Protective Capacity of bicycle helmets

Introduction A recent editorial in the Lancet points out the need for bicyclists to wear helmets to reduce the fatality rate of 300 per year in the UK. The epidemiology of head injuries to bicyclists has shown that the most frequent impact sites are the front and sides of the head. A variety of helmet designs have been produced in an attempt to reduce the number of severe injuries and fatalities. At the same time, bicycle helmet test standards have been developed to identify those helmets with an adequate protective capacity. The standards have been developed by adapting existing standards for other helmet types. Thus Bishop and Briard describe the use of a modified NOCSAE American football helmet standard (1973). Hodgson describes how in this standard the NOCSAE-Wayne State University headform attempts to match the stiffness of cadaver skulls. The surface struck (a flat steel plate covered with hall an inch of rubber) presumably simulates the turf or other objects struck. Although this rubber covered surface would appear much more compliant than road surfaces, Bishop and Briard concluded that helmets fitted with polystyrene foam liners are superior to those with soft foam liners. In contrast, Hurt and Thomb described bicycle helmet testing to the American National Standard Z 90.4 (1984). In this standard, the falling headform strikes a rigid flat steel plate or a steel hemisphere of 50mm radius. There is no requirement for the headform rigidity to match that of the human skull; rather 'headforms shall be made of low-resonantfrequency material and shall exhibit no resonant frequencies below 3000Hz'. The impact tests in the recent British Standard BS 6863 (1987) are similar, except that the hemispherical anvil is replaced by a simulated kerbstone (a cylinder of radius 18mm). The headform requirement is the same. If a helmet passes one of these tests, it is a guarantee of a certain minimum level of protection. However, it is not clear what the human equivalent of a rigid headform falling one metre is, nor is it possible to infer the ultimate protective capacity of any design from the test result of a maximum headform acceleration. Therefore, the helmet test conditions have been analysed, and the performance of helmets related to the thickness, geometry and type of foam liner and shells used.