Heavy infantry originated in the Middle East, with the Sumerian and Assyrian infantry. The idea that Persian armies were untrained, undisciplined, poorly-equipped levy hordes with cavalry (which the internet likes to think must all be horse-archers, which is nonsense--Xenophon and Arrian describe enormous formations of heavily-armored cavalry with praise) doing the work is pure nonsense, and no self-respecting classicist would suggest such a thing (although people like Hanson apparently would despite knowing better than to do so). The core of the Persian army was its heavy infantry corps, spearheaded by the Persian and Median troops. Herodotus and Arrian both refer to large elite contingents--Herodotus speaks of the Immortals and Arrian mentions the royal guard, which many scholars consider to be the same thing. In both cases the formations are enormous--Herodotus numbers the Immortals alone at ten thousand, a full army right there, with numerous contingents from outside Media and Persia, and Arrian lists the royal guard as composing almost the entirety of the native Persian infantry that engaged Alexander throughout his campaign (so that would exclude the Greek mercenaries, which were not Persian, and the Asiatic levies, which did not see combat--the only other important native Persian infantry to see battle were the Cardaces, who were not normal troops but an experiment). The only important levied infantry (apart from much of Xerxes' enormous army, which must have relied on a lot of levies, though few probably saw combat) that appears in any of our sources is the Asiatic levies at Gaugamela--and these were brought by Darius' barons, not Darius. Darius didn't want the Asiatic levies there and he stuffed them in the rear, where they did nothing except run away. Herodotus says that the native Persian troops under Xerxes, who made the majority of his combat infantry, were armed with swords, spears, and bows, carried wicker shields (about as effective as the leather shields the Romans had) and wore scaled armor--though no helmets, wearing instead the tiara, the traditional headdress of the Persians. Xenophon goes to great lengths to describe in detail the equipment of the various Persian contingents at Cunaxa, noting with praise their good armor and weapons, as well as repeatedly praising the discipline of the Persians, particularly their cavalry (Xenophon also expresses surprise at the Persian custom of going into battle with no helmet, though this might be because Cyrus didn't wear a helmet and took a javelin to the face).

EDIT: Oh, I forgot, a lot of satraps had their own "Immortals," satrapial contingents of essentially professional heavy infantry, although we don't know too much about them

Yes it is a misconception as Xenophon said, Heavy infantry originated in the Middle East, and what do you know Muslim armies like the Turks and Arabs did employ heavy infantry, one example would be the Umayyad Empire which basically copy and pasted Persian style infantry with a helmet, face shown while wearing heavy scaled hauberk. They usually be in the front. Same thing during the Abbasid Era where Byzantine-Abbasid had clashes in the frontier zone. Abbasid Infantry would be wearing lamellar armour and mail armour, the style was actually not that far from the Byzantine.

Also there are more ethnic groups like Azerbeijanis, Turcomans, Kurds and Armenians.

My Source: Osprey Books

A good image MINUS the Creepy Mask and title. That is how heavy it can get during the medieval.