# Evaluation of Snowboarding Helmets in Mitigation of the Biomechanical Responses of Head Surrogate
## 4 Discussion  
In this work, the mechanical response of the head surrogate with various helmet configurations (conventional and anti-rotational) was investigated under blunt impact (impactor velocity 4.6 \(\pm\) 0.2 m/s). The results elucidate interesting paradigms in head kinematics and kinetics. Compared to the no helmet, the conventional helmets did not significantly reduce angular kinematics (Figure 4a,b,d,e), equivalent stress (Figure 8a,b), and MPS (Figure 8c,d). Conventional helmets are not designed to mitigate angular motion. On the other hand, helmets equipped with anti-rotational technologies significantly reduced angular kinematics (Figure 4a,b,d,e), equivalent stress (Figure 8a,b), and MPS (Figure 8c,d) with respect to the no helmet and conventional helmet. With reference to the no helmet, linear accelerations, and coup pressures were reduced significantly for all helmet configurations (Figures 4c,f and 5a,b). These trends in kinematics and kinetics are logical. Numerous investigations in the literature [27; 36; 37] have demonstrated that angular kinematics of the head predominately governs the equivalent stress and MPS, whereas linear kinematics of the head mainly governs brain pressures [34; 35]. Compared to the conventional helmet, the WaveCel helmet performed the best, followed by the silica pad helmet and MIPS helmet, respectively. Further, WaveCel and silica pad helmets significantly reduced most of the biomechanical parameters (except contrecup pressure) (Figures 4 and 5). The MIPS helmet did not significantly reduce the aforementioned parameters regarding the front orientation but significantly reduced them regarding the side orientation (except contrecup pressure) (Figures 4c,f and 5a,b). The performance of the newly developed, low-cost silica pads was comparable to the WaveCel and MIPS helmets. Silica pads are also lighter (total weight 0.17 kg) compared to the cellular liner of the WaveCel helmet (0.22 kg). These trends are reasonable considering how each helmet has been designed. The wavy cellular liner of the WaveCel helmet was specifically designed to mitigate the effects of both linear and angular motions. This liner acts as both a impact mitigation liner and anti-rotational liner. Silica pads may naturally provide additional damping, facilitating significant reductions in linear motion (and hence linear acceleration). The MIPS anti-rotational liner (i.e., a plastic sheet) was not specifically designed to mitigate linear motion. Interestingly, none of the anti-rotational helmets significantly reduced the contrecup pressures compared to the conventional helmet (Figure 5a,b). This might be attributed to the fact that contrecup pressures are generated due to a lag in the motion of the brain with respect to the skull [46; 64]. The skull moves faster than the brain (due to the higher wave speeds), causing a lag. Anti-rotational helmets are not designed to prevent the lag in the motion of the brain and the skull. The inability of anti-rotational helmets to reduce contrecup pressure may have major implications as CSF cavitation has been proposed as one of the mechanisms of mild TBI [65; 66; 67]. In the case of anti-rotational helmets (MIPS and WaveCel), the removal of a comfort liner increased the angular kinematics (Figure 6a,b,d,e). This underscores that in addition to the anti-rotational element, a comfort liner plays an important role in mitigating angular kinematics. This might be attributed to the fact that a comfort liner facilitates additional sliding between the anti-rotational liner and comfort liner. Interestingly, friction coefficients of an anti-rotational element/liner and a comfort liner are similar in the range of 0.25-0.35 [68; 69]. For similar impact conditions, [PERSON] et al. [22] have observed significantly lower angular kinematics in a snowboarding helmet compared to a bicycle helmet. They attributed this reduction potentially to the lack of a comfort liner in the bicycle helmet.