# Radial and Oblique Impact Testing of Alpine Helnets onto Snow Surfaces
## 1 Introduction
Samples were removed from the freezer and allowed to thaw at room temperature for approximately 2.5 h before testing. For soft snow impacts to the crown, no significant protective effect was observed in the helmeted tests, and all peak linear headform accelerations remained below 42 \(g\) for all soft snow impacts, and no significant differences were found between the helmeted and unhelmeted conditions. In contrast, hard snow impacts to the crown of the unhelmeted and helmeted headform resulted in peak linear acceleration ranges of 138-165 g and 79-98 \(g\), respectively; therefore, the presence of the ski helmet was found to significantly reduce peak linear headform accelerations by 32-48%. [PERSON] et al. [21] stated that the quality and consistency of snow samples were a limitation and suggested that future studies investigate snow hardness at ski resorts. Such drop tests evaluate the effectiveness of alpine helmets for radial impacts, which are associated with a focal head injury, such as a skull fracture [18]. Few studies have investigated alpine helmets in oblique impacts, which comprise a rotational component and are associated with a diffuse head injury, such as intracranial haematoma [22]. [PERSON] et al. [23] dropped a helmeted headform from a height of 1.7 m onto a snow slope, from which the acceleration and high-speed video data were used to reconstruct impacts and validate a finite element snow model. In a similar study, [PERSON] et al. [24; 25] obtained the damping properties of hard and soft snow by performing drop tests on ski slopes using a rigid headform from various heights. It was concluded that a relevant impacting surface and more demanding acceleration criteria should be considered for inclusion in performance standards for ski and snowboard helmets. [PERSON] et al. [30] used the KTH Oblique Test Rig to evaluate the performance of alpine helmets with the Multi-directional Impact Protection System (MIPS), which features a low-friction layer inside the helmet that allows multi-directional relative movement of 10-15 mm between the helmet and the head. For a resultant impact speed of 7.4 m/s and an impact angle of approximately 30\({}^{\circ}\), the MIPS helmet reduced peak angular acceleration and velocity by 38% and 23%, respectively, compared to a standard helmet. Using a 45\({}^{\circ}\) anvil to induce oblique impacts from vertical drop tests, [PERSON] et al. [31] compared a standard alpine helmet to helmets with rotation-damping systems (RDS): MIPS and WaveCel. The latter comprises a cellular structure that collapses to provide rotational attenuation. It was found that the alpine helmets with RDS reduced peak angular headform velocity and acceleration compared to the standard alpine helmet. Both [PERSON] et al. [30] and [PERSON] et al. [31] tested the helmets against rigid anvils covered in grip tape, which provides a consistent surface but does not represent an impact on the snow. To the authors' knowledge, no study has used an oblique impact test rig to bridge the knowledge gap between real helmeted impacts onto alpine snow slopes and laboratory helmet impacts onto rigid surfaces. Therefore, the aim of the current study was to develop a method to assess the performance of alpine helmets for radial and oblique impacts onto snow surfaces in a laboratory setting and compare peak headform kinematics across headform conditions. For radial impacts, it was hypothesised that helmeted tests have lower peak linear acceleration compared to unhelmeted tests. For oblique impacts, it was hypothesised that the helmeted tests with RDS would reduce peak linear acceleration compared to standard helmets.