In terms of a medieval sword, that is. Is there an accepted way to measure stiffness of a blade?

I see statements in a review that say things like ' the blade flexes a couple of inches and returns to true', but how do you measure how much a blade flexes under a particular load? Doesn't it depend on the length of the blade?

I have often wondered how stiff modern reproductions are in comparison to their historic counterparts or models - I expect that some of the 14th and 15th century XV and XVIII swords with pronounced mid ribs were very stiff compared with most of the swords we see with that blade shape today. Thoughts?

Gordon