بسم الله الرحمن الرحيم
If someone is mazur (excused) to such an extent that he cannot do ruku and sajdah and it is also impossible for him to offer salah sitting on ground then due to compulsion he shall be allowed to sit on chair and do ruku and sajdah with gesture. And though it is lawful to offer nafl salah by sitting without an excuse but if one is able to do ruku and sajdah then doing ruku and sajdah with gesture is not lawful.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.