Objective. Reducing surface friction has been shown to reduce head injury risk in some scenarios. However, rigid body mechanics suggests that other scenarios may exist in which reducing surface friction increases head injury risk. The goal of this study is to demonstrate that reducing friction on the surface of a helmet decreases the rotational acceleration of the head in some scenarios and increases it in other scenarios and to discuss the implications for helmet design.

Methods. A series of drop tests were conducted to simulate normal and oblique impacts and assess head injury risk with conventional helmets and helmets that had been modified to reduce friction.

Results. The low friction modification had little influence on peak linear head acceleration or HIC. However, it decreased peak rotational acceleration by 55% in one oblique impact scenario and increased it by 83% in another oblique impact scenario. In normal impacts, the low friction modification had no effect on peak rotational acceleration.

Conclusions. The relationship between rotational acceleration and surface friction is sensitive to the impact scenario. A single oblique impact test may be misleading when assessing the overall performance of a low friction helmet. The implications of these findings for helmet design are discussed.