Just How Heavy was Medieval Armor?

Their paper, " Limitations imposed by wearing armour on Medieval soldiers' locomotor performance," just published in the Proceedings of the Royal Society B: Biological Sciences, approaches the problem from the point-of-view of energy. By having the interpreters wear oxygen masks while on the treadmill, Askew, Formenti, and Minetti were able to measure how much oxygen they needed to stay in motion. A typical suit of armor could weigh as much as one hundred pounds, but they found that it's not the weight itself that matters -- it's the distribution of that weight. Because each arm and leg is weighed down, it's harder to move, and that makes walking and running more difficult, even as the face mask makes it harder to breathe. Walking with one hundred pounds in a backpack, you'd use 1.7 times as much energy as you would were you not weighed down -- but wearing that weight as armor, you'll use 2.3 times as much.

"The significant energetic cost of moving in armour," the researchers conclude, "is likely to have had a profound limitation on soldiers’ performance, and may have contributed to the outcome of certain battles":

For example, during the Battle of Agincourt (1415), heavily armoured French knights advanced towards the English men-at-arms across terrain made extremely muddy from recent ploughing, over-night rain and an earlier French cavalry charge. Exhaustion of the French knights is cited as a contributing factor to their demise at the hands of the more lightly armoured English archers. Similarly, it has been suggested that exhaustion of the French men-at-arms resulting from several days of marching may have impaired their subsequent performance (in armour) and contributed to their defeat by the English army in the Battle of Crecy (1346).

Not terribly surprising, maybe -- but it's satisfying to see the question settled in such a concrete, empirical way!