Related for Berakhot 56:31

Just like the Torah established a set time to read the Shema, so too the Sages established a set time for [the] prayer [of Shemoneh Esreh]. Why did they say [that] the morning prayer [of Shmoneh Esreh can be prayed only] until noon? Because the morning Tamid (daily offering) [sacrifice] could be brought [only] until noon. Rebbi Yehudah says, “[The morning prayer of Shmoneh Esreh can be prayed only] until [the end of the first] four hours [of the day].” And why did they say that the Mincha (afternoon) prayer [of Shemoneh Esreh can only be prayed] until the evening? Because the afternoon Tamid [sacrifice] could be brought [only] until the evening. Rebbi Yehudah says, “[The Mincha prayer of Shmoneh Esreh can be prayed only] until Plag Hamincha.” And when is Plag Hamincha? Eleven minus one quarter hours [of the day] (i.e. 10 ¾ hours). And why did they say that the evening prayer [of Shmoneh Esreh] does not have a set time? Because the limbs and fats [of the sacrifices] could be brought the whole night. And why did they say that the Mussaf prayer [of Shmoneh Esreh can be prayed] the whole day? Because the Mussaf (additional offering) sacrifice could be brought the whole day. Rebbi Yehudah says, “[The Mussaf prayer can be prayed] until [the end of the first] seven hours [of the day].”

Rebbi Simon said, corresponding to the 18 bones in the spine since when a person prays he has to bend all of them bowing down. What is the reason? (Ps. 35:10) “All my bones shall say, o Eternal, who is like You?¹⁴¹In the Babli (28b), this reason for the number 18 is given by Rebbi Tanḥum in the name of R. Joshua ben Levi who notes that, when bowing down in prayer, all joints must be moved.”