There is a very good writeup here: http://www.thearma.org/essays/2HGS.html#.VmWSK8lOnqA

It focuses on the 2 handers used by Swiss and German soldiers during the late Renaissance, like the iconic Zweihander of the flamboyant and feared Landsknecht.

First off, true greatswords that are not knock offs or bad replicas are far lighter than many would expect, often between 5-7 lbs. Yes it requires a strong arm, but these were relatively rare weapons, probably wielded by only a handful of specially selected and trained troops in a unit. So we can expect that this type of blade would be given to a large and strong soldier and he could probably wielded it quite gracefully.

But what was the purpose? Where did this specially equipped soldier(s) fit into an organized unit of pikemen or halberdiers? I often see people stating that the great sword would be used to hack the points off enemy pikes. Personally I don't buy that- standing against a pike wall and flailing your great sword in front of you seems like a terrible idea, especially because you would have little room to operate in a crowded formation. A great sword in close formation basically would just be wielded like a spear for long thrusts. You would need 5-6 feet minimum on every side to slash safely. Maybe the sword was used to push aside enemy pikes, or allow the wielder to crash into a hole in the enemy ranks. But as long as the stuff hasn't hit the fan, the Zweihander is really just an expensive, short pike, or less powerful halberd.

My view is this: a Zweihander functions as a spear or pike when you are in formation. The weilder probably isn't supposed to be a first line soldier. But in special situations, he can utilize his weapon differently. You can act as a shock troop attacking the flanks, or fend off multiple attackers when your lines break and you need to defend the banner or an officer. This would explain why the great sword morphed into a ceremonial item as well.

This is obviously my opinion and you will find a number of others, but the article I cited has a good number of primary and secondary sources, plus they are very skilled in Renaissance sword fighting and have a good understanding of what works in a fight and what does not.