# Radial and Oblique Impact Testing of Alpine Helnets onto Snow Surfaces
## 1 Introduction  
Head injuries are common in skiing and snowboarding at both the recreation and elite levels [1; 2; 3]. Helmet use by recreational skiers and snowboarders has increased in recent decades [4]. Several earlier studies reported that helmets were associated with a reduction in the risk of head injury in skiing and snowboarding [5; 6; 7]. In contrast, [PERSON] et al. [8] found no significant decrease in severe traumatic brain injury among alpine skiers despite an increase in helmet use. Similarly, [PERSON] et al. [9] observed an unexpected reduction in the protective effect of helmets in alpine skiing and snowboarding, which was suggested to be a result of new skiing trends. For paediatric skiers and snowboarders, [PERSON] et al. [10] found that wearing a helmet was significantly associated with intensive care admission; however, injury severity was significantly lower compared to those not wearing a helmet. [PERSON] et al. [11] found that helmet wearers were less likely to sustain any head injury; however, the effect of helmets on preventing traumatic brain injury was non-significant. More recently, [PERSON] et al. [12] found helmet use was associated with a significant reduction in skull fracture but a significant increase in intracranial haemorrhage. Alpine helmet standards require helmets to be drop tested on flat anvils at impact speeds ranging from 4.5 to 6.8 m/s with peak linear headform acceleration remaining below 250 or 300 \(g\)[13; 14; 15; 16; 17]. Some alpine helmet standards also require helmets to be drop tested on hemispherical and edge anvils [13; 16; 17], which are intended to represent hazards, such as a rock or tree stump. Currently, no alpine helmet standard includes an oblique impact test with rotational criteria, such as angular acceleration and/or angular velocity. It is unknown how drop tests onto rigid anvils relate to impacts on snow and ice surfaces; however, several studies have investigated head impacts on snow surfaces to bridge the knowledge gap between standards tests and real-world impacts [18; 19; 20; 21; 22; 23; 24; 25]. Anthropomorphic test devices (ATDs) have been used to simulate rearward falls onto snow slopes resulting in occipital head impact [18; 19; 20], which has been identified as the mechanism of over half of all major head injuries to snowboarders [26; 27; 28; 29]. A Hybrid III ATD was accelerated along a cable and released at approximately 8 m/s onto a snow-covered ramp with a gradient of 20\({}^{\circ}\), which was used to replicate a snow slope. The ATD was outfitted with snowboarding attire and equipment, which comprised boots, bindings and a snowboard. For soft snow conditions, the ramp was covered in 300 mm of snow. For scipy snow conditions, the soft snow on the ramp was allowed to freeze and covered in an additional 100 mm of soft snow. A small mound of snow was formed on the ramp so that the rear edge of the snowboard caught and resulted in the ATD falling rearwards down the ramp. For soft snow impacts to the occupt, all peak linear headform accelerations remained below 83 \(g\) for all soft snow impacts, and no significant differences were found between the helmeted and unhelmeted conditions. In contrast, icy snow impacts to the occupt of the unhelmeted and helmeted headform resulted in mean peak linear accelerations of 391 and 162 \(g\), respectively. Therefore, the presence of the ski helmet was found to significantly reduce peak linear headform accelerations by a factor of over two for icy snow impacts. [PERSON] et al. [21] investigated the protective potential of a ski helmet for 4 m/s drop tests into 150 mm deep snow samples using a Hybrid III head-neck system attached to a carriage with a total mass of 16 kg to represent the torso. Hard snow samples were prepared by filling the trays with snow and allowing them to freeze overnight, whereas soft snow samples were prepared by allowing a 60 mm layer to freeze overnight and adding 90 mm of soft snow prior to testing.