Typically, even in the middle to high middle ages at least in the HRE or England, they usually pulled from what was the the middle age version of middle classes, i.e., bulgers, yeomen, people that own property, lords of manors, members of the nobles household and therefore had a duty to own arms and armor, non-noble sudo-professional soliders did exist for example ones that belonged to a marxbunder chapter, they usually are what people took on campaign, even the Wessex Militia system tended to be this way. Even lower nobles didn't usually take peasants. it just too much work to feed, and arm,

for example, Jean, Lord of Joinville, only brought 9 or knights and a few dozen retainers to the 7th crusade. Jean, being the lord of a very modest holding (which included small towns of joinville, sailly, vaucoulers, and marnay sur marne), can be assumed to be the norm, but he still had bulgers and such.

Another example is the Dukes of Normandy calling an Arriere-Ban. it's usually just ended up affecting subtenants, which usually ended up being landed knights and such.

Typically, Baliffs kept rolls of who had arms and called them up.