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." A furious debate has arisen over whether bicycle helmets do any good in protecting the brain from more serious injury and whether bike helmets should remain mandatory in all Australian states, including NSW since 1990. Rosenfeld's opinion is candid. "I don't know if [helmets] do much to protect the inner part of the brain," he says. "That's where people might be saying, 'Well, if you're going to get a serious head injury, it's going to happen whether you're wearing a helmet or not.' Well, there's some truth in that, but it depends on the velocity [with which] you come off the bike, what the force of the impact is, and the way that you land, the way that your head hits the ground … "[A helmet is] certainly offering protection to the outer part of the head, scalp, the skull and to some degree the brain as well. How much [helmet] cushioning there is … also may lessen some of the impact forces on the brain itself; dampening down that effect on the brain from the impact." Rosenfeld is convinced a helmet protects the "outer part" of the head if, say, a cyclist's head hits a rock, pavement or grass strip at low speed, lessening their risk of a ripped scalp, which can lead to infection, while also minimising the chance of a depressed skull fracture, although cyclists who get away with superficial injuries such as concussion are treated in emergency rather than admitted to hospital, so Rosenfeld never sees them.

Sedentary Australia continues to lie around as the helmet debate rages. John Pucher, a professor who studies transport at Rutgers University in New Jersey, tells the Herald that for an individual, wearing a helmet is "clearly safer", but on a population level, helmet laws discourage about 30 per cent of people from cycling, thus "the increased health benefits of those who continue cycling are far offset by the reduced health benefits to those who do not cycle at all". Pucher's team recently compared bike use and walking data from 2000 to 2006 in 14 countries: Australia, Canada, Denmark, Finland, France, Germany, Ireland, the Netherlands, Norway, Spain, Sweden, Switzerland, Britain and the United States (although the Australian and Canadian figures were limited to work trips). The lowest levels of bike riding and walking were in the US, Australia and Canada, according to their study published online last month at the American Journal of Public Health website before its print publication next month. Those three countries also had the worst levels of overweight, obesity and diabetes. The results were similar to a 2008 study that Pucher also co-wrote. There are no adult bicycle helmet laws in the US, although most US states require helmets for children. In Canada, Vancouver has a helmet law, but Montreal and Quebec do not. Europe, by contrast, is laissez-faire about alfresco bicycling heads. Pucher says helmet laws have discouraged people from using city bike sharing programs in Vancouver, Brisbane and Melbourne. Almost no one wears helmets in the Netherlands, Germany, Denmark, Belgium and Sweden, yet those countries have the "safest cycling in the world in terms of fatalities and injuries per kilometre cycled", he argues, though the issues are admittedly "complex and controversial".

As a public protest, Chris Rissel, an associate professor in Sydney University's school of public health, has stopped wearing his cycling helmet. Last month Rissel co-wrote a paper published in the Journal of the Australasian College of Road Safety that looked at the number of cycling-related injuries admitted to NSW hospitals between 1989 - a year before helmets were made mandatory in this state - and in 2008. It found an initial small drop in the number of cycling head injuries, with figures flat ever since. Rissel is pleased that a District Court judge recently reflected that helmet wearing ought to be a matter of choice. Rissel says the original studies championed in Victoria by Frank McDermott, who led the campaign for Australia to have compulsory helmet laws, can be challenged on "methodological weaknesses". "It's a very murky area," Rissel says. "On the basis of so much doubt and confusion around the science, it's questionable whether there should be mandatory legislation for helmets." Rissel says the argument that there may be fewer severe head injuries is often brought up, but a West Australian study shows severity rates "didn't really change". "We've got this massive climate of fear that cycling is such a dangerous activity, and we overstate the case and insist on helmets, when in fact the danger is probably exaggerated," Rissel says.

But McDermott, a former head of the Royal Australasian College of Surgeons' Victorian Road Trauma Committee, says Rissel's paper on the NSW figures is "fairly flimsy", with inconsistent injury data, no observational figures on helmet wearing before and after legislation, and including only cyclists admitted to hospital, which leaves out patients who died before getting to hospital, were treated as outpatients, or were so protected they did not need to go to a hospital. The Rissel paper also relied on the ratio of head to arm injuries, but that ratio can be altered by changes to either category. "It's a limited paper which didn't go to a very notable journal," McDermott says. "I think if it went to any reputable journal they would have sent it back and said, 'Rewrite it.' '' McDermott says Victorian cycling head injury inpatients dropped by 37 per cent in 1991, the year after legislation was introduced, although better road conditions also played a part. In 1992, the number of inpatient head injuries dropped by 70 per cent. The director of Monash University's accident research centre, Rod McClure, says there is room for debate, but "most people would agree that helmets are a good thing". "If Australia was prepared to put into place the sorts of quite radical measures that already exist in places like the Netherlands to reduce the speeds [of traffic] and to give cyclists priority, then not wearing a helmet would be safer than it would be in the current environment," he says.

The forces in an accident don't have to be high to cause serious head injuries. "At 30 or 40 km/h, which a cyclist can quite readily reach, if you hit a brick wall, that's a potentially fatal injury,'' he says. ''So some protection at that point is in the cyclist's interests. "The disadvantage for the cyclist is that's a relatively rare event, and they have to wear the helmet all the time when they don't want to, with a very low probability of a situation where it will come in use. "But it's a higher probability you will need a helmet if you're trying to compete with a motor car, which travels routinely at 60 to 80 km/h, in a space where you're only one foot apart sometimes, where a bump in the road may throw you off." Bill Curnow, the president of the Cyclists Rights Action Group in Canberra, reviewed the scientific literature for a 2008 book, Transport Accident Analysis and Prevention, and concluded science had not been applied to assess the efficacy of helmets to protect the brain, against diffuse injury in particular, arguing the medical profession and helmet industry have been "unduly influential" in the introduction of compulsory helmet wearing.

Scientific circles had "widely discredited" the theory that linear acceleration is the main cause of brain injury, yet helmet makers had made a huge investment in this theory and designed their helmets accordingly, says Curnow, while ignoring the role of angular acceleration in causing brain injury, due to rotation of the head. Curnow quotes a 1994 National Health and Medical Research Council report that suggested the use of helmets, by increasing the size and mass of the head, "may result in an increase in brain injury by a number of mechanisms. Blows that would have been glancing become more solid and thus transmit increased rotational force to the brain". The council's report, however, looked at football injuries rather than cycling injuries. McClure says he accepts that helmets are clearly better for some sorts of force directions than others. "If you got a blow right on the helmet you'll do much better if you get a blow underneath the helmet, for instance, but the rotational force, while relevant, is probably not the important force," he says. "There is no evidence to support the case [that helmets make rotational forces worse], but I'd qualify that by saying no evidence is not necessarily the same as saying there is evidence that is not. It's certainly an area worthier of further exploration." Rosenfeld, the neurosurgeon, dismisses any suggestion helmets potentially make head and brain injuries worse. "I haven't seen any evidence of that," he says. "It's counter-intuitive to me; why would it make things worse? That doesn't make any sense to me." He wants to keep the legal status quo. "We should not bend the rules on helmets. The helmet laws should remain."