Would the battle of Agincourt have turned out differently if the French had worn lighter armour? Perhaps, say researchers who have discovered that the heavy steel-plate armour worn by the French would have exhausted them before the fight with the English had even started.

No self-respecting medieval knight went into battle without a suit of shining steel armour. A typical suit would have comprised steel plates covering the chest and back, plus leg and arm components, all weighing at least 30kg. Compared with wearing no armour, the steel plates would have doubled the effort required to move around and fight, according to Graham Askew, a lecturer in biomechanics at Leeds University, who led the research.

He asked staff at the Royal Armouries Museum in Leeds to walk and run on a treadmill while wearing different types of armour. All were exact replicas of armour made in the 15th century and included English, Milanese and German designs.

"While they were doing that, we were collecting the air they breathed out," said Askew. "We were able to measure how much oxygen they were using and that tells us how much energy they're using as they're moving at each of those speeds."

The results, published on Wednesday in the Proceedings of the Royal Society B: Biological Sciences, showed that people wearing armour expended up to 2.3 times as much energy while walking and 1.9 times as much energy while running compared with wearing no armour.

The doubling effect was greater than could be accounted for by the extra weight of the armour alone. The researchers worked out that if the knights had carried the total weight of the armour in a backpack, they would have experienced just 1.7 times their unloaded energy expenditure.

"Carrying a load of about 30kg spread around the body requires more energy than carrying the same load in a backpack," said Askew. "This is because, in a suit of armour, the limbs are loaded with weight which means it takes more effort to swing them with each stride. If you're wearing a backpack, the weight is all in one place and swinging the limbs is easier."

In addition, the armour constricts breathing. "Being wrapped up in a tight shell of thick steel makes one feel invincible, but also unable to take a deep breath," said Federico Formenti of the University of Auckland, who was a co-author of the research. "You feel breathless as soon as you move around in medieval armour, and this would likely limit soldiers."

He speculated on how the burden of all the armour might have affected the course of the battle of Agincourt in 1415, in which Henry V's lightly armoured soldiers defeated the French army. A key feature of the battle was that the French knights had to advance across a very muddy field towards English archers.

Formenti said there would have been a very high cost associated with moving through mud in heavy armour, suggesting that the French knights were exhausted by the time they reached the English. "[This] contributed to the killing of a lot of the French knights, despite the fact that there were many more French than there were English soldiers."