Collars that inflate like air bags when they sense a crash occurring are available from a Swedish company, but they don’t meet U.S. safety standards. (Hovding)

Whether helmet use should be mandatory for bicycle riders is a heated topic.

You’re undoubtedly better off wearing a helmet if an accident occurs, and the importance of wearing a helmet increases with the significance of the accident. But while helmets have come a long way since the strappy leather head coverings some cyclists wore before the 1970s, they still don’t do much to protect us from concussions.

Helmet experts are calling for radically new designs to improve safety.

Studies have found that in an accident, you’re much less likely to suffer a severe brain injury if you’re wearing a helmet than if you’re not, but your odds of experiencing minor brain trauma are similar. That’s because modern helmets are lined with hard foam. The design can withstand a serious impact, protecting your skull from fracturing against a hard surface such as pavement, but the rigid foam doesn’t absorb as much energy as a softer liner, such as those found in football helmets.

The best protection for a bicyclist would be a helmet made from a softer material thick enough to absorb any impact, but “nobody wants to bike around with a mushroom on their head,” said Mehmet Kurt, who studies head injury prevention at the Stevens Institute of Technology in Hoboken, N.J. The No. 1 reason people don’t want to wear a helmet relates to self-image — how cool it looks or whether it will cause helmet hair, he added.

Consumer preference has often driven helmet design, and not always in the direction of safety. Randy Swart, director of the Bicycle Helmet Safety Institute, a nonprofit based in Arlington, said the transition from round helmets to elliptical or oval-shaped ones in the 1990s was “certainly not an improvement” safety-wise. But people wanted to look like Lance Armstrong.

A round helmet with a smooth surface is preferable, Swart said, because if a helmet snags during an accident, a rider’s head will be whipped around, possibly causing a concussion.

According to Roy Burek, a visiting professor at the Concussion and Traumatic Brain Injury Prevention Group at Cardiff University in Wales, cyclists can face four basic types of brain injury: skull fractures, interior brain bruising and swelling, brain bleeding, and twisting or distortion of the brain.

Skull fractures and brain bruising result from direct impact and linear energy — the sort you would experience if you fell and hit your head on a curb. Bike helmets protect from these injuries quite well. The hard-to-crack polycarbonate layer on the outside of helmets prevents skull fractures. Microscopic air pockets in the hard foam lining inside helmets burst in a crash, allowing the lining to compress to about 25 percent its volume and absorbing much of the impact that would send the skull smashing into the brain.

When a rider goes flying and skids to a stop, the brain experiences the effects of rotational energy, which can produce internal bleeding and contortion. “The brain is a little bit like an orange in a glass of water,” said Burek. “If you twist the glass quickly, the orange won’t follow immediately behind.”

Now imagine that the orange is attached to the glass by little vessels. Those vessels will tear when the glass moves but the orange doesn’t (or when the orange moves much more slowly). That’s what happens with the brain inside the skull. As the skull whips around, the small blood vessels that stretch from the brain to the skull break, causing bleeding in the brain.

In addition to tearing blood vessels, the brain itself also twists. “Brains are incompressible. No matter how hard you push on them, they won’t change size, like Jell-O,” said Kurt, of the Stevens Institute, “but they change shape easily.”

Back to the orange in a glass of water: Imagine you turn the glass and then suddenly stop. The orange will lag behind, causing tension like that feeling you get in your stomach when you crest a big hill on a roller coaster. This tension is what causes a knockout when a boxer is punched and his neck stops his head from spinning all the way around. This is probably also what causes concussions.

Helmets do a pretty good job of protecting riders from skull fractures and brain bruising. They don’t do much to prevent injuries resulting from rotational energy.

Helmet design could go in many directions. Swart envisions a smooth, round helmet with a softer material that survives more-dramatic impacts but wouldn’t need to be impractically thick.

Burek noted that most bicycle accidents occur at low speeds, so an ideal material for a helmet would be soft when you land at low speeds, to allow the brain to move and thus decrease damage from rotational energy. But that material would also be “smart” — firming up when a high-speed crash occurred, thus preventing a skull fracture.

Kurt thinks the most promising smart ingredient is, actually, air. Earlier this year, Kurt worked with scientists at Stanford University to test inflatable helmets. Something similar — collars that inflate like air bags when they sense a crash occurring — are available from the Swedish company Hovding, but they don’t meet U.S. safety standards. And for good reason, argues Swart: While Kurt’s research showed that these devices can withstand the same impact as helmets, they had to be overinflated to do so; they did not automatically inflate enough to protect against serious impacts.

Kurt worries that federal standards aimed particularly at preventing skull fractures may impede innovation of helmets that might be better at preventing concussions. For example, helmets are required by the Consumer Product Safety Commission to withstand water (think rain) and extreme heat, which he said rules out anything with sensors that might be needed for a softer helmet.

Burek is hoping for technologies that more closely mimic the scalp. If you press your fingertips against your head and move them around, you can feel that your scalp wiggles around relative to your skull. This wiggle room is important: It helps protect our brains from rotational energy in minor impacts by allowing our heads to move a bit in those cases.

Helmets are not designed to move when you crash, because you really don’t want them falling off. “We need to come up with materials that don’t collapse head on, but twist and move in different directions like a second scalp,” said Burek. It’s a bit like landing on a water bed instead of a firm mattress.

Increased awareness about the long-term consequences of brain injuries for professional football players has led to an uptick in traumatic brain injury research and the development of new materials, Kurt said in an email. Bicycle helmets pose a unique challenge, though, because the impact speeds of an accident, especially if a car is involved, can be much greater than that of colliding athletes. But research on football helmets does help us better understand concussions, and given the attention being paid to preventing such injuries, Kurt says he is “fairly optimistic” we could see a better bicycle helmet in the not-too-distant future.

health-science@washpost.com