It is well known that Islam is the religion of easiness. The person who finds it extremely difficult to pray while standing can pray while sitting, and he will get a complete reward, Insha Allah. However, one should keep in mind that a slight difficulty is not a valid excuse to perform prayer while sitting.
Under the title Standing During the Obligatory Prayers, the late Azharite scholar Sheikh Sayyed Sabiq, states the following in his book Fiqh As-Sunnah:
“One must stand during the prayer, if it is possible for one to do so. Referring to this, Allah Almighty says “Guard and preserve the prayers and the mid-most prayer and stand for Allah with devotion.” (Al-Baqarah: 238)
`Umar Ibn Hussain reported that he had some physical problem, so he asked the Prophet, peace and blessings be upon him, about the prayer, and he said, “Pray standing; if you are not able to; pray sitting, if you are not able to; pray (while lying) on your side.” (Reported by Al-Bukhari)
For voluntary prayers, one can pray sitting even if he can stand, but one who stands receives a larger reward than one who sits.
`Abdullah Ibn ‘Umar, may Allah be pleased with him, quoted the Prophet, peace and blessings be upon him, as saying: “The prayer of one who sits is half of the prayer.” (Reported by Al-Bukhari and Muslim)
If one cannot stand, he may pray according to what he is capable of doing, as Allah does not burden a soul beyond what it can bear. In this case, he will get a complete reward for the prayer.
Abu Musa, quoted the Prophet, peace and blessings be upon him, as saying: “If a slave (of Allah) is sick or travels, he will get a reward for those acts similar to what he would get if he was healthy and at home.””
Thus, if one finds it extremely difficult to pray while standing, they can pray while sitting, and there is no blame on them as the issue is beyond their ability and they find it extremely difficult to pray while standing. Allah Almighty says: “…He hath chosen you and hath not laid upon you in religion any hardship…” (Al-Hajj: 78)