A review of impact testing methods for headgear in sports: Considerations for improved prevention of head injury through research and standards

Peaks and Distributions of White Matter Tract-related Strains in Bicycle Helmeted Impacts: Implication for Helmet Ranking and Optimization

Traumatic brain injury (TBI) in cyclists is a growing public health problem, with helmets being the major protection gear. Finite element head models have been increasingly used to engineer safer helmets often by mitigating brain strain peaks. However, how different helmets alter the spatial distribution of brain strain remains largely unknown. Besides, existing research primarily used maximum principal strain (MPS) as the injury parameter, while white matter fiber tract-related strains, increasingly recognized as effective predictors for TBI, have rarely been used for helmet evaluation. To address these research gaps, we used an anatomically detailed head model with embedded fiber tracts to simulate fifty-one helmeted impacts, encompassing seventeen bicycle helmets under three impact locations. We assessed the helmet performance based on four tract-related strains characterizing the normal and shear strain oriented along and perpendicular to the fiber tract, as well as the prevalently used MPS. Our results showed that both the helmet model and impact location affected the strain peaks. Interestingly, we noted that different helmets did not alter strain distribution, except for one helmet under one specific impact location. Moreover, our analyses revealed that helmet ranking outcome based on strain peaks was affected by the choice of injury metrics (Kendall's Tau coefficient: 0.58-0.93). Significant correlations were noted between tract-related strains and angular motion-based injury metrics. This study provided new insights into computational brain biomechanics and highlighted the helmet ranking outcome was dependent on the choice of injury metrics. Our results also hinted that the performance of helmets could be augmented by mitigating the strain peak and optimizing the strain distribution with accounting the selective vulnerability of brain subregions and more research was needed to develop region-specific injury criteria.

Padded Helmet Shell Covers in American Football: A Comprehensive Laboratory Evaluation with Preliminary On-Field Findings

Protective headgear effects measured in the laboratory may not always translate to the field. In this study, we evaluated the impact attenuation capabilities of a commercially available padded helmet shell cover in the laboratory and on the field. In the laboratory, we evaluated the padded helmet shell cover's efficacy in attenuating impact magnitude across six impact locations and three impact velocities when equipped to three different helmet models. In a preliminary on-field investigation, we used instrumented mouthguards to monitor head impact magnitude in collegiate linebackers during practice sessions while not wearing the padded helmet shell covers (i.e., bare helmets) for one season and whilst wearing the padded helmet shell covers for another season. The addition of the padded helmet shell cover was effective in attenuating the magnitude of angular head accelerations and two brain injury risk metrics (DAMAGE, HARM) across most laboratory impact conditions, but did not significantly attenuate linear head accelerations for all helmets. Overall, HARM values were reduced in laboratory impact tests by an average of 25% at 3.5 m/s (range: 9.7 to 39.6%), 18% at 5.5 m/s (range: - 5.5 to 40.5%), and 10% at 7.4 m/s (range: - 6.0 to 31.0%). However, on the field, no significant differences in any measure of head impact magnitude were observed between the bare helmet impacts and padded helmet impacts. Further laboratory tests were conducted to evaluate the ability of the padded helmet shell cover to maintain its performance after exposure to repeated, successive impacts and across a range of temperatures. This research provides a detailed assessment of padded helmet shell covers and supports the continuation of in vivo helmet research to validate laboratory testing results.

An Assessment of Sikh Turban's Head Protection in Bicycle Incident Scenarios

Due to religious tenets, Sikh population wear turbans and are exempted from wearing helmets in several countries. However, the extent of protection provided by turbans against head injuries during head impacts remains untested. One aim of this study was to provide the first-series data of turbans' protective performance under impact conditions that are representative of real-world bicycle incidents and compare it with the performance of bicycle helmets. Another aim was to suggest potential ways for improving turban's protective performance. We tested five different turbans, distinguished by two wrapping styles and two fabric materials with a size variation in one of the styles. A Hybrid III headform fitted with the turban was dropped onto a 45 degrees anvil at 6.3 m/s and head accelerations were measured. We found large difference in the performance of different turbans, with up to 59% difference in peak translational acceleration, 85% in peak rotational acceleration, and 45% in peak rotational velocity between the best and worst performing turbans. For the same turban, impact on the left and right sides of the head produced very different head kinematics, showing the effects of turban layering. Compared to unprotected head impacts, turbans considerably reduce head injury metrics. However, turbans produced higher values of peak linear and rotational accelerations in front and left impacts than bicycle helmets, except from one turban which produced lower peak head kinematics values in left impacts. In addition, turbans produced peak rotational velocities comparable with bicycle helmets, except from one turban which produced higher values. The impact locations tested here were covered with thick layers of turbans and they were impacted against flat anvils. Turbans may not provide much protection if impacts occur at regions covered with limited amount of fabric or if the impact is against non-flat anvils, which remain untested. Our analysis shows that turbans can be easily compressed and bottom out creating spikes in the headform's translational acceleration. In addition, the high friction between the turban and anvil surface leads to higher tangential force generating more rotational motion. Hence, in addition to improving the coverage of the head, particularly in the crown and rear locations, we propose two directions for turban improvement: (i) adding deformable materials within the turban layers to increase the impact duration and reduce the risk of bottoming out; (ii) reducing the friction between turban layers to reduce the transmission of rotational motion to the head. Overall, the study assessed Turbans' protection in cyclist head collisions, with a vision that the results of this study can guide further necessary improvements for advanced head protection for the Sikh community.

Evaluating the Intracranial Pressure Biofidelity and Response Repeatability of a Physical Head-Brain Model in Frontal Impacts

Headforms are widely used in head injury research and headgear assessment. Common headforms are limited to replicating global head kinematics, although intracranial responses are crucial to understanding brain injuries. This study aimed to evaluate the biofidelity of intracranial pressure (ICP) and the repeatability of head kinematics and ICP of an advanced headform subjected to frontal impacts. Pendulum impacts were performed on the headform using various impact velocities (1-5 m/s) and impactor surfaces (vinyl nitrile 600 foam, PCM746 urethane, and steel) to simulate a previous cadaveric experiment. Head linear accelerations and angular rates in three axes, cerebrospinal fluid ICP (CSFP), and intraparenchymal ICP (IPP) at the front, side, and back of the head were measured. The head kinematics, CSFP, and IPP demonstrated acceptable repeatability with coefficients of variation generally being less than 10%. The BIPED front CSFP peaks and back negative peaks were within the range of the scaled cadaver data (between the minimum and maximum values reported by Nahum et al.), while side CSFPs were 30.9-92.1% greater than the cadaver data. CORrelation and Analysis (CORA) ratings evaluating the closeness of two time histories demonstrated good biofidelity of the front CSFP (0.68-0.72), while the ratings for the side (0.44-0.70) and back CSFP (0.27-0.66) showed a large variation. The BIPED CSFP at each side was linearly related to head linear accelerations with coefficients of determination greater than 0.96. The slopes for the BIPED front and back CSFP-acceleration linear trendlines were not significantly different from cadaver data, whereas the slope for the side CSFP was significantly greater than cadaver data. This study informs future applications and improvements of a novel head surrogate.

Soft-shell headgear in rugby union: a systematic review of published studies

Objectives

To review the rate of soft-shell headgear use in rugby union, consumer knowledge of the protection potential of soft-shell headgear, incidence of concussion reported in rugby headgear studies, and the capacity of soft-shell headgear to reduce acceleration impact forces.

Design

A systematic search was conducted in July and August 2021 using the databases SPORT Discus, PubMed, MEDLINE, CINAHL (EBSCO), Scopus, and Science Direct. The review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The protocol for this systematic review was registered on PROSPERO (registration number: CRD42021239595).

Outcome measures

Rates of headgear use, reports of estimated protection of headgear against head injury, incidence of concussion and magnitude of impact collisions with vs. without headgear, impact attenuation of headgear in lab studies.

Results

Eighteen studies were identified as eligible: qualitative (N = 4), field (N = 7), and lab (N = 7). Qualitative studies showed low rates of headgear use and varying understanding of the protection afforded by headgear. Field studies showed negligible association of headgear use with reduced impact magnitude in headgear vs. non-headgear cohorts. Lab studies showed increased energy attenuation for thicker headgear material, poorer performance of headgear after repetitive impacts and increased drop heights, and promising recent results with headgear composed of viscoelastic polymers.

Conclusions

Rates of adoption of soft-shell headgear remain low in rugby and any association between its use and reduction in acceleration impact forces remains unclear. Lab results indicating improved impact attenuation need to be validated in the field. Further headgear-related research is needed with youth and female rugby players.

In-Depth Bicycle Collision Reconstruction: From a Crash Helmet to Brain Injury Evaluation

Traumatic brain injury (TBI) is a prevalent injury among cyclists experiencing head collisions. In legal cases, reliable brain injury evaluation can be difficult and controversial as mild injuries cannot be diagnosed with conventional brain imaging methods. In such cases, accident reconstruction may be used to predict the risk of TBI. However, lack of collision details can render accident reconstruction nearly impossible. Here, we introduce a reconstruction method to evaluate the brain injury in a bicycle–vehicle collision using the crash helmet alone. Following a thorough inspection of the cyclist’s helmet, we identified a severe impact, a moderate impact and several scrapes, which helped us to determine the impact conditions. We used our helmet test rig and intact helmets identical to the cyclist’s helmet to replicate the damage seen on the cyclist’s helmet involved in the real-world collision. We performed both linear and oblique impacts, measured the translational and rotational kinematics of the head and predicted the strain and the strain rate across the brain using a computational head model. Our results proved the hypothesis that the cyclist sustained a severe impact followed by a moderate impact on the road surface. The estimated head accelerations and velocity (167 g, 40.7 rad/s and 13.2 krad/s2) and the brain strain and strain rate (0.541 and 415/s) confirmed that the severe impact was large enough to produce mild to moderate TBI. The method introduced in this study can guide future accident reconstructions, allowing for the evaluation of TBI using the crash helmet only.

A human surrogate neck for traumatic brain injury research

Properties of the human neck such as range and resistance to motion are considered important determinants of the kinematic response of the head pre, during and post-impact. Mechanical surrogate necks (i.e., anthropomorphic test device necks), have generally been limited to a single anatomical plane of motion and an artificially high resistance to motion. The aim of this study was to present the Loughborough University Surrogate Neck that is representative of the 50th percentile human male neck, developed for motion in and between each of the anatomical planes with inertial and flexural stiffness properties matching those of a passive elastic (i.e., negligible active tension) neck muscle state. The complex intervertebral joints were reduced to three encapsulated ball joints with appropriate locations, orientations and distributed range of motion to precisely position and orientate the head with respect to the torso at the neutral position and end range of motion. A plain bearing sub-assembly was incorporated at the C1-C2 vertebral level to permit 50% of the axial rotation with negligible resistance to motion, as exhibited by humans. Detachable elastomeric elements provided resistance to motion across each ball joint and permit any orientation of the head within the physiological range of motion of the joints. The mass of the surrogate neck (1.62 Kg) was in agreement with the typical human range and similar agreement was found for the principal moments of inertia (I_xx 26.8 kg cm², I_yy 20.5 kg cm² and I_zz 14.3 kg cm²). Quasi-static bending moment and dynamic torque tests characterised the surrogate neck in flexion/extension, lateral flexion and axial rotation. With respect to commercial surrogate necks, the surrogate neck presented here was in closer agreement to the reported human responses, for equivalent loading conditions. The applications of a surrogate neck that can appropriately constrain the head relative to the torso are far reaching in the areas of brain injury mechanism research, and for the development and assessment of protective equipment to reduce the risk of such injuries.

A Review of the Compressive Stiffness of the Human Head

Synthetic surrogate head models are used in biomechanical studies to investigate skull, brain, and cervical spine injury. To ensure appropriate biofidelity of these head models, the stiffness is often tuned so that the surrogate's response approximates the cadaveric response corridor. Impact parameters such as energy, and loading direction and region, can influence injury prediction measures, such as impact force and head acceleration. An improved understanding of how impact parameters affect the head's structural response is required for designing better surrogate head models. This study comprises a synthesis and review of all existing ex vivo head stiffness data, and the primary factors that influence the force-deformation response are discussed. Eighteen studies from 1972 to 2019 were identified. Head stiffness statistically varied with age (pediatric vs. adult), loading region, and rate. The contact area of the impactor likely affects stiffness, whereas the impactor mass likely does not. The head's response to frontal impacts was widely reported, but few studies have evaluated the response to other impact locations and directions. The findings from this review indicate that further work is required to assess the effect of head constraints, loading region, and impactor geometry, across a range of relevant scenarios.

A Review of Head Injury Metrics Used in Automotive Safety and Sports Protective Equipment

Despite advances in the understanding of human tolerances to brain injury, injury metrics used in automotive safety and protective equipment standards have changed little since they were first implemented nearly a half-century ago. Although numerous metrics have been proposed as improvements over the ones currently used, evaluating the predictive capability of these metrics is challenging. The purpose of this review is to summarize existing head injury metrics that have been proposed for both severe head injuries, such as skull fractures and traumatic brain injuries (TBI), and mild traumatic brain injuries (mTBI) including concussions. Metrics have been developed based on head kinematics or intracranial parameters such as brain tissue stress and strain. Kinematic metrics are either based on translational motion, rotational motion, or a combination of the two. Tissue-based metrics are based on finite element model simulations or in vitro experiments. This review concludes with a discussion of the limitations of current metrics and how improvements can be made in the future.

Oblique impact responses of Hybrid III and a new headform with more biofidelic coefficient of friction and moments of inertia

New oblique impact methods for evaluating head injury mitigation effects of helmets are emerging, which mandate measuring both translational and rotational kinematics of the headform. These methods need headforms with biofidelic mass, moments of inertia (MoIs), and coefficient of friction (CoF). To fulfill this need, working group 11 of the European standardization head protection committee (CEN/TC158) has been working on the development of a new headform with realistic MoIs and CoF, based on recent biomechanics research on the human head. In this study, we used a version of this headform (Cellbond) to test a motorcycle helmet under the oblique impact at 8 m/s at five different locations. We also used the Hybrid III headform, which is commonly used in the helmet oblique impact. We tested whether there is a difference between the predictions of the headforms in terms of injury metrics based on head kinematics, including peak translational and rotational acceleration, peak rotational velocity, and BrIC (brain injury criterion). We also used the Imperial College finite element model of the human head to predict the strain and strain rate across the brain and tested whether there is a difference between the headforms in terms of the predicted strain and strain rate. We found that the Cellbond headform produced similar or higher peak translational accelerations depending on the impact location (−3.2% in the front-side impact to 24.3% in the rear impact). The Cellbond headform, however, produced significantly lower peak rotational acceleration (−41.8% in a rear impact to −62.7% in a side impact), peak rotational velocity (−29.5% in a side impact to −47.6% in a rear impact), and BrIC (−29% in a rear-side impact to −45.3% in a rear impact). The 90th percentile values of the maximum brain strain and strain rate were also significantly lower using this headform. Our results suggest that MoIs and CoF have significant effects on headform rotational kinematics, and consequently brain deformation, during the helmeted oblique impact. Future helmet standards and rating methods should use headforms with realistic MoIs and CoF (e.g., the Cellbond headform) to ensure more accurate representation of the head in laboratory impact tests.

Drop Test Kinematics Using Varied Impact Surfaces and Head/Neck Configurations for Rugby Headgear Testing

World Rugby employs a specific drop test method to evaluate headgear performance, but almost all researchers use a different variation of this method. The aim of this study was, therefore, to quantify the differences between variations of the drop testing method using a Hybrid III headform and neck in the following impact setups: (1) headform only, with a flat steel impact surface, approximating the World Rugby method, (2 and 3) headform with and without a neck, respectively, onto a flat MEP pad impact surface, and (4) headform and neck, dropped onto an angled MEP pad impact surface. Each variation was subject to drop heights of 75-600 mm across three orientations (forehead, side, and rear boss). Comparisons were limited to the linear and rotational acceleration and rotational velocity for simplicity. Substantial differences in kinematic profile shape manifested between all drop test variations. Peak accelerations varied highly between variations, but the peak rotational velocities did not. Drop test variation also significantly changed the ratios of the peak kinematics to each other. This information can be compared to kinematic data from field head impacts and could inform more realistic impact testing methods for assessing headgear.

Computational prediction of head-ground impact kinematics in e-scooter falls

E-scooters are the fastest growing mode of micro-mobility with important environmental benefits. However, there are serious concerns about injuries caused by e-scooter accidents. Falls due to poor road surface conditions are a common cause of injury in e-scooter riders, and head injuries are one of the most common and concerning injuries in e-scooter falls. However, the head-ground impact biomechanics in e-scooter falls and its relationship with e-scooter speed and design, road surface conditions and wearing helmets remain poorly understood. To address some of these key questions, we predicted the head-ground impact force and velocity of e-scooter riders in different falls caused by potholes. We used multi-body dynamics approach to model a commercially available e-scooter and simulate 180 falls using human body models. We modelled different pothole sizes to test whether the pothole width and depth influences the onset of falls and head-ground impact velocity and force. We also tested whether the e-scooter travelling speed has an influence on the head-ground impact velocity and force. The simulations were carried out with three human body models to ensure that the results of the study are inclusive of a wide range of rider sizes. For our 10 in. diameter e-scooter wheels, we found a sudden increase in the occurrence of falls when the pothole depth was increased from 3 cm (no falls) to 6 cm (41 falls out of 60 cases). When the falls occurred, we found a head-ground impact force of 13.2 ± 3.4kN, which is larger than skull fracture thresholds. The head-ground impact speed was 6.3 ± 1.4 m/s, which is the same as the impact speed prescribed in bicycle helmet standards. All e-scooter falls resulted in oblique head impacts, with an impact angle of 65 ± 10° (measured from the ground). Decreasing the e-scooter speed reduced the head impact speed. For instance, reducing the e-scooter speed from 30 km/h to 20 km/h led to a 14% reduction in the mean impact speed and 12% reduction in the mean impact force, as predicted by the models. The models also showed that the median male riders were sustaining higher head-ground impact force and speed compared with the small female and large male riders. The findings of this study can assist authorities and e-scooter hiring companies to take more informed actions about road surface conditions and speed limits. These results can also help define representative impact test conditions for assessing the performance of helmets used by e-scooter riders in order to reduce head and brain injuries in e-scooter falls.

Potential of Soft-Shelled Rugby Headgear to Reduce Linear Impact Accelerations

The purpose of this study was to examine the potential of soft-shelled rugby headgear to reduce linear impact accelerations. A hybrid III head form instrumented with a 3-axis accelerometer was used to assess headgear performance on a drop test rig. Six headgear units were examined in this study: Canterbury Clothing Company (CCC) Ventilator, Kukri, 2nd Skull, N-Pro, and two Gamebreaker headgear units of different sizes (headgears 1-6, respectively). Drop heights were 238, 300, 610, and 912 mm with 5 orientations at each height (forehead, front boss, rear, rear boss, and side). Impact severity was quantified using peak linear acceleration (PLA) and head injury criterion (HIC). All headgear was tested in comparison to a no headgear condition (for all heights). Compared to the no headgear condition, all headgear significantly reduced PLA and HIC at 238 mm (16.2-45.3% PLA and 29.2-62.7% HIC reduction; P < 0.0005, η p 2 = 0.987-0.991). Headgear impact attenuation lowered significantly as the drop height increased (32.4-5.6% PLA and 50.9-11.7% HIC reduction at 912 mm). There were no significant differences in PLA or HIC reduction between headgear units 1-3. Post hoc testing indicated that headgear units 4-6 significantly outperformed headgear units 1-3 and additionally headgear units 5 and 6 significantly outperformed headgear 4 (P < 0.05). The lowest reduction PLA and HIC was for impacts rear orientation for headgear units 1-4 (3.3 ± 3.6%-11 ± 5.8%). In contrast, headgear units 5 and 6 significantly outperformed all other headgear in this orientation (P < 0.0005, η p 2 = 0.982-0.990). Side impacts showed the greatest reduction in PLA and HIC for all headgear. All headgear units tested demonstrated some degree of reduction in PLA and HIC from a linear impact; however, units 4-6 performed significantly better than headgear units 1-3.

Effect of Compressive Strain Rate on Auxetic Foam

Auxetic foams have previously been shown to have benefits including higher indentation resistance than their conventional counterparts, due to their negative Poisson’s ratio, making them better at resisting penetration by concentrated loads. The Poisson’s ratio and Young’s modulus of auxetic open cell foams have rarely been measured at the high compressive strain rates typical during impacts of energy absorbing material in sporting protective equipment. Auxetic closed cell foams are less common than their open cell counterparts, and only their quasi-static characteristics have been previously reported. It is, therefore, unclear how the Poisson’s ratio of auxetic foam, and associated benefits such as increased indentation resistance shown at low strain rates, would transfer to the high strain rates expected under impact. The aim of this study was to measure the effect of strain rate on the stiffness and Poisson’s ratio of auxetic and conventional foam. Auxetic open cell and closed cell polymer foams were fabricated, then compression tested to ~80% strain at applied rates up to 200 s−1, with Poisson’s ratios obtained from optical full-field strain mapping. Open cell foam quasi-static Poisson’s ratios ranged from −2.0 to 0.4, with a narrower range of −0.1 to 0.3 for closed cell foam. Poisson’s ratios of auxetic foams approximately halved in magnitude between the minimum and maximum strain rates. Open cell foam quasi-static Young’s moduli were between 0.02 and 0.09 MPa, whereas closed cell foams Young’s moduli were ~1 MPa, which is like foam in protective equipment. The Young’s moduli of the auxetic foams approximately doubled at the highest applied strain rate of 200 s−1.

Quantitative and qualitative analysis of head and body impacts in American 7v7 non-tackle football

Objectives

Non-tackle American football is growing in popularity, and it has been proposed as a safer alternative for young athletes interested in American football. Little is known about the nature of head contact in the sport, which is necessary to inform the extent to which protective headgear is warranted. The objective of this study was to identify the location, types and frequency of head and body contacts in competitive 7v7 non-tackle American football.

Methods

Video analysis was used to document the type, frequency and mechanism of contacts across a series of under 12, under 14 and high school non-tackle tournament games. A subset of impacts was quantitatively analysed via 3-D model-based image matching to calculate the preimpact and postimpact speed of players’ heads and the change in resultant translational and rotational velocities.

Results

The incidence rate of head contact was found to be low (3.5 contacts per 1000 athlete-plays). Seventy-five per cent of head contacts were caused by a head-to-ground impact. No head-to-head contacts were identified. Most contacts occurred to the rear upper (occiput) or side upper (temporal/parietal) regions. Head-to-ground impact was associated with a maximum preimpact velocity of 5.9±2.2 m/s and a change in velocity of 3.0±1.1 m/s.

Conclusion

Non-tackle football appears to represent a lower contact alternative to tackle football. The distribution of head impact locations, mechanisms and energies found in the present study is different than what has been previously reported for tackle football. The existing tackle football standards are not appropriate to be applied to the sport of non-tackle football, and sport-specific head protection and headgear certification standards must be determined.

An improved method for obtaining rotational accelerations from instrumented headforms

The following compares the effect of differentiation methods used to acquire angular acceleration from three types of un-helmeted headform impact tests. The differentiation methods considered were the commonly used 5-point stencil method and a total variation regularization method. Both methods were used to obtain angular acceleration by differentiating angular velocity measured by three angular rate sensors (gyroscopes), and a reference angular acceleration signal was obtained from an array of nine linear accelerometers (that do not require differentiation to obtain angular acceleration). For each impact, three injury criteria that use angular acceleration as an input were calculated from the three angular acceleration signals. The effect of the differentiation methods were considered by comparing the criteria values obtained from gyroscope data to those obtained from the reference signal. Agreement with reference values was observed to be greater for the TV method when a user-defined tuning parameter was optimized for the impact test and cutoff frequency of each condition, particularly at higher cutoff frequencies. In this case, mean absolute error of the five-point stencil ranged from 1.0 (the same) to 11.4 times larger than that associated with the TV method. When a constant tuning parameter value was used across all impacts and cutoff frequencies considered in this study, the TV method still provided a significant improvement over the 5-point stencil method, achieving mean absolute errors as low as one-tenth that observed for the five-point stencil method.