# Influence of Chinstrap Stiffness on Cerebrospinal Fluid Dynamics and Brain Stress in Helmet Impacts
## 4 Discussion  
The findings from this study reveal a nuanced relationship between chinstrap stiffness and helmet performance during impact. Both extremes, i.e., too stretchable and too non-stretchable, pose significant risks. Highly stretchable chinstraps, while effective in absorbing energy through elongation, may allow excessive helmet displacement, reducing their ability to maintain proper alignment with the skull and potentially increasing the risk of rotational injuries. On the other hand, non-stretchable chinstraps, though they ensure helmet stability, transmit undampened forces directly to the skull base, leading to higher intracranial stress concentrations and a greater likelihood of traumatic brain injury. However, these results also highlight a critical intermediate range of stiffness that introduces a unique hazard. Semi-non-stretchable configurations, which fall between these extremes, create a mechanical conflict: they are stiff enough to partially decouple the chin guard from the mandible, allowing the guard to slide laterally across the jaw, yet not elastic enough to absorb significant energy through strap elongation. This partial decoupling reduces the effective contact area between the chin guard and the jaw, leading to localized stress concentrations at bony interfaces rather than distributing forces evenly through soft tissues. This instability mechanism explains why the semi-non-stretchable configuration exhibits the largest angular displacement (\(\theta=45.13^{\circ}\)) and the highest residual stress (45.96%), making it a clear outlier in this dataset. This phenomenon underscores the importance of balancing stiffness and elasticity in chinstrap design to avoid detrimental intermediate behaviors that can compromise helmet stability and increase injury risk. Notably, Figure 11 reinforces these conclusions by demonstrating how stress propagation evolves across designs: all configurations localized peak stresses in sulcal and brainstem regions (i.e., key sites for shear-induced injuries) but stiffer straps (minimally and non-stretchable) exhibited expanded stress patterns into adjacent cortical folds and deeper cranial base structures. This spatial divergence aligns with the temporal escalation of peak stresses (Figure 9), showing that increased stiffness not only amplifies stress magnitude but also distributes damaging forces to broader neuroanatomical regions when energy dissipation through strap elongation is compromised. This study contributes to understanding the biomechanical impact of chinstrap design on helmet performance during high-velocity impacts, leveraging advanced computational modeling techniques such as SPH and FSI. One of the key strengths of this research is the use of GPU, which significantly reduced computational time and allowed for the analysis of complex scenarios that would have been infeasible with traditional methods. The study's focus on chinstrap stiffness as a critical factor in helmet design addresses a notable gap in the literature, providing new insights into how different chinstrap materials influence intracranial stress distribution. Furthermore, the computational model was validated against previous studies, including cadaveric data [29], which enhances the credibility of the findings. This validation ensures that the model accurately represents real-world biomechanical interactions, even though the validation did not include helmets. However, the study has several limitations that warrant consideration. One significant limitation is the anatomical simplification inherent in the computational model, specifically the omission of the skin, hair, and meningeal layers (including the dura mater, archnotid mater, and pia mater). These tissues possess distinct viscoelastic and damping properties that can absorb and redistribute impact energy before it reaches the skull and intracranial contents.