Bicycle Helmet Size, Adjustment, and Stability

Kim T. Thai, Andrew S. McIntosh, Toh Yen Pang

ABSTRACT Objectives One of the main requirements of a protective bicycle helmet is to provide and maintain adequate coverage to the head. A poorly fitting or fastened helmet may be displaced during normal use or even ejected during a crash. The aims of the current study were to identify factors that influence the size of helmet worn, identify factors that influence in use helmet position and adjustment, and examine the effects of helmet size worn and adjustment on helmet stability. Methods Recreational and commuter cyclists in Sydney were surveyed to determine how helmet size and/or adjustment affected helmet stability in the real world. Anthropometric characteristics of the head were measured and, to assess helmet stability, a test analogous to the requirements of the Australian bicycle helmet standard was undertaken. Results 267 cyclists were recruited across all age groups and 91% wore an AS/NZS 2063 compliant helmet. The main ethnic group was Europeans (71%) followed by Asian (18%). The circumferences of the cyclists' heads matched well the circumference of the relevant ISO headform for the chosen helmet size, but the head shapes differed with respect to ISO headforms. Age and gender were associated with wearing an incorrect sized helmet and helmet adjustment. Older males (<55 years) were most likely to wear an incorrectly sized helmet. Adult males in the 35-54 year age group were most likely to wear a correctly adjusted helmet. Using quasi-static helmet stability tests, it was found that the correctness of adjustment, rather than size, head dimensions or shape significantly affected helmet stability in all test directions. Conclusions Bicycle helmets worn by recreational and commuter cyclists are often the wrong size and are often worn and adjusted incorrectly, especially in children and young people. Cyclists need to be encouraged to adjust their helmets correctly. Current headforms used in standards testing may not be representative of cyclists' head shapes. This may create challenges to helmet suppliers if on one hand they optimise the helmet for meeting tests on ISO related headforms, while on the other seek to offer greater range of sizes.