Turning Heads: Bontrager’s WaveCel putting a new spin on rotational acceleration

A few weeks ago, Trek and their subsidiary company Bontrager dropped what they described as a game changer: their new WaveCel helmet design. Their claims certainly sounded amazing, with Bontrager stating that WaveCel is 48 times more effective than standard foam helmets at reducing concussions caused by cycling accidents.

We had many questions: how does the WaveCel act to reduce concussions and traumatic injuries? What do these claimed numbers mean? Is this technology the real deal?

A basic understanding of head injuries was key to unpacking how this helmet technology has been developed and assessed. In this article, we explore head injury physiology, look at existing technologies and how WaveCel seeks to overcome some of the limitations of existing products. We dive into some literature produced by Bontrager, and the resulting helmet wars between Bontrager and competing helmet protection heavyweight, MIPS; look at what the gaps may be in their testing and claims; and finally what this means for your head and helmet choices.

Head injuries and traumatic brain injuries

Head injuries and brain injuries are distinct, but overlapping terms describing damage occurring to the external head (skull, scalp) and the internal brain and structures respectively (oedema or swelling, diffuse axonal injuries, subdural bleeds and concussions). Head injuries such as lacerations and skull fractures require the head to be struck by an external force to damage the head and skull directly. Helmets have long been used as a safety measure to mitigate these type of injuries, however reduction of internal traumatic brain injuries (TBI) occurring inside the head has been highlighted as a potential area of improvement for cycling helmet safety.

Why the distinction? Well this is because of the nature of the brain itself. The brain and it’s vasculature exists within the rigid outer bucket of the skull: essentially like a slightly gelatinous baked custard in a rigid baking dish. The case of a rapid deceleration, head strike, and repeated shaking (or dropping the custard…) results in the soft, squishy brain striking the outer shell of the skull resulting in injuries to the brain.

Previous studies have shown the even when the custard remains in the bowl…errr skull, the brain is highly susceptible to damage from shearing strains induced by angular head acceleration. In the instance of a bike crash, it’s rare that one crashes directly onto one’s head, and hence the head is subjected to both oblique and linear forces in the case of a bike crash.

Extensive research on helmets have long focused on external damage from linear accelerations, however, the development and use of a standardised oblique testing matrix in order to test for internal head injury litigation through helmets in a lab setting has been largely ignored until recent years.

Outcomes of a traumatic brain injury can range from a mild concussion to death, and the chance of acquiring a brain injury when on the bike directly relates to impact height and speed, impact location and the peak linear acceleration. Lab-based testing seeks to translate these forces into brain-injury risk using the Abbreviated Injury Score (AIS) and Brain Injury Criterion (BrIC)1.

With the discovery of repeated concussions or mild sub-concussive hits being linked to a degenerative brain condition called chronic traumatic encephalopathy (CTE)2, concussions and head injuries have come into mainstream consciousness, as we now understand even ‘mild’ concussions can have devastating long-lasting effects.

1 Takhounts, E. G., Craig, M. J., Moorhouse, K., Mcfadden, J., & Hasija, V. (2013). Development of Brain Injury Criteria (BrIC). SAE Technical Paper Series. doi:10.4271/2013-22-0010

2 Stein TD, Alvarez VE, McKee AC. Concussion in Chronic Traumatic Encephalopathy. Curr Pain Headache Rep. 2015;19(10):47. doi:10.1007/s11916-015-0522-z

2. Helmet Technologies: new vs old

Australia was the first country to make helmet use on bicycles mandatory, and our own ‘Australian Standards’, protocols for AS/NZS is much more rigorous than EU and CPSC standards: the European and US standards of testing, respectively1. AS/NZS testing features a lower threshold for peak linear acceleration (250g vs 300g) and features multiple impact testing on the same helmet as well as retention testing and load distribution testing, and a number of different temperatures and conditions.

EPS

Expanded polystyrene is the most common type of helmet on the market. Featuring an outer shell and a layer of EPS and comfort foam padding on the interior, this idea of a layer of foam to protect the head as been proven to reduce head injuries in linear testing time and time again1. Prior to the advent of MIPS and WaveCel-equipped helmets, this is what we all rode.

The Giro Chronicle has MIPS, but also good coverage with EPS foam.

EPS helmets have been evolved with the oblique effects of crashing in mind previously; in 2010 Australian physicist Don. E Morgan identified failings of existing EPS in reduction of force transference in the event of a crash and invented ‘Conehead’ technology; essentially interlocking pyramids of EPS foams of different densities, allowing for compression of the foam and force absorption in the case of a crash2. This allowed the forces of the head to be spread laterally and marked a departure in helmet technology from a single compound EPS structure.

Other recent papers have suggested an anisotropic (ie: a having a physical property which has a different value when measured in different directions) foam may address some of the concerns raised in this article about rotational acceleration, by building a helmet that is lower in density and with anisotropic values that acts to absorb linear and oblique forces in the case of impact.

MIPS Slip-Liners

If you have been to purchase a helmet in the past few years you have probably seen the little yellow MIPS label. MIPS, or Multi-Directional Impact Protection System, began development in Sweden in 2001 and has since been licensed to multiple helmet manufacturers.

MIPS sought to answer the problems of rotational acceleration in the case of a crash, and developed what we now see as the yellow liner inside the MIPS-equipped helmets.

The MIPS liner seeks to reduce rotational acceleration in the case of a crash by allowing the head to move 10-15mm in case of crashing, allowing the head to continue in a linear manner in the case of angled impacts, therefore reducing rotational motion of the head and brain. MIPS also acts to dissipate these forces over a larger surface area of the head.

MIPS’ own testing has been undertaken at 6.0m/s at 45 degree impact angle against grinding paper at three helmet test points using the Hybrid III headform.

WaveCel

WaveCel was released in March 2019 and was advertise in the weeks leading up to the release as a game changer. The design, from Bontrager, is limited to their line of helmets. WaveCel is essentially a collapsible cellular honeycomb liner made of plastic inserted inside the EPS shell. The idea is for the WaveCel to flex, crumple and glide to transfer forces and reduce rotational acceleration in the case of an impact.

Compared to the MIPS slip liner, the idea behind WaveCel is to allow the cellular structure to flex, allowing a reduction in shearing forces, and crumple the cellular structure, a key feature absent from MIPS helmets. However the glide property of the WaveCel is similar to that of a MIPS slip liner.

WaveCel claims to reduce the instance of cycling-related concussions by up to 48 times when compared to a standard foam EPS helmet, and have certainly been advertised as the new best thing in helmet technology.

1 Olivier, Jake & Creighton, Prudence. (2017). Bicycle injuries and helmet use: A systematic review and meta-analysis. International Journal of Epidemiology, 46, dyw153. 10.1093/ije/dyw153.

2 ‘Conehead Helmets’. (accessed April 4 2019) from http://coneheadhelmets.com.au

36. Vanden Bosche, K., Mosleh, Y., Depreitere, B., Vander Sloten, J., Verpoest, I., & Ivens, J. (2017). Anisotropic polyethersulfone foam for bicycle helmet liners to reduce rotational acceleration during oblique impact. Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine, 231(9), 851–861. https://doi.org/10.1177/0954411917711201

4 TECHNOLOGY. (n.d.). Retrieved April 05, 2019, from http://mipsprotection.com/technology/

5 Bontrager WaveCel Helmet. (n.d.). Retrieved April 01, 2019, from https://wavecel.trekbikes.com/au/en_AU/

1 ACCC. (2016). Review of the safety standard for bicycle helmets (accessed April 4 from 2019) from https://consultation.accc.gov.au/product-safety/bicycle-helmets/supporting_documents/ACCC%20review%20of%20the%20mandatory%20standard%20for%20bicycle%20helmets%20%20consultation%20paper.pdf

3. What are the Claims?

When WaveCel busted onto the market, their key claim was that the new technology would reduce your chance of a head injury by up to 48 times, and reduce your risk of a concussion in a case of crash to around 1%.

These are fighting numbers indeed, but what does it mean?

The study itself was published in the Journal Accident Analysis and Prevention, and relies heavily on modifying existing technology described and tested by Hansen. et.al in1 his paper “Angular Impact Mitigation system for bicycle helmets to reduce head acceleration and risk of traumatic brain injury”. Hansen et.al developed an aluminium liner, similar to the WaveCel, and tested this in oblique impacts.

4. What are the limitations?

While we know that oblique impacts have an effect on the forces transmitted to the brain, and can result in devastating traumatic brain injury. There is yet to be a universally accepted test criteria for assessing this. It’s widely accepted that a drop test, used in CPSC, AS/NZS and Virginia Tech testing, is the gold standard in assessing peak linear acceleration, however despite multiple oblique tests in existence there is yet to be a gold standard in angular testing.

The best data and testing would be real world data from a large sample of riders from commuters, road racers and mountain bikers; assessing speed and angle of crashes and relating head injuries in real world situations. However, the time and technology to carry out this testing would be largely prohibitive and would require more steps to ensure the emerging technology isn’t in any case likely to worsen head in jury outcomes in the case of a crash.

TBI are a heterogenous occurrence, even when specifically looking at bicycle injuries, hence attempting to reproduce these real world impacts in a lab would be near impossible.

The study from WaveCel used one brand and model of helmet but with different specifications: fitted with MIPS, WaveCel or a standard EPS helmet.2 They studied impacts at 4.8 and 6.2m/s (relating to approximately 17 and 23km/hr) in a mid-sagittal (central line) location at 30-, 45- and 60-degree impact angles.

Issues with the studies:

The WaveCel study had certain features that made it, well, less encouraging for mountain bikers and those who ride at speeds >23km/hr (those racing and engaging in higher speed downhill-style pursuits).

-While the speeds tested are regarded as a gold standard in linear acceleration tests (4.8m/s and 6.2m/s—relating to 17.3km/hr and 22.3km/hr, relating from broad studies of general cycling crashes) they certainly don’t cover all crashes; especially those in a competitive setting and speeds reached when riding or racing, in particular gravity fuelled events. There are no testing standards that will look at what happens if you hit the deck at 40km/h on a downhill track, or at 60km/h at the end of a road criterium in a bunch sprint, so consider the limitations of these studies if you are in one of these populations.

-The study looked at a single impact per helmet tested. While this is possible in crashes, this doesn’t give us real world data in the case of multiple impact in, for example, uneven terrain (think crashing and rolling down a rock-garden).

-The study was approached heavily using existing studies on city-based riding (in terms of impact speed, angle and surface)—which makes sense as most cycling safety studies would be approached from a commuter and city-riding standpoint. This doesn’t address the difference for mountain bikers of striking a different surface to asphalt (ie: landing and sliding down a muddy chute, on a loose surface, or colliding with trees or other trail furniture).

1 Hansen, K., Dau, N., Feist, F., Deck, C., Willinger, R., Madey, S. M., & Bottlang, M. (2013). Angular Impact Mitigation system for bicycle helmets to reduce head acceleration and risk of traumatic brain injury. Accident Analysis & Prevention, 59, 109-117. doi:10.1016/j.aap.2013.05.019

2 Bliven, Emily, et al. (2019) Evaluation of a Novel Bicycle Helmet Concept in Oblique Impact Testing, Accident Analysis & Prevention. 124, 58–65, doi:10.1016/j.aap.2018.12.017.

Issues with claims, continued

-The testing used mid-sagittal impact locations at a range of different angles: given the evidence of severity of rotational injury in non-central impact points (ie: temporal impacts) this means the study has omitted potential significant testing that could yield significant yet discrete results from the testing undertaken.

-The study used a Hybrid III 50th percentile headroom and neck, which has been acknowledged to have issues with lateral neck flexion: i.e.: the unit is overly stiff and may not adequately represent the movement and forces of oblique impacts.

-The claimed numbers WaveCel bases their advertising on, is the 6.2m/s at 45-degree angle, and is referring to the WaveCel vs a standard EPS foam helmet: not a MIPS equipped helmet (though must be noted, in their testing WaveCel tested significantly better than the MIPS helmet in this test).

-WaveCel’s reduction in linear acceleration—essentially what all standards seek to test—is 26% compared to the EPS helmets (at a 30 degree impact, 4.8ms/s)—but no significant linear acceleration reduction between helmets at 6.2m/s so for certain types of crash there may be nil significant difference between the basic EPS and WaveCel helmets, their claim of concussion reduction is specifically linked to a specific test outcome within a host of different tests.

-Reproducibility: the integrity of finding and reproducibility of the data acquired has come into question with MIPS issuing a statement shortly after the release of the WaveCel “Preliminary test results of WaveCel helmets by MIPS cannot substantiate these claims. While further testing is warranted, MIPS cannot see that the helmets perform in a way that the claims Bontrager/WaveCel makes in the comparison between WaveCel and other helmets/technologies”.

The WaveCel vs MIPS affair

MIPS’ statement initially addressed some of the concerns we had when looking at testing data, namely acknowledging the limitations of lab-based studies on real-world applications;

“MIPS’ position on evaluating the possibility of a concussion resulting from a crash is that it is a highly variable event and unique to the individual impact and rider physiology. No two crashes are the same and no two people are the same, so the risk of concussion is a near-impossible claim to make. However, rotational motion itself can be measured objectively, so that is the metric MIPS can actually report and address”.

MIPS goes on to deliver a statement on their findings; “MIPS tested the helmets with commonly used test methods for helmets in angular fall and following the same test protocol as WaveCel. When tested at the speed of 4.8m/s MIPS found no difference in risk injury reduction between helmets equipped with WaveCel and those helmets just equipped with EPS foam. In the 6.2m/s impact case, there was a slight reduction, but not consistent with the communicated claims.”

MIPS stated that the use of the Hybrid III neck was likely what caused the inconsistencies with testing, but begs the question about their reproducibility of studies; if they used the headform without the neck or surrogate neck, then the study has been modified from its original form; surely some variation would be expected?

MIPS state that the neck, as acknowledged in the WaveCel paper, is overly laterally stiff and state that they use a different neck. We reached out for MIPS for further comment and access to their own bicycle helmet testing papers, however, at this point haven’t received a response.

5. Our take and where to now with head technology?

Despite the discussed limitations of the WaveCel study and the ongoing battle between the two new heavyweights in the helmet industry: MIPS and WaveCel, it should be stated that Virginia Technology—an independent helmet tester (using linear testing)—has the Bontrager Specter WaveCel rated the highest out of a range of other tested helmets; including a range of MIPS-equipped helmets and EPS helmets, with the entire range of WaveCel helmets rated 5/53.

This doesn’t tell us about the rotational acceleration that WaveCel seeks to reduce with the technology, but given the paper that we do have, we can assume that in certain situations WaveCel does have the upper hand in TBI prevention compared to other helmets on the markets.

Is it the be all and end all? Do we put our head safety in the claims of one manufacturer and start doing backflips with gay abandon because we have the new WaveCel technology on board?

Well not exactly, as mentioned there are multiple tests within the WaveCel study where all the helmets performed very similarly. In the real world, when pin-balling off trees, flopping through rock gardens or travelling at a higher speed than the standardised tests, you are of course more likely to attain a head injury. At certain speeds, and certain angles of crashing WaveCel may reduce your chance and severity of a TBI, but it’s probably best to still ride within your ability.

It’s an exciting and encouraging development that brands are spending money on safety and, in particular, looking towards better ways of protecting our heads, as we are beginning to understand the broad implications of concussions and TBI and their effects on everyday functioning. In the future, we look forward to oblique standardised testing that can be used to assess rotational acceleration and velocity in the case of bicycle crashes, with realistic and agreed upon head and neck models to replicate a range of ‘real world’ experiences in a lab-based scenario.

1 Bland, M. L., Zuby, D. S., Mueller, B. C., & Rowson, S. (2018). Differences in the protective capabilities of bicycle helmets in real-world and standard-specified impact scenarios. Traffic Injury Prevention, 19(Sup1). doi:10.1080/15389588.2017.1388915

2 MIPS EVALUATION OF WAVECEL TECHNOLOGY SHOWS RESULTS FAR BELOW THE PUBLISHED CLAIMS. (2019, April 03). Retrieved from http://mipsprotection.com/mips-evaluation-of-wavecel-technology-shows-results-far-below-the-published-claims/

3 Bike Helmet Ratings. (n.d.). Retrieved April 01, 2019, from https://www.helmet.beam.vt.edu/bicycle-helmet-ratings.html