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A helmet is a form of protective gear worn to protect the head from injuries. Ceremonial or symbolic helmets (e.g. UK policeman's helmet) without protective function are sometimes used. The oldest known use of helmets was by Assyrian soldiers in 900BC, who wore thick leather or bronze helmets to protect the head from blunt object and sword blows and arrow strikes in combat. Soldiers still wear helmets, now often made from lightweight plastic materials. In civilian life, helmets are used for recreational activities and sports (e.g. jockeys in horse racing, American football, ice hockey, cricket, baseball, and rock climbing); dangerous work activities (e.g. construction, mining, riot police); and transportation (e.g. Motorcycle helmets and bicycle helmets). Since the 1990s, most helmets are made from resin or plastic, which may be reinforced with fibers such as aramids. The word ''helmet'' is diminutive from ''helm'', a Medieval word for protective combat headgear. The Medieval great helm covers the whole head and often is accompanied with camail protecting throat and neck as well. Originally a ''helmet'' was a helm which covered the head only partly. ==Design== All helmets attempt to protect the user's head by absorbing mechanical energy and protecting against penetration. Their structure and protective capacity are altered in high-energy impacts. Beside their energy-absorption capability, their volume and weight are also important issues, since higher volume and weight increase the injury risk for the user's head and neck. Anatomical helmets adapted to the inner head structure were invented by neurosurgeons at the end of the 20th century. Helmets used for different purposes have different designs. For example, a bicycle helmet must protect against blunt impact forces from the wearer's head striking the road. A helmet designed for rock climbing must protect against heavy impact, and against objects such as small rocks and climbing equipment falling from above. Practical concerns also dictate helmet design: a bicycling helmet should be aerodynamic in shape and well ventilated, while a rock climbing helmet must be lightweight and small so that it does not interfere with climbing. Some helmets have other protective elements attached to them, such as a face visors or goggles or a face cage, or an ear cage or ear plugs and other forms of protective headgear, and a communications system. Sports helmets may have an integrated metal face protector (face cage). * Baseball batting helmets have an expanded protection over the ear, which protects the jaw from injury. * Motorcycle helmets often have flip-down face screens for rain and wind protection, and they may also have projecting visors to protect the eyes from glare. * Hard hats for construction workers are worn mainly to protect the wearer from falling objects such as tools. * Helmets for riot police often have flip-down clear visors and thick padding to protect the back of the neck. * Modern firefighter's helmets protect the face and back of the head against impact, fires and electricity, and can include masks, communication systems, and other accessories. * Welding helmets protect the eyes and face and neck from flash burn, ultraviolet light, sparks and heat. They have a small window, called a lens shade, through which the welder looks at the weld; for arc welding this window must be much darker than in blowtorch goggles and sunglasses. *People with some medical conditions must wear a helmet to protect the brain, due to a gap in the braincase, e.g. because of cleidocranial dysostosis or in separated craniopagus twins. * Mixed martial arts helmets have ear pads to prevent serious injuries to the athletes, who do not usually endure such force to the ears. * Some watersports helmets, such as for underwater hockey or water polo, have ear-cages fitted which are designed to help prevent burst eardrums caused by an excessive water pressure resulting from a contact or percussion from other equipment involved in the sport. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「helmet」の詳細全文を読む スポンサード リンク
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