Despite various designs and requirements, helmets attempt to protect the user's head through a mechanical energy-absorption process. Therefore, 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.
Classical helmets from the ancient Greeks to today treat the head uniformly and are currently tested on rigid headforms. Anatomical helmets adapted to the inner head structure were invented by neurosurgeons at the end of the 20th century. Since the materials are disposed according to the anatomical structure of the head, they are smaller and lighter than the classical helmets.
Types of helmets
Helmets used for different purposes differ greatly in their design. For example, a bicycle helmet would chiefly need to protect against blunt impact forces. A helmet designed for rock climbing, however, would need to protect against objects (e.g. small rocks and climbing equipment) falling from above. Consequently, bicycling and rock climbing helmets little resemble each other. Practical concerns also dictate helmet design: a bicycling helmet would preferably be aerodynamic in shape and probably well ventilated, while a rock climbing helmet would be lightweight and with a minimum of bulk to reduce any detrimental effect on the climber's technique.
Goggles, face guards and ear plugs are other forms of protective headgear. Hard hats are typically preferred in modern times for construction workers. Helmets are often used by riot police.
Sometimes medical conditions need a helmet to be worn to protect the brain because of a gap in the braincase, e.g. because of cleidocranial dysostosis or in separated craniopagus twins.
