A single blow to the head is bad enough. But what about a series of blows? That's more complicated, and standard computer models of the brain don't account for it: they treat each impact as if it were the first. But the brain isn't a steel ball: it remembers past strain.
Brain tissue is like a rubber band that has been stretched many times. With each stretch, it becomes a bit softer and stretches more easily. The researchers added this softening effect to a computer model of the head to see what happens with a series of impacts, like in mixed martial arts. They took data from real impacts and ran it through the model.
The result was surprising: with five identical impacts in a row, brain deformation grew from one hit to the next. The model without softening predicted a lower risk of injury, while the model with softening predicted a higher risk. The effect was especially pronounced with twenty random impacts from different directions: by the end of the series, the brain deformed more than at the beginning, even though the force of the impacts didn't change.
This means that old models may have underestimated the risk of repeated injuries. For athletes, military personnel, or people who frequently take blows to the head, this is an important signal: the brain gets tired, and each subsequent hit can be more dangerous than the last, even if it feels the same.
The effect of material softening after repeated stretching was first described for rubber back in the 1960s. It's called the Mullins effect, and now it's helping to model the brain.
In the movie 'Limitless,' the hero takes a pill that enhances brain function, but repeated strain on the brain in real life works the opposite way: it impairs it.