A few days ago on Vent, Oberon brought up the old issue concerning Medieval knights on horseback wearing heavy armour and how it negatively effected their performance in battle. I forget the specific arguments made off-hand, although I do know the discussion snowballed into one about Landsknechts and other aspects of Renaissance-era warfare(jenocyde was nerding me).
Anyways.....here's something that should clarify the issue, on the general level at least:
4. Knights in full plate armor were clumsy and slow.
False. The popular belief in untutored knights clumsily swinging crude swords while awkwardly lumbering around in heavy armor is inaccurate and uninformed. Mistaken claims that Medieval armored horsemen had become clanking tanks or that unhorsed a knight was at his foe's mercy have become common even among some medieval historians. A warrior in plate armor was far from being the sluggish lobster so frequently mischaracterized by military writers. While an armored man was not as agile as an unarmored one, plate armor overall was well balanced and ingeniously designed to permit considerable maneuverability and nimbleness. This fact is clearly expressed in the fighting literature on armored combat and born out by modern experiments in both antique armor specimens and historically accurate reproductions. Unlike what has been notoriously misrepresented in popular culture, a well-trained and physically conditioned man fighting in full harness was typically a formidable opponent (and there were many different kinds of armor for foot or mounted combat). But this is not to say that fighting in full plate armor was not tiresome or stifling. Armor restricted breathing and ability to ventilate body heat, as well as limited vision and hearing. If armor did not work well it would not have been around for so long in so many different forms. (For more on this see: "Medieval Armor: Plated Perfection" in Military History, July 2005).
Top Myths of Renaissance Martial Arts & Swords