Rotational Injury Mitigation
Summary: Impacts that rotate the brain inside the skull and stretch or damage brain connective tissues have been identified as the main cause of concussions. Helmet designers have struggled to keep pace with consumer demand for better protection from this type of injury. Standards still emphasize protection from the highest translational impact forces. Manufacturers have been forced to develop a marketing story about rotation, but it is not clear that the measures introduced to date are effective. The best-known mitigation system is MIPS, but there are many others. We have a separate page up on MIPS.
Nature of the injury
Every impact to the head that is not exactly in line with the center of gravity of the brain involves some tangential force to the brain. That would include almost all impacts that a bicycle helmet protects against. The brain floats in the skull and can rotate in response to an impact, straining, stretching or tearing the connecting tissues. Damage to those tissues has been associated with concussions (mild traumatic brain injury) for about four decades. Research over that time has established that the effects are more serious and much longer-lasting than was originally suspected.
Design responses: MIPS, Trek, 6D, Leatt, Kali, POC, LEM, HEXR, Louis Garneau
Helmet designers initially ignored the concussion problem, focussing on prevention of catastrophic injuries that cause death and permanent disability. They also pointed out that reducing the peak severity of the blow reduces both rotational and translational energy, reducing all brain injury. As concussion awareness has risen, many manufacturers have been working on technology to address the rotation problem. Helmet companies now say "you have to have a marketing story on concussion protection."
The best-known rotational injury management technology is the slip plane, a layer that permits the head to slip sideways in an impact to reduce peak energy to the head. The Swedish company MIPS obtained a patent on it and began aggressively promoting it. (We have
a page up on MIPS) Although some doubt that MIPS is actually effective in field crashes, publicity about it was instrumental in spurring innovation in the field.
6D has demonstrated that its technology performs well in lab testing. Leatt and Kali also have helmets with liner "doughnuts" designed to permit head motion on impact. POC, the first MIPS user, introduced a similar alternative in 2018 that it calls
SPIN pads, placed around the interior of the helmet to promote lateral movement of the helmet in a crash. (A MIPS patent-infringement lawsuit against POC has now been settled.) Trek introduced their WaveCel liner, with a mesh that can collapse at an angle to reduce rotational force. LEM has "GelMotion" impact pads. Custom helmet producer HEXR has
lab tests on their site showing that their 3D printed honeycomb interior reduces rotational energy. Bell, once a part-owner of MIPS, has a MIPS layer between two helmet layers (the original POC concept) that it calls ball-and-socket or "Spherical Technology." Louis Garneau has
ROTEXX pads with a fluid layer. One company has a European grant to develop a technology that changes the structure of an EPS liner to permit movement on impact. The MIPS company has purchased Glidewear and Fluid Inside, saying they wanted to expand their offerings. There are more that we don't have on this page yet.
Test results
There are lots of test results published by manufacturers, all showing that their particular brand is best. Among the organizations we look to for comparisons:
Virginia Tech's Biomechanical Engineering Department has been involved in concussion research for a number of years. They developed a program called STAR ratings based on that research, rating helmets on their ability to prevent concussions. Although there is no agreement among helmet and concussion experts that the ratings are based on exact concussion parameters, we think they represent a valid attempt to use lab testing to highlight concussion-level performance. Their
STAR ratings page has all the details. Almost all of their top 72 Five Star helmet models have MIPS. The others have WaveCel or SPIN. The next rank of 37 Four Star helmets include 17 with MIPS, another 17 without MIPS and three with other rotational energy management systems.
The Snell Foundation has published two rounds of testing of either MIPS or another unspecified rotational energy mitigation technology. They are sceptical. You can read about their first round of MIPS testing on
our MIPS page or
the Snell site.
In 2022 Snell published a paper on its website with
new findings on rotational testing.
They did not specifically identify the technology they tested, MIPS or other. When they tested bicycle helmets with a method similar to ASTM/EN/MIPS/Folksam/CERTIMOOV testing, they found some improvement in angular force in one location on a helmet, but no improvement in angular velocity. For a motorcycle helmet they did find improvement in both measures. But testing of motorcycle helmets with a wig on the headform to add some realism showed no improvement at all in any of the impacts. They conclude that the need for additional technology to control angular forces by adding slip plane layers is not demonstrated, since the coupling of helmet and head in actual crashes is not known. See the link above or
our Update newsletter report for details.
Consumer Reports
After testing helmets with and without MIPS and WaveCel, Consumer reports now says: "Both technologies are comparable in reducing the risk of brain injury, according to researchers at Virginia Tech's Helmet Lab, but is's difficult to prove that any helmet will protect against concussion because there's no single objective test, such as a CT scan or an MRI, that can
determine whether someone has one. Instead, concussions are diagnosed based on symptoms and the results of a neurological exam. But there may be some added benefit, so even if it's not definitive, a MIPS or WaveCel helmet may be worth the extra cost." Note that this is an abrupt change in their position, since the magazine had earlier recommended MIPS without reservation.
The University of Strasbourg in France has a testing program called CERTIMOOV. It tests mainly European models, and the results would be interesting if you live in a country where the EN1078 Standard is relevant. You can find their ratings on the
CERTIMOOV site.
These are not US helmet models. They normally are built to the EN1078 European helmet standard, even if the model names are the same as US models. The helmets would typically be thinner, lighter and less protective than the US model.
Folksam is a respected Swedish consumer magazine. Their ratings are normally for products available on the Swedish market, but recently they expanded to include helmets available in the UK.
These are not US helmet models. They normally are built to the EN1078 European helmet standard, even if the model names are the same as US models. The helmets would typically be thinner, lighter and less protective than the US model. You can find Folksam's 2020 ratings without a pay wall
here.
Helmet standards
Standards makers are struggling to define reliable, repeatable test methods and determine pass-fail thresholds for rotational injury. ASTM has a task group working on that problem, led by Dr. Terry Smith, an experienced standards developer who helped 6D win a million dollar prize for their design. ASTM has now adopted the first of two test methods, similar to the MIPS test protocols, but with no failure thresholds. The European Community also has a task group, led by MIPS co-founder Peter Halldin. They are making progress on the basic test method, but are not optimistic about achieving consensus soon on failure thresholds. Meantime, consumers will continue to find it difficult to sort out the claims of manufacturers about the performance of their helmets.
While waiting, BHSI would like to see the ASTM bicycle helmet standard add a requirement to measure sliding resistance of the shell. That is an easily measured parameter that has been demonstrated in lab testing to affect both neck strain and g's to the brain, but has been ignored.
A Swedish view
This appeared in a study by the Swedish insurance company magazine
Folksam:
"One of the helmets, Yakkay, which is available in a version in which it is possible to fit a cover in the form of a hat/cap, was tested both with and without the cover. A major difference was measured in the oblique tests between the Yakkay with and without the cover, which indicates that this cover provides good protection against rotational forces, similar to MIPS; Figure 17 and Figure 18. The difference is that the sliding shell in the case of the Yakkay is fitted on the outside of the shell of the helmet. It is probably not an intentional rotational protection, but shows that a surface-mounted layer can provide similar protection as a sliding layer fitted on the inside. There is a similar concept among motor cycle helmets, known as SuperSkin, which has been shown to reduce the rotational forces in oblique impact tests (Phillips Helmets. 2015). Another example is the 6D helmet that consists of two layers of EPS linked with "dampers" that allow energy absorbing shear between the layers (6D Helmet. 2015). The Hovding also obtains very good results in the rotational tests; Figure 17. When it is inflated, the exterior fabric can slide sideways in relation to the fabric on the inside against the head. Thus two shearing layers are created that considerably reduces the rotational acceleration. The above examples clearly demonstrate that there are several ways to design a helmet to manage rotational forces."
Brain models and strain measurements
The growing use of computer brain models has led to many modeling studies and promotions of helmet effectiveness based on indications of tissue strain. But the state of computer brain models is at best unsettled. A study
interpreting the same impact dataset with eight different brain models concluded that the results could be highly misleading, with one helmet rated two stars by one brain model and four stars by another. The authors included two early MIPS company founders. They recommended "Therefore, we suggest that the biomechanics community should create a norm or recommendation for future ranking and rating methods." But that misses the main question: is there a brain model today that will produce consistently valid results? We don't know the answer to that.
Rotational crash energy and shell sliding resistance
Although MIPS marketing has concentrated attention on inner layers to address rotation, lab testing has shown conclusively that a smooth outer surface makes a difference. In a crash you want an interface with the road that is smooth, hard, round and slick. That keeps your head from snagging, adding to the severity of the impact and putting more strain on your neck. The first defense against rotational energy is to avoid it in the beginning of the impact.
The optimal bicycle helmet for that moment when you hit the pavement has a round shape and a hard, or at least smooth, shell. The first ones we saw in the US market were skate-style helmets. Most of them have minimal vents and are hot for warm weather bicycle riding. But now road and mountain bike helmets are improving every year. Some are lumpy and have styling ridges, but most manufacturers are moving toward more rounded designs in keeping with current style.
For sliding, the rounder the helmet the better. In addition to possibly adding sliding resistance if a rear projection digs into the pavement, the rear of the elongated helmet could shove the helmet aside when you hit, leaving your head unprotected. Although it has been debated ever since the elongated designs appeared before the turn of the century,
Professor Hugh Hurt raised this question again in 2005, based on both testing problems and field reports of injury from helmets being pushed aside. You probably do not want a helmet that could only be tested by duct taping it onto the test headform. Fortunately, the current trend is to "compact" helmets, and the aero shape looks dated now.
Vents are necessary, but make sure they are smoothly faired into the helmet shell, and avoid any helmet with unnecessary fashion ridges on the outside, or protruding snaps for visors, or little animal ears and noses, or any other feature that could cause the shell to snag. This is an easy issue for a consumer to assess, as long as you keep in mind that you want your head to slide on impact with pavement. It should be evident that you don't want to add any accessory or cover to the exterior of a helmet that adds to its sliding resistance. That includes lights and cameras. Many of the ones we see have mounts that are much too strong to break away easily when you need to slide. ASTM tried to develop a standard for the force that should trigger an accessory to break away, but concluded after testing that
the jerk is not a problem. Some manufacturers already have their own internal standard. In the meantime you are on your own to figure it out.
Conclusion
Knowledge of rotational injury management is evolving. No consensus standards have been adopted yet for the effectiveness of any helmet technology to mitigate rotation. Some technologies may help prevent injury in some impacts. Based on lab data we believe that rounder, smoother helmets that slide well on pavement are essential.