he cannot find any person who would raise the mohr etc. for him, then as precaution, he should raise it with his h& & do Sajdah on it.
(As regards to raising the mohr with his h& & do Sajdah on it, Hujjatul Islam Sayyid Muhammad Rizvi has commented: The basis of your answer is correct; however, when I compared the statement in the Tawzihul Masa'il with what the Sayyid has written in Minhajus Salihiyn (which I consider to be more updated & comprehensive), it seems that the emphasis in the Minhaj is doing it by ishara rather than bring the muhr by h& to the forehead. In any case, the issue of bringing the muhr to the forehead is based on ihtiyat istihbabi, & not ihtiyat wujubi; and, therefore, I would go with the sajda by ishara as the first choice.)
If a person cannot perform Sajdah at all, he should make a sign for it with his head, & if he cannot do even that, he should make a sign with his eyes. & if he cannot make a sign even with his eyes he should, on the basis of obligatory precaution, make a sign for Sajdah with his hands etc. & should make a niyyat for Sajdah in his mind
, & recite the obligatory Zikr.