There are a lot of misconceptions about Plate Armor that exist among the modern population, Gamers included. Many believe that a Knight when knocked over was rendered all but helpless, unable to stand back up because his movement was so restricted by by his armor. That just isn't true. Was their movement and vision impaired because of their armor and helm? Of course. But if it restricted it to that degree a Knight would have had a different set of Plate commissioned. Remember Armor smiths made the armor to fit the man. It was precision work. Many pieces on the armor were articulated so that the Knight could stay mobile even when fully armored. A Knight needed to move and be mobile on the battlefield, and that mobility could mean the difference between life and death.
Here is a video demonstrating what I'm talking about. Mike Loades is a noted military historian and in this video he demonstrates how quickly a Knight can be knocked from his horse, stand back up, and be ready to fight all while wearing Plate Armor.