  The minimum possible value of the average deceleration of the head of a
player engaged in an impact sport such as football or hockey, as a result of an
impact of his/her helmet with a surface or another player, is inversely
proportional to the helmet padding thickness. Since there are practical limits
to its maximum thickness, it is difficult to significantly reduce the average
acceleration. There is evidence, however, that the peak, rather than the
average, acceleration is the most significant cause of brain injury. It is
proposed here that brain injuries that occur as a result of an impact, could be
reduced by using as padding a hydrophilic porous material swollen with fluid.
The friction experienced by the fluid as it is squeezed out of the porous
material in an impact can "tune" the acceleration of the skull so that it is
never significantly higher than its average during the impact.
