A helmet increases rotational force by effectively increasing the radius of the head + helmet vs. head alone. It also increases rotation by increasing friction, thus rotation. A scalp provides lubricant, reducing the frictional coefficient.

OK... so what's your source for, "...the main cause of TBI, which is rotational injury..."? Specifically regarding cycling crashes where TBI occurs.

Here you go.

J Head Trauma Rehabil. 2003 Jul-Aug;18(4):307-16.
Diffuse axonal injury in head trauma.
Smith DH, Meaney DF, Shull WH.
Source

Department of Neurosurgery, University of Pennsylvania, 3320 Smith Walk, 105 Hayden Hall, Philadelphia, PA 19104, USA. smithdou@mail.med.upenn.edu
Abstract

BACKGROUND: Diffuse axonal injury (DAI) is one of the most common and important pathologic features of traumatic brain injury (TBI). The susceptibility of axons to mechanical injury appears to be due to both their viscoelastic properties and their high organization in white matter tracts. Although axons are supple under normal conditions, they become brittle when exposed to rapid deformations associated with brain trauma. Accordingly, rapid stretch of axons can damage the axonal cytoskeleton resulting in a loss of elasticity and impairment of axoplasmic transport. Subsequent swelling of the axon occurs in discrete bulb formations or in elongated varicosities that accumulate transported proteins. Calcium entry into damaged axons is thought to initiate further damage by the activation of proteases. Ultimately, swollen axons may become disconnected and contribute to additional neuropathologic changes in brain tissue. DAI may largely account for the clinical manifestations of brain trauma. However, DAI is extremely difficult to detect noninvasively and is poorly defined as clinical syndrome. CONCLUSIONS: Future advancements in the diagnosis and treatment of DAI will be dependent on our collective understanding of injury biomechanics, temporal axonal pathophysiology, and its role in patient outcome.

Exp Neurol. 2012 Jan 20. [Epub ahead of print]
Axonal pathology in traumatic brain injury.
Johnson VE, Stewart W, Smith DH.
Source

Penn Center for Brain Injury and Repair and Department of Neurosurgery, University of Pennsylvania, Philadelphia, USA.
Abstract

Over the past 70years, diffuse axonal injury (DAI) has emerged as one of the most common and important pathological features of traumatic brain injury (TBI). Axons in the white matter appear to be especially vulnerable to injury due to the mechanical loading of the brain during TBI. As such, DAI has been found in all severities of TBI and may represent a key pathologic substrate of mild TBI (concussion). Pathologically, DAI encompasses a spectrum of abnormalities from primary mechanical breaking of the axonal cytoskeleton, to transport interruption, swelling and proteolysis, through secondary physiological changes. Depending on the severity and extent of injury, these changes can manifest acutely as immediate loss of consciousness or confusion and persist as coma and/or cognitive dysfunction. In addition, recent evidence suggests that TBI may induce long-term neurodegenerative processes, such as insidiously progressive axonal pathology. Indeed, axonal degeneration has been found to continue even years after injury in humans, and appears to play a role in the development of Alzheimer's disease-like pathological changes. Here we review the current understanding of DAI as a uniquely mechanical injury, its histopathological identification, and its acute and chronic pathogenesis following TBI.


J Neurotrauma. 2012 Sep 19. [Epub ahead of print]
A Multiscale Computational Approach to Estimating Axonal Damage under Inertial Loading of the Head.
Wright RM, Post A, Hoshizaki B, Ramesh KT.
Source

Johns Hopkins University, Department of Mechanical Engineering, Baltimore, Maryland, United States; rika@jhu.edu.
Abstract

A computational modeling framework is developed to estimate the location and degree of diffuse axonal injury (DAI) under inertial loading of the head. DAI is one of the most common pathological features of traumatic brain injury and is characterized by damage to the neural axons in the white matter regions of the brain. We incorporate the microstructure of the white matter (i.e. the fiber orientations and fiber dispersion) through the use of diffusion tensor imaging (DTI), and model the white matter with an anisotropic, hyper-viscoelastic constitutive model. The extent of DAI is estimated using an axonal strain injury criterion. A novel injury analysis method is developed to quantify the degree of axonal damage in the fiber tracts of the brain and identify the tracts that are at the greatest risk for functional failure. Our modeling framework is applied to analyze DAI in a real-life ice hockey incident that resulted in concussive injury. To simulate the impact, two-dimensional finite element (FE) models of the head are constructed from detailed MRI and DTI data and validated using available human head experimental data. Acceleration loading curves from accident reconstruction data are then applied to the FE models. The rotational (rather than translational) accelerations are shown to dominate the injury response, which is consistent with previous studies. Through this accident reconstruction, we demonstrate a conceptual framework to estimate the degree of axonal injury in the fiber tracts of the human brain, enabling the future development of relationships between the computational simulation and neurocognitive impairment.


Biomed Sci Instrum. 2012;48:239-45.
Experimental biomechanical study of head injuries in lateral falls with skateboard helmet.
Kumar S, Herbst B, Strickland D.
Source

University of Mississippi Medical Center.
Abstract

FROM THE ABSTRACT: Although the linear head accelerations...were reduced, the angular head accelerations even with the helmet were above nearly all proposed rotational head injury threshold in the literature. The higher angular head accelerations indicate a higher probability of concussion, acute subdural hematoma and diffuse axonal injuries. The present study is an additional step to better understand the biomechanics of TBI and the role of protective headgear systems in sports and recreational accidents.

Neuroscience. 2010 Aug 11;169(1):357-69. Epub 2010 May 6.
Diffuse axonal injury induced by simultaneous moderate linear and angular head accelerations in rats.
Li XY, Li J, Feng DF, Gu L.
Source

Department of Neurosurgery, No.3 People's Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, PR China.
Abstract

Diffuse axonal injury (DAI) is one of the most common and important pathologic features of human traumatic brain injury (TBI), accounting for high mortality and development of persistent post-traumatic neurologic sequelae. Although a relatively high number of therapies have been shown to be effective in experimental models, there are currently few treatments that are effective for improving the prognosis of clinical DAI. A major reason is the failure of current models to validly reproduce the pathophysiological characteristics observed after clinical DAI. In the present study, we employed a specially designed, highly controllable model to induce a sudden rotation in the coronal plane (75 degrees rotation at 1.6x10(4) degrees/s) combined with lateral translation (1.57 cm displacement at 3.4x10(2) cm/s) to the rat's head. We were interested in discovering whether the combined accelerations could reproduce the pathophysiological changes analogous to those seen in human DAI. The axonal injury as assessed with amyloid protein precursor (APP) as a marker was consistently present in all injured rats. The commonly injured brain regions included the subcortical regions, deep white matter, corpus callosum and brain stem. The evolution of APP accumulations in brain sections depicted the detailed progression of axonal pathology. Ultrastructural studies gave further insights into the presence and progression of axonal injury. All injured rats exhibited transient physiological dysfunction, as well as immediate and dramatic neurological impairment that still persisted at 14 days after injury. These results suggest that this model reproduced the major pathophysiological changes analogous to those observed after severe clinical TBI and provides an attractive vehicle for experimental brain injury research.



Neurol Res. 2001 Mar-Apr;23(2-3):144-56.
Biomechanics of neurotrauma.
Zhang L, Yang KH, King AI.
Source

Bioengineering Center, 818 W. Hancock, Wayne State University, Detroit, MI 48202, USA.
Abstract

This paper reviews the traditional areas of impact biomechanics as they relate to brain injury caused by blunt impact. These areas are injury mechanisms, human response to impact, human tolerance to impact and the use of human surrogates. With the advent of high-speed computers, it is now possible to add computer models to the list of human surrogates that used to be limited to animals and human cadavers. The advantages and shortcomings of current computer models are discussed. One of the computer models was used to predict the pressures and shear stresses developed in the brain and the extent of stretch of the bridging veins in the brains of American football players who sustained severe helmet-to-helmet head impact during the game. It was found that increases in intracranial pressure were more dependent on translational acceleration while the primary determinant for the development of shear stresses in the brain is rotational acceleration. Although the current head injury criterion is based almost entirely on translational acceleration, it is recommended that any new criterion should reflect the contribution of both translational and rotational acceleration.

It's been awhile since I last posted to this thread...and I am still helmetless and pretty! Apparently, I must really be a God! :D

At least one manufacturer has made an effort to address this. (POC?) Using scalp and hair as a lubircant to reduce tortional acceleration is very surface dependent- sometimes a helmet may slide better.

Lots of studies for sure. Most mention DAI -- a few mention rotational injury specifically, but others don't. A bit of digging reveals that mechanisms which cause DAI are still not completely understood, that along with rotational forces as a cause, it can also be caused by sudden linear deceleration.

Far from conclusive, especially when you consider what you left out of this gem:

The part you left out seems to indicate that a helmet helps with angular/rotational forces. And the part you left in -- when read in context with what you left out -- indicates that even without a helmet, angular head accelerations were higher than expected where rotational forces are concerned. Besides: they're studying skateboard helmets...

The issue is still far from clear, with the studies you've posted here doing nothing to clear up anything regarding the bicycle helmet debate... There's nothing bicycle helmet specific in these studies, nothing helmet specific in many of them, and the ones which are citing helmet use, are citing skate and football helmets.

In fact, if we are going to equate skate and bike injuries, far from increasing likelihood of DAI TBI caused by rotational energy, the study you posted indicates that helmets can help in such situations.

You didn't ask for anything on bicycle helmets. You asked for information on why I said rotational forces were instrumental in TBI. I answered that. You asked for how that relates to bicycle crashes. I provided that.

I did redact the abstract because it was confusing and, unlike you, I've read that entire study, not just an abstract of it. What I provided was the information most germane to the question you were asking.

Now you ***** about how "the studies you've posted here doing nothing to clear up anything regarding the bicycle helmet debate." Holy crap. That's not what you asked for and therefore that wasn't my goal.

I have posted study after study after study on this thread. If you want more research, just ****ing search the thread and then go actually read the goddamn research I've already provided. Or, what, you want me to read it to you?

I'm beginning to understand why people wear helmets. They're too goddamn lazy to think for themselves.

Wait, I just remembered something. I have a paper, buried somewhere, that was from Bell's own research people. It showed that their bicycle helmet actually *increased* rotational injury.

I remember laughing like hell when I read it, because in the discussion part of the paper, they tried to explain it away by saying that the DAI theory of TBI was just wrong, because they **know** their helmets reduce injuries. Like, yeah, neuropathologists across the western hemisphere who have been publishing this data for a dozen years are all wrong because a couple of chuckleheads from Bell's R&D said so.

Gimme a couple of days, I'll dig it up.

I've never been hit with a puck or hockey stick while riding my bike.

nah, I've found that more casual cyclists - mostly women, but sometimes men - may be concerned about messing up their hair as ONE reason they choose not to wear a helmet. But it's not silly because at least they're riding, and have made a perfectly rational choice that riding a bicycle is not so dangerous as to require the use of a helmet to be reasonably safe. I'd rather see them riding than not, and I happen to understand that the choice they're making is perfectly reasonable, not silly.

To me the silly ones are those that actually think cycling is so dangerous that one would have to be thoughtless or an idiot to choose not to wear a helmet.

That might be embarassing, but not very likely in the big picture. It's also embarassing and painful to slip on the ice, but people don't wear helmets walking in the winter. Silly.

Don't go to Canada. :eek:

Mikael Colville-Andersen on bike helmets and the culture of fear:

https://www.youtube.com/watch?featur...&v=07o-TASvIxY

Must-see regardless of your position on helmets.

A summary of legislatively-induced helmet wearing and the results worldwide.

Australia
Australian Capital Territory
Casualties: 89 hospital admissions year before law, 87 and 88 in
two years after.
Cycle use: fell 33% weekdays, 50% weekends.
Net result: risk of injury increased relative to cycle use.


New South Wales
Casualties: child cyclists 21% more likely to suffer death or serious
injury post-law.
Cycle use: fell 36% - 44% (but 90% among girl teenagers in
Sydney)
Net result: risk of serious/fatal injury increased.


Northern Territory
Casualties: pre/post law unknown.
Cycle use: fell 22% - 50%
Subsequent changes: The law was modified after 2 years, the
practical effect of which was to reduce helmet wearing to only about
20% and cycle use has increased again. Despite low helmet use, in
2001 Northern Territory had the lowest casualty rate in Australia
and also the highest per-capita cycle use.
Net result: Greatest benefit after fall in helmet use.


Queensland
Casualties: no change in intra cranial injuries, increase in
concussions.
Cycle use: fell 22% - 30%.
Net result: risk of injury may have increased relative to cycle use.


South Australia
Casualties: no change in trends.
Cycle use: fell approx 38%.
Net result: large decrease in cycle use without casualty benefit.


Tasmania
Casualties: possibly slightly fewer head injuries to children under 9
years. No change for children 10 - 14 years. Adults unknown.
Cycle use: no data.
Net result: minimal benefit, and only if cycle use unchanged.


Victoria
Casualties: small initial decline in % head injuries less than for
pedestrians. Over first 4 years, no net decline in %HI.
Cycle use: fell 36% - 46%.
Net result: risk increased relative to cycle use.


Western Australia
Casualties: head injuries fell, but by less than decline in cycling.
Cycle use: fell 30% - 50%.
Subsequent changes: number of cyclists recovered following
extensive promotion and population growth, but cycle use still lower
than pre-law for children and utility journeys. Casualties in 2000
were at an all-time high.
Net result: risk of head injury increased; utility cycle use not
recovered.


Canada
Alberta
Casualties: head injuries apparently doubled in first 6 months.
Cycle use: no data.
Net result: possible increase in risk of head injury.

British Columbia
Casualties: no change in overall % head injuries compared with
non-law provinces. % head injury in motor vehicle crashes
increased.
Cycle use: fell 28% - 30%.
Net result: no obvious benefit for fall in cycle use.

New Brunswick
Casualties: no data.
Cycle use: no data.
Net result: not known.

Nova Scotia
Casualties: no change in head injuries relative to cycle use, but
total injuries doubled.
Cycle use: fell 40% - 60%; greatest fall among teenagers.
Net result: no obvious benefit for fall in cycle use.


Ontario
Casualties: reductions in head injuries but not linked to law, which
is not enforced.
Cycle use: no reliable data. Helmet wearing rate similar in 2001/2
to pre-law, probably because of lack of enforcement.
Net result: no benefit.


Czech Republic
Casualties: no data.
Cycle use: no data. The law is not enforced and appears to be
widely unknown and disregarded.
Net result: no known benefit.


Iceland
Casualties: no data.
Cycle use: no data. Enforcement is understood to be lax and child
helmet use has fallen.
Net result: no known benefit.


New Zealand
Casualties: head injuries fell 19%, less than cycle use and not
more than for population at large.
Cycle use: fell approx 22%.
Net result: risk of head injury relative to cycle use increased. Report
showed cost outweighed benefit by a large factor.


Spain
Casualties: contradictory data.
Cycle use: no data but cycle use low. Helmet use remains unusual
except among sports cyclists. Enforcement very lax.
Net result: no known benefit.


USA
In the USA, little monitoring of the effect of helmet laws on injuries
or bicycle use has been undertaken. Helmet laws are seldom
enforced. One analysis of jurisdictions with helmet laws found no
significant change in fatalities, which were subject to large
year-to-year variation. Cycle use generally fell with laws, but
recovered if the law was not enforced.

Nationally, an increase in helmet use from 18% to 50% of cyclists
was not accompanied by any fall in the proportion of head injuries.
An analysis of 8 million cases of injury and death to cyclists over 15
years found no benefit from helmets, with a small increase in risk of
fatality for helmet wearers.

So can the anti helmet cult tell me in how many accidents that rotational stress comes into play??

Thanks for the info on TBI, and while the abstracts are informative, I'd hesitate to say, that rotational forces are instrumental in TBI. Contributory, certainly, but since DAI can be caused by non-rotational forces, I'd stop short of linking rotational forces to TBI as "instrumental."

You listed a bunch of studies on DAI, one had something regarding football helmets, the one that actually focused on helmets concerned skateboard helmets. I fail to see how you related this to bicycle crashes; you provided no link at all between the studies listed and bicycle helmets.

No, you redacted the abstract because it said something you don't want to hear: "Results suggest that the helmet reduced... angular head acceleration compared to the impact without a helmet." <-- that's pretty clear and unambiguous. And while it might have been the most germane, it's still far from what I queried.

Imagine that, expecting abstracts posted in the bicycle helmet thread to have something to do with bicycle helmets... But what I did ask, you only half answered.

You've provided very little research, but a great deal of abstracts. Obviously I do go actually read the abstracts you link to, but I don't have a free link to the actual research papers and, sorry, not going to shell out money for research papers where the abstracts indicate they will be of little value.

What I have noticed is that you post quotes from abstracts out of context to support claims you make but which are not made in the actual study you quote. Which is why I feel compelled to follow-up on your links and call you out for engaging in the same tactics and misrepresentation of quoted studies which many bare-head brigadiers are quick to call out when pro-helmeteers do it instead.

Trouble is, some of us do think and recognize you as an unreliable source due to your partisan leanings.

The thought is that accidents involving the head are a low percentage and the chance that rotational injuries might contribute to these injuries makes the use of a helmet insignificant?

Wait, wait, wait -- so now that there's something that goes your way, widely discredited helmet-industry sponsored studies are now legitimate...?

No, the thought is that a helmet increases rotational forces in the event of a bike crash where the helmeted head suffers angular forces which translate to rotational forces acting on the brain, leading to DAI as a mechanism of TBI. I.e. supposedly, wearing a helmet is more dangerous because the brain will suffer higher rotational forces in the event of a crash.

Although that one abstract regarding skate helmets that Skye posted seems to contradict that basic bare-head brigade arguing point.

Not legitimate, just hilarious.

Well, hilarious in a nerd kinda way.

When it come right down to common sense, the anti helmet cult is on the wrong side. Look at it this way----------anything that is anything that is between you and hard pavement or dirt and gravel is a good thing.

Here's a pointy rock. Do with it what you will.

I don't know about accidents, there are very, very few accidents. However, when it comes to crashes, any crash that is not a direct, head-on collision will result in a rotational component. That is, most of them.

Acknowledged. Not to mention hypocritical...

Bull****. Show me one time on this thread where I criticised a journal article due to its source rather its methodological shortcomings.

You say I'm a hypocrite? Prove it or STFU.

The risk of cycling is so small that death from any cause, let alone head trauma, amounts to one cyclist death per 33 million km of cycling. It would take the average cyclist 21,000 years to cycle this distance (Cavill & Davis, 2007)