No wonder physicists are interested in shocks in sport. But that it is specialists in fluid mechanics is more surprising. A team from the Ecole Polytechnique thus presented at the Sports Physics conference in Lyon in December 2021 its daring, but plausible hypothesis for the explanation of mild brain trauma (better known by the common name of “cerebral concussion”) and their detection.
“About 7% to 10% of the 35,000 to 50,000 annual traumas that we see in emergencies in Ile-de-France are related to sportrecalls Philippe Deck, head of the neurosurgery department at Beaujon Hospital (AP-HP), and member of several working groups on the subject at the French Federation and the International Rugby Federation. In rugby, it is about two per day of the Top 14 championship which are listed. »
The diagnosis is not simple, because the symptoms and criteria are numerous and can appear twenty-four to forty-eight hours after the shock. The loss of consciousness is thus not systematic. Other dysfunctions of cerebral functions, identified for example in rugby by questionnaires in the locker room, are listed as memory problems, a disturbed mental state, or loss of balance.
But the origin of these evils is poorly known. “Shock leads to cellular dysfunction in the brain. This one, which consumes a lot of energy, runs out of it and takes time to recover », believes Philippe Deck. Others evoke the consequences of contact with the cranial box. Physicists from the Ecole Polytechnique tested another idea, put forward in the 1970s by other physicists: the appearance of bubbles in the cerebrospinal fluid, which, by imploding, would damage the neurons.
Bubble energy modeling
No sportsman was shaken to test the hypothesis, but an experimental analog of their brain was built: a cylinder filled with water falls on different more or less hard objects in order to vary the intensity and the time of the shock. impact. During the impact, a depression appears between the bottom and the top of the cylinder because of the inertia of the fluid. This depression creates a bubble about one millimeter in diameter. This implodes in a shock wave capable of damaging the neurons. It can also propel a fairly strong jet of liquid against the brain and damage it.
After modeling the energy released by these bubbles, Juliette Amauger, the doctoral student on this project led by Caroline Cohen and Christophe Clanet, found a result that has been well known since automobile accidentology work in the 1960s. According to these studies, the risk of concussion depends on the intensity of the shock (from 50 to 150 times the earth’s gravity) and time (from two to twenty milliseconds). A short shock is only dangerous at high intensity, while a long shock can be harmful even for weak shocks. And it is this law that physicists have found. “Even if we have no in vivo proof that this cavitation phenomenon exists in these circumstances, the hypothesis is plausible and the work is exciting”notes Philippe Deck.
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