Read this excerpt from "Heads Up on Sports Safety." Your neck can also get hurt while playing football, so many helmet designers are using materials that prevent neck injuries. One common injury is whiplash, which is a sudden hit to the head that moves the neck very quickly. Neck padding can help, but materials that prevent the head from turning can decrease athletic performance. One alternative is to use a thermoplastic material that responds differently to different kinds of pressure. This material is soft when moved or pressed slowly but stiffens into a hard version of its shape when hit quickly. It has been used in bulletproof vests for at least a decade. One problem and its solution presented in this article is