What is the ruling on performing obligatory prayer when one is physically unable to stand fully upright?
Chapter on the Times When Prayer is Prohibited
Al-Mughni
Book of Prayer
Primary text
If an individual is capable of standing but only in a posture resembling bowing, such as someone severely hunched (Al-Ahdab), or if one is in a low-ceilinged house preventing full standing, or aboard a ship, or fears discovery if raising one's head, the obligation depends on the cause. If the inability to stand fully is due to a physical defect like a hump or old age, then standing in a manner commensurate with their ability is obligatory upon them. This is supported by the Prophet's command: "Pray standing, and if you are unable, then sitting." This scenario implies inability to perform the complete standing posture.
Supporting text
If the inability to stand fully is due to reasons other than a physical defect, there is a possibility that standing is not obligatory, drawing an analogy to the case of the hunched person. Imam Ahmad (may Allah have mercy on him) stated regarding one on a ship unable to stand fully due to the low ceiling, that such a person should pray sitting, unless the inability is slight, in which case a similar ruling applies to comparable situations.