Bicycling accidents account for 3.5% of unintentional injury in children.1 It is estimated that 66 per 100,000 U.S. children use emergency department services for bicycle-related injuries each year.2 Additionally, bicycle-related injuries total approximately $200 million in related inpatient hospital costs.2 Enforcement, modeling, and legislation, as well as community- and school- based interventions, have demonstrated increased helmet use, resulting in fewer head injuries and decreased bicycle-related mortality.3 While the benefits of advocating helmet use are clear, there could be further safety improvements in helmet design.

Generally, there are two categories of helmet foam: 1) Stiff and crushable foams intended for a single hard impact, 2) Softer foams meant for multiple softer impacts.4 In 1957, expanded polystyrene (EPS) was first used in automobile racing helmets.4 In addition to being stiff and crushable, this foam was chosen because it was inexpensive, lightweight, and relatively easy to manufacture.4 Currently, most bicycle helmets are made of EPS foam.4 However, helmets designed for sports with a high risk of head injury, such as football, hockey, and skateboarding, typically use softer foams. In just feeling the foam, bicycle helmet foam feels hard and rigid, while the foam used in other helmets feels much softer.

Adding layers of polyolefin foam to the exterior surface of a football helmet demonstrated reduced impact forces.5 our hypothesis is that bicycle helmets using a softer polyolefin foam will reduce the force of impact better than bicycle helmets using the more rigid EPS foam which could reduce the injury potential when using commercial, mainstream bicycle helmets. The purpose of this study is to compare the G-forces generated from an impact to a manikin's head wearing an off-the-shelf “standard” bicycle helmet (stdBH) compared to a modified bicycle helmet (modBH) (original foam replaced with softer polyolefin foam).

Pairs of 5 different bicycle helmets were purchased from a wide price range: Bell (Vista Outdoor, Anoka, MN) Cruiser ($15, Walmart, Bentonvill, AR), Bell Summit Adult Bike Helmet ($25, Walmart), Bell Avenue LED ($70, Bell Helmets), Giro (Giro, Scotts Valley, CA) Agilis MIPS Bike Helmet ($100, REI, Seattle, WA), and Smith (Smith Optics, Ketchum, ID) Convoy MIPS Bike Helmet ($80, REI). Additionally, a youth baseball helmet was purchased. The pairs of helmets were used to create a stdBH group and modBH group. In the modBH group, the EPS foam was removed with an electric hot knife foam cutter. Next, sheets of polyolefin foam were folded and sewed together to match the thickness of the original EPS foam. The folded and sewn polyolefin foam was attached to the inner surface of the plastic outer shell of the bicycle helmets using Shoe GOO (Eclectic Products, Eugene, OR) as an adhesive (Figure 1).

ADXL326 - 5V ready triple-axis accelerometers (Analog Devices, Inc., Norwood, MA) were attached to the forehead, apex of the head, and right ear of a Century Body Opponent Bag (BOB®) manikin (Century, LLC, Oklahoma City, OK). This head and torso manikin is a martial arts / boxing manikin that mounts onto a weighted base via a hollow plastic tube. The system is non-rigid to better mimic a martial arts / boxing strike. The test helmet was attached to the manikin’s head. To keep the striking force consistent, a pendulum was created using a conventional football helmet (4.6 kg additional weight added) swung from a 1.2 meter rope and released from an angle of 45º serially for multiple data points as shown in Figure 2. Each helmet was struck twelve times in each of the 3 locations (frontal, occipital, and parietal). Maximum linear acceleration in G's (9.8 m/sec/sec) was obtained from the three accelerometer locations on the manikin's head for each striking position. To minimize manikin head strike location variance, the stdBH and modBH were changed without changing the manikin’s position. The manikin’s position was then changed only after all strikes for a given location were completed.

The maximum linear accelerations (in G-forces 9.8 m/sec/sec) recorded for each trial were averaged for each individual helmet. 95% confidence intervals of the mean (95%CIM) were calculated for each of the three accelerometer locations, for each of the three strike types (frontal, parietal, occipital), for each of the helmets, for comparison.

The mean linear accelerations across the 5 different helmet pairs and 1 baseball helmet are summarized in Table 1 and graphed for visible comparison in Figures 3a (frontal strikes), 3b (parietal strikes), and 3c (occipital strikes). For each of the three striking locations, there were statistically lower striking G-forces sustained with the modified softer foam bicycle helmet (modBH) compared to the standard bicycle helmet (stdBH) and baseball helmet. Comparing the stdBHs and modBHs, the greatest reductions were observed in the apical accelerometers when the manikin was struck in the occipital and parietal region.

This study compared the difference in severity of a uniform strike to the head as simulated by a manikin wearing a hard-foam (off- the-shelf, EPS; stdBH) helmet versus a soft-foam helmet (modified, polyolefin; modBH). Additionally this study compared a baseball helmet, which has a soft foam lining made of ethylene-vinyl acetate, to the stdBH and modBH.

A previous study demonstrated that a mean linear acceleration of 98 G’s is a key factor in the development of concussions.4 the linear accelerations in both groups were below the threshold for a concussion injury. Future studies should increase the angle or weight of the football helmet to better mimic the force sustained in a significant traumatic brain injury (TBI). Although the rigid EPS foam of the stdBHs are designed for single use, our serial strike data did not show worsening forces sustained for the serial stdBH strikes. In this study, we measured linear acceleration G-forces. However, rotational acceleration caused by oblique impacts should also be considered. Studies show that translational acceleration is related to pressure and rotational acceleration is dominant in shear deformations, suggesting that they contribute to different types of traumatic brain injuries.6 Since rotational acceleration is a large contributor to concussion, diffuse axonal injury, hemorrhage, and other brain parenchymal injuries7, a future study should include rotational acceleration measurements and a model to better mimic rotational acceleration.

Figures and Tables

Figure 1: Left: Standardized bicycle helmet (EPS Foam); Right: Modified Bicycle Helmet (Polyolefin Foam).

Figure 2: Simulation model setup showing the weighted football helmet and the manikin wearing the bicycle helmet.

Figure 3a, 3b, and 3c: Average peak G-force accelerationcategorized by strike location and helmet type. The error bars represent the 95% CIM.

Table 1: Mean linear accelerations in G’s (9.8 m/secsec) across the 5 different helmet pairs and 1 baseball helmet.

Legend; 1 - Bell Cruiser, 2 - Bell Summit, 3 - Bell Avenue, 4 - Giro Agilis MIPS, and 5 - Smith Convoy MIPS.

Acknowledgement

None of the authors identify a conflict of interest. Abstracts of this study were presented at the Pediatric Academic Societies meeting (Denver, CO), American Academy of Pediatrics conference (Anaheim, CA), and the Biomedical Sciences & Health Disparities Symposium (University of Hawaii, John A. Burns School of Medicine).

Funding

This research work was supported by a research grant form the Hawai’i Pediatric Association Research and Education Foundation (HPAREF) which provided the funds to purchase the helmets, polyolefin foam, electric hot knife foam cutter, and shoe goo adhesive.

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