various designs and requirements, all 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. 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 would chiefly need to protect against blunt impact forces from the wearer’s head striking the road or a car hood. A helmet designed for rock climbing, however, would need to protect against objects (e.g. small rocks and climbing equipment) such as an ice axe falling from above. 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 so that it would not interfere with climbing.
Some helmets have other protective elements attached to them, such as a face visors or goggles or a face cage, and ear plugs and other forms of protective headgear, and a communications system. American football, hockey, and lacrosse helmets usually have an integrated face protector (face cage) made from metal.