<Li> to prevent penetration of the helmet by a pointed object that might otherwise puncture the skull, and </Li> <Li> to provide structure to the inner liner so it does not disintegrate upon abrasive contact with pavement . This is important because the foams used have very little resistance to penetration and abrasion . </Li> <P> The purpose of the foam liner is to crush during an impact, thereby increasing the distance and period of time over which the head stops and reducing its deceleration . </P> <P> To understand the action of a helmet, it is first necessary to understand the mechanism of head injury . The common perception that a helmet's purpose is to save the rider's head from splitting open is misleading . Skull fractures are usually not life - threatening unless the fracture is depressed and impinges on the brain beneath and bone fractures usually heal over a relatively short period . Brain injuries are much more serious . They frequently result in death, permanent disability or personality change and, unlike bone, neurological tissue has very limited ability to recover after an injury . Therefore, the primary purpose of a helmet is to prevent traumatic brain injury while skull and face injuries are a significant secondary concern . </P>

When did wearing a crash helmets become compulsory