A football helmet prototype that reduces linear and rotational acceleration with the addition of an outer shell

Abstract

OBJECTIVE Amid the public health controversy surrounding American football, a helmet that can reduce linear and rotational acceleration has the potential to decrease forces transmitted to the brain. The authors hypothesized that a football helmet with an outer shell would reduce both linear and rotational acceleration. The authors' objectives were to 1) determine an optimal material for a shock-absorbing outer shell and 2) examine the ability of an outer shell to reduce linear and/or rotational acceleration. METHODS A laboratory-based investigation was undertaken using an extra-large Riddell Revolution football helmet. Two materials (Dow Corning Dilatant Compound and Sorbothane) were selected for their non-Newtonian properties (changes in viscosity with shear stress) to develop an outer shell. External pads were attached securely to the helmet at 3 locations: the front boss, the side, and the back. The helmet was impacted 5 times per location at 6 m/sec with pneumatic ram testing. Two-sample t-tests were used to evaluate linear/rotational acceleration differences between a helmet with and a helmet without the outer shell. RESULTS Sorbothane was superior to the Dow Corning compound in force reduction and recovered from impact without permanent deformation. Of 5 different grades, 70-duro (a unit of hardness measured with a durometer) Sorbothane was found to have the greatest energy dissipation and stiffness, and it was chosen as the optimal outer-shell material. The helmet prototype with the outer shell reduced linear acceleration by 5.8% (from 75.4 g to 71.1 g; p < 0.001) and 10.8% (from 89.5 g to 79.8 g; p = 0.033) at the side and front boss locations, respectively, and reduced rotational acceleration by 49.8% (from 9312.8 rad/sec to 4671.7 rad/sed; p < 0.001) at the front boss location. CONCLUSIONS Sorbothane (70 duro) was chosen as the optimal outer-shell material. In the outer-shell prototype helmet, the results demonstrated a 5%-10% reduction in linear acceleration at the side and front boss locations, and a 50% reduction in rotational acceleration at the front boss location. Given the paucity of publicly reported helmet-design literature and the importance of rotational acceleration in head injuries, the substantial reduction seen in rotational acceleration with this outer-shell prototype holds the potential for future helmet-design improvements.

Sport-Related Concussion: Evaluation, Treatment, and Future Directions

“…Specifically, Breedlove and colleagues evaluated the Guardian Cap and demonstrated that the device reduced force at velocities high enough to cause skull fracture, but failed to mitigate impact forces at the low and medium velocities typically involved in concussion [110]. One study designed a helmet prototype that significantly reduced rotational acceleration with the addition of an outer shell [111], suggesting a potential promising direction for future helmet design. Further study is necessary to evaluate how implementation of various designs may affect the incidence of concussion during real-time play.…”

Brain Injuries in American Football: Understanding the Injury, Difficulty in Helmet Optimization, and Current Communication Practices – A Narrative Review

“…NOCSAE testing methods only require tolerances on helmet performance based on linear accelerations (Levy et al, 2004;Lloyd & Conidi, 2016). While this may be impactful for catastrophic injuries, as some speculate are influenced greater by linear acceleration and high velocities (Broglio et al, 2012), many studies have reflected the even greater influence of rotational accelerations on injury risk, especially in cases of mTBI (Ford et al, 2018;Hernandez, Shull, & Camarillo, 2015;Levy et al, 2004;Lloyd & Conidi, 2016;Merchant-Borna et al, 2016;Zuckerman et al, 2019). Yet, many still support the hypothesis that rotational accelerations provide a superior indication of concussion risk as well as catastrophic injury risk (Levy et al, 2004;Lloyd & Conidi, 2016).…”

“…While many researchers have focused on comparing current helmets designs to one another and subjecting them to newer testing procedures in hopes of altering NOCSAE standards; other research groups have started developing helmet designs that can better attenuate impact forces, so the head undergoes less linear and rotational movement. These researchers have highlighted the potential use of exterior padding or shells for greater attenuation with optimistic results (Nakatsuka & Yamamoto, 2014;Zuckerman et al, 2019). However, the skull and brain move very differently relative to one another.…”

A Decision-Analytic Approach to Addressing the Evidence About Football and Chronic Traumatic Encephalopathy

“…60 • Improved protective equipment, particularly new helmet designs that might reduce the deceleration the brain undergoes following an impact. 61 • Development of reliable sensors that could measure the relevant forces each player's head area is exposed to, so that if a biologically relevant index of risky exposure could be developed, employers would be able to gauge whether each player was approaching an unacceptable amount of exposure and needed to have time to "recover" before additional exposure. h • Development of devices that might reduce individual susceptibility to the consequences of RHI.…”