The World Federation One Stop Fiqh

Ruling 1022

If a person cannot bend forward to the extent of rukūʿ, he must lean on something and perform rukūʿ. If when he leans on something he still cannot perform rukūʿ in a normal manner, he must bend forward to the extent that it can be commonly considered to be rukūʿ. If he cannot bend forward to even this extent, he must perform rukūʿ by indicating with his head.