Have you ever read about the Battle of Nicopolis?http://en.wikipedia.org/wiki/Battle_of_ ... The_battle"The French charge crushed the untrained conscripts in the Turkish front line and advanced into the lines of trained infantry, though the knights came under heavy fire from archers and were hampered by rows of sharpened stakes designed to skewer the stomachs of their horses. Chroniclers write of horses impaled on stakes, riders dismounting, stakes being pulled up to allow horses through, and the eventual rout of the Turkish infantry, who fled behind the relative safety of the sipahis. Coucy and Vienne recommended that the French pause to reform their ranks, give themselves some rest and allow the Hungarians time to advance to a position where they could support the French. They were overruled by the younger knights who, having no idea of the size of the Turkish force, believed that they had just defeated Bayezid's entire army and insisted on pursuit.
The French knights thus continued up the hill, though accounts state that more than half were on foot by this point, either because they had been unhorsed by the lines of sharpened stakes or had dismounted to pull up stakes. Struggling in their heavy armor, they reached the plateau on the top of the slope, where they had expected to find fleeing Turkish forces, but instead found themselves facing a fresh corps of sipahis, whom Bayezid had kept in reserve. As the sipahis surged forward in the counterattack sounding trumpets, banging kettle drums and yelling "God is great!", the desperation of their situation was readily apparent to the French and some knights broke and fled back down the slope. The rest fought on "no frothing boar nor enraged wolf more fiercely," in the words of one contemporary chronicler. Admiral de Vienne, to whom was granted the honor as the eldest knight of carrying the French standard into battle, was wounded many times as he attempted to rally the morale of his countrymen, before being struck down dead. Other notable knights who were slain include Jean de Carrouges, Philippe de Bar and Odard de Chasseron. The Turks threatened to overwhelm Nevers and his bodyguard threw themselves to the ground in silent submission to plead for the life of their liege lord. Notwithstanding the declaration of jihad, the Turks were as interested in the riches that could be gained by ransoming noble captives as anyone else, and took Nevers prisoner. Seeing Nevers taken, the rest of the French yielded."
This type of warfare is a very physical activity. You have done HEMA so you know how exhausting even 5 minutes of fighting can be. Now imagine a crazy type of HEMA tournament, in which you walk up a very long staircase, stopping to fight every few minutes, in 60 or 80 lbs of armor. I believe heat also plays a major role here as armor, especially plate armor, does not breathe and heats up the wearer badly.
I think it was normal in battles of this period for troops, especailly armored troops fighting on foot to come out of the line and rest if they could, as you can see some Polish Knights doing in this contemporary (i.e. 16th Century) painting of the Battle of Orshahttp://upload.wikimedia.org/wikipedia/c ... -08%29.jpg
Also remember that when heavy cavalry are fighting dismounted, they are not wearing armor which is designed for fighting on foot.
But armor is not a necessary component of this type of scenario, I am aware of battles in WW II and Korea in which vast numbers of American soldiers just reached the point of physical exhaustion and surrendered or simply quit fighting and let themselves be slaughtered. Same thing happened to the Japanese fighting against American Marines at Guadalcanal and New Guinea on several occasions. Romans also exploited similar morale collapses of poorly armed indigenous European warriors in vast numbers on several doccumented occasions. So did the Mongols. I think the rain of missiles has a lot to do with it and so do the fluid dynamics in cases like Agincourt. But it's basically a morale collapse combined with exhaustion which can and does happen to almost any military unit under the right kind of combat stress for long enough.
"In the case of an ailing social order, the absence of an adequate diagnosis... is a crucial, perhaps decisive part of the disease." -Zygmunt Bauman
"With any luck we'll be in Stalingrad by winter. " - Anyonymous German soldier