As weapons became more and more sophisticated in medieval Europe armour also evolved, and by the 15th century soldiers would have been clad top-to-toe in up to 50 kg of bulky steel.
Researchers have always suspected that this would have been tough to move around in. But now with the help of volunteers wearing replica medieval armour - and a treadmill - they've been able to confirm this.
They found that walking or running in it used up huge amounts of energy, restricted breathing and bore intense pressure on the legs.
The effect was so great the team suspects it could have played a part in France's defeat in the battle of Agincourt.
In this famous conflict of 1415 - despite heavily outnumbering the English - the French were beaten.
The scientists say the fact that the French knights had to trek through a muddy field while wearing their heavy armour - to meet a stationary English line - would have left them so exhausted, it probably led to their downfall.