Explanation of the Hadeeth "Pray Standing"

The Prophet ﷺ said in the hadeeth of 'Imraan bin Husayn: "Pray standing, if you are not able (to do that) then pray sitting, if you are not able (to do that) then pray lying down." And here I am amongst you who can say that I can pray standing, make ruku' and sujud or that I cannot. No one can make the judgement, I alone judge for myself. And Indeed Allah has spoken the truth, {Nay! Man will be a witness against himself (as his body parts [skin, hands, legs, etc.] will speak about his deeds).} [Al-Qiyamah: 14]