One of the common misconceptions about medieval armor was that it was incredibly cumbersome. An oft-repeated tale was that knights had to use a crane to get themselves onto their horses. This seems to come from Henry VIII, who at his fattest could barely move himself, let alone a load of armor.
As this video briefly and clearly shows, armor had a lot more movement than generally thought.
In The Face of Battle, John Keegan points out that the load for the average infantryman stayed the same for much of history. The Tommies going "over the top" in World War One carried just as much weight as an armed and armored medieval knight. Both of these warriors could climb, get up from a prone position, and manage a lumbering run.
The real problem for knights was heat exhaustion. With the faceplate down and most of the body covered, medieval knights often passed out from their exertions. This was an especially serious issue when fighting the Crusades in the Middle East.
For more detailed coverage, check out this lecture by Dirk H. Breiding, Assistant Curator, Department of Arms and Armor, The Metropolitan Museum of Art.