At first glance, it might have appeared that the two men were battling in a purely physical battle of clashing swords, but magic was at the essence of their actions. They could not produce such destructive force simply by increasing their physical strength to a ridiculous extent. After passing a certain line, only a path of self-destruction where one’s own muscles crushed one’s own organs awaited.
At the same time as they were producing such overwhelming destruction with their bodies, they were also predicting what the various negative effects and other side effects of creating the ridiculous strength and speeds were and then using assisting magic to meticulously deal with those effects. Hundreds and even thousands of those negative effects were constantly being created in battle and overlooking even one while the situation changed from instant to instant would lead to death in that high speed fight.

Toaru Majutsu no Index vol.18 ch.5 pt.3