The myth that bicycle helmets protect against brain injury


Brain injury is caused by rotational acceleration.
Bicycle helmets are not designed to protect against it.
In some circumstances they can increase it, causing permanent disability.


What surgeons say

This article reports from a surgeon who operates on cyclists:

“The ones with brain swelling, that’s diffuse axonal injury, and that’s bad news” …

the whole brain is shaken up, creating many little tears in its inner structure …

Such patients undergo personality change, can contract epilepsy and have difficulty controlling their anger. They might become unemployable. Depression is a common accompaniment to brain injury. Rosenfeld sees patients’ families shattered, too. “They’re never the same. It often leads to marriage disharmony and family breakdown.” …

Rosenfeld’s opinion is candid.  “I don’t know if [helmets] do much to protect the inner part of the brain,”

A neurosurgeon from the UK admits:

“I see lots of people in bike accidents and these flimsy little helmets don’t help.”

According to another Australian surgeon who operates on cyclists:

“The best evidence is that [a helmet] doesn’t make any difference to serious head injury when riding a bicycle …

initial research used to back the mandatory laws was “deeply flawed and criticised”. Some newer findings, he said, showed that these laws could increase the chance of serious injury. “

Sport cyclists are having doubts

Helmets are part of the sport cyclist uniform. Yet fatalities have doubled since mandatory helmets:

The helmet rule for professional cyclists was brought by the UCI in 2003 following the death of Andrei Kivlev during the Paris-Nice race.

Since then deaths of professional cyclists while racing have doubled, so where is the protection that helmets are supposed to give a rider?

Sport cyclists are having doubts, as reflected in a recent edition of Bicycling Magazine:

new research is finding that concussions could be as dangerous as splitting open your skull. And that brain bucket you own? It was never designed to prevent concussions.

Distinguishing between focal brain injury and diffuse brain injury

What’s going on? Why do people believe that bicycle helmets protect against brain injury while surgeons say they don’t? Much of the confusion comes from a lack of distinction between focal brain injury and diffuse brain injury.

An old popular belief is that brain injury is caused by a direct hit the head, like a head hitting a wall, causing linear acceleration. This is focal brain injury. Bicycle helmets are designed to reduce focal injury. The polystyrene reduces linear acceleration by compressing on impact.

Scientific research done in the 1970’s has revealed that the main cause of severe brain injury is not focal injury but diffuse axonal injury:

“The experiments gave special attention to direct comparison of the contributions of linear and angular acceleration because the widely accepted Head Injury Criterion for head injury is based on measurements of the former – see below. The essential role of angular acceleration in producing cerebral concussion was shown, the threshold being estimated as 2000-3000 rad/sec². Translation was also responsible for brain injuries, albeit only focal, and did not produce concussion.  …

Along with the support which research has provided for the new theory, it has discredited the notions of coup and contre-coup and of linear acceleration of the brain being a major factor in injury to it.”

Diffuse axonal injury, also called rotational injury, is caused by angular acceleration, for example when the head rotates quickly. The skull may be intact, but there can be severe internal brain damage. This is what the surgeon Dr.  Rosenfield was describing in the article mentioned above.

Some people believe helmets protect against brain injury. Not only is this not true, but the opposite is more likely to be the case:

“Protecting the brain from injury that results in death or chronic disablement provides the main motivation for wearing helmets. Their design has been driven by the development of synthetic polystyrene foams which can reduce the linear acceleration resulting from direct impact to the head, but scientific research shows that angular acceleration from oblique impulse is a more important cause of brain injury. Helmets are not tested for capacity to reduce it and, as Australian research first showed, they may increase it.

Even though helmets do not protect against brain injury, fear of chronic disability has been used to promote them. This has lead to an exaggerated opinion of the protection provided by helmets. A false sense of safety can lead to increase risk taking, as reported in the New York Times:

“the increased use of bike helmets may have had an unintended consequence: riders may feel an inflated sense of security and take more risks. …

The helmet he was wearing did not protect his neck; he was paralyzed from the neck down. …

”I definitely felt safe. I wouldn’t do something like that without a helmet.” 

Research on bicycle helmets

Soft-shell helmets, the most common type of bicycle helmet, are helmets without a hard shell. They have a thin layer of plastic on top, less than 1 mm.

soft-shell-helmet A soft-shell helmet is a piece of polystyrene covered by a layer of plastic less than 1mm thick.

In 1987, an Australian government agency released research that highlighted deficiencies with bicycle helmets:

The substantial elastic deformation of the child head that can occur during impact can result in quite extensive diffuse brain damage. It is quite apparent that the liner material in children’s bicycle helmets is far too stiff …

rotational accelerations were found to be 30% higher than those found in similar tests using a full face polymer motorcycle helmet. More work needs to be done in this area as there would seem to be a deficiency in rotational acceleration attenuation that may be the result of insufficient shell stiffness

Helmets can increase brain injury, according to research done in Sweden:

“The non-shell helmet did in all trials grab the asphalt surface, which rotated the head together with
the helmet. The consequences were in addition to the rotating of the head, a heavily bent and compressed neck, transmitted on through the whole test dummy body after the impact.  …

This gives an average angular acceleration of 20800 rad/s² for rotating the head from 0 to 0.26 rad during the 5 ms. Löwenhielm proposes 4500 rad/s² to be the maximum angular acceleration that can be tolerated for a limited time period”

Soft-shell helmets amplified rotational acceleration to four times higher than the tolerable maximum.

On impact, the larger head volume amplifies rotational acceleration. 3cm increase in helmet circumference increases rotational acceleration by 150%:

“the 3000rad/s² to 8500rad/s² measured during abrasive and projection oblique tests with size 54cm (E) helmeted headforms. However, for the most severe cases using a size 57cm (J) headform, rotational acceleration was typically greater than 10,000rad/s² and increased to levels of 20,000rad/s², a level at which a 35% – 50% risk of serious AIS3+ injuries is anticipated.”

The difference between a helmeted and non-helmeted head is about 20cm.

The mechanics of bicycle helmets

Helmets increase the volume of the head.  In the event of an accident, this increases the risk of the head hitting the road. Post-crash studies found that most helmets show impacts to the side, where a bare head is protected by the shoulders.


Helmets increase volume of the head, doubling  chance of the head hitting the road in an accident.
The larger volume amplifies rotational acceleration, the main cause of brain injury.

Is this just a theory?

Unfortunately it doesn’t seem like it.

doctor from New Zealand reports:

“cycle helmets were turning what would have been focal head injuries, perhaps with an associated skull fracture, into much more debilitating global head injuries”

In Canada, the length of stay in hospital increased increased following helmet laws, from 4.3 days to 6.9 days. The number of serious head injury admissions increased by 46%.

Helmeted riders are more than twice as likely to hit their head in an accident, with more impacts to the sides. A 1988 study reports that helmeted riders hit their heads seven times more often than un-helmeted riders.

In the US, a strong rise in helmet wearing was followed by an increase in head injuries:

“Bicycle helmets might not protect cyclists much at all.  And, in fact, in some cases, they might actually be more dangerous than going lidless. …

head injuries had increased even though the use of helmets had skyrocketed throughout the 1990s.  The risk of injury per cyclist had gone up by 51%. …

We don’t know what’s going on,” said one political appointee who should know.  Well, I’ll offer my idea.  People accepted the idea that helmets work, and then created studies to “prove” that they do. “

It is easy to take things for granted, and to assume that a device labeled a “helmet” can only protect, and cannot make things worse.

What about the studies that claim that helmets protect against 80% of brain injuries?

There have been many “studies” claiming that helmets protect against brain injuries. The most famous one was done by helmet lobbyists and funded by the helmet industry. This “study” had methodological errors. It was the basis for a US government claim that helmets reduce 85% of head injuries. The US government has since dropped this claim.

Many studies assume that helmets are effective and attempt to “prove” it. Such studies jump to a predetermined conclusion, with a disturbing lack of scientific discipline. Often the claims are contradicted by the study’s own data.

Governments who enacted helmet law have funded policy-driven studies defending their policy. Such “studies” use biased statistics, resulting misleading claims. Bill Curnow, a scientist from the CSIRO, wrote as a conclusion of a scientific article:

“Compulsion to wear a bicycle helmet is detrimental to public health in Australia but, to maintain the status quo, authorities have obfuscated evidence that shows this.”

In 2011, the New South Wales government funded a study trying to deny that helmets can aggravate brain injury. This risk exists for soft-shell helmets at high speed. The “study” set up unrealistic conditions at low speed, then magically generalized the results. This is deceitful as the unrealistic conditions are not representative of real life accidents.

The dilemma: minor skull injuries vs brain injury

Bicycle helmets are designed to protect cyclists if they fall on top of their heads at speeds below 20 km/h. They mitigate against minor skull injuries like bruises and lacerations. They are not tested for side impacts.

Is it worth to increase the risk of brain injury to mitigate minor skull injuries? An Australian cyclist challenged a helmet fine as helmets can increase brain injury. After reviewing evidence in a court of law, District Court Judge Roy Ellis concluded:

“”Having read all the material, I think I would fall down on your side of the ledger …

I frankly don’t think there is anything advantageous and there may well be a disadvantage in situations to have a helmet  and it seems to me that it’s one of those areas where it ought to be a matter of choice.”

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2 thoughts on “The myth that bicycle helmets protect against brain injury”

  1. Great article. Here in the Netherlands there’s also talk of helmet laws (for as long as I can remember) but I never heard the counterargument of helmets actually making things worse. The only thing I don’t understand is, why is it bad that the plastic layer is very thin on bike helmets?

    1. Is the thin plastic layer bad?

      It’s not bad for helmet manufacturers. It makes the helmet looks shiny and attractive, masking that it is mostly a piece of styrofoam. Some helmets sell for $200. That’s a nice profit.

      On the other hand, this shiny bit of plastic can give the impression that the helmet is stronger than it really is. That can lead to a false sense of safety, leading people to ride less carefully. Unfortunately some people find out too late that the “helmet” doesn’t protect much in a serious accident.

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