As an avid off-road motorcyclist and skier I’m no stranger to sports requiring helmets. As a huge advocate of helmet use, I make it a habit. In the past I have generally ascribed to the common wisdom that when purchasing a helmet, the more expensive (sometimes much more expensive), the safer the helmet. I didn’t give the helmet design much thought other than to make sure it had passed one or more of the major compliance certifications.
Buying the safest helmet possible makes sense. You can live a relatively normal life with an injured limb, but if your brain is severely injured, it can be a catastrophic, life altering experience. Recently, my daughter and I started downhill mountain biking together, which typically carries more speed than cross country riding. When shopping for helmets for her, I became disappointed in most of the ones offered, especially for kids. I wanted her to be as safe as possible. With more digging, I learned that for the first time in 30 years a major shift in helmet design was coming and with it, significant improvements. This experience changed the way I looked at helmets.
For more than 30 years helmet design has been static. Helmets for both motorcycle and bicycles have consisted of little more than a hard shell with a foam liner and padding for comfort. Many bicycle helmets are little more than a foam cap. Typically helmets came in one size only, adding or reducing volume with padding. Advances in both technology and brain science are finally making the helmets of the past 30 years obsolete.
Notably, new discoveries in brain science have driven the development of better helmets. The current science indicates that the brain is much more susceptible to injury during lower speed impacts than previously believed. Additionally, oblique or rotational injuries are also much more dangerous to the brain than doctors anticipated even a decade ago. The combination of these forces is particularly dangerous, causing a shearing force inside the skull where the brain continues to rotate in its fluid membrane after the skull has stopped. Only recently have we discovered that the helmets of yesterday did almost nothing to protect the brain from such impacts.
Most current helmets and all helmets of the past decades were designed to simply protect the skull from severe traumatic linear impacts. The helmets were designed to address frontal impacts almost exclusively. The hard shell and foam lining did a fair job of protecting the skull from crushing impacts with rocks, trees and roadways, generally preventing skull fractures. The thought behind the design was that rigidity equaled protection. Unfortunately, this was not always the case and in fact rigidity may have actually put the brain at greater risk of injury.
To really understand how most current helmets are designed and tested, the tests conducted by the U.S. Department of Transportation (DOT) as well as the Snell Memorial Foundation provide a lot of insight. There is endless debate as to which test is better and provides a safer helmet. Generally, a helmet which meets or exceeds Snell requirements also meets DOT requirements. Both outfits test helmets to meet minimum impact criteria before certifying them for use. Both conduct a test where the helmet is positioned on a test headform and then dropped or rammed through two guided falls onto a fixed steel anvil. The test is repeated so that each helmet is impacted on at least four different sites on its surface against either a flat or hemispherical shaped anvil. Snell testing also uses a steel edge to reproduce the effect of hitting a sharp edged surface.
Each test measures:
- Mechanical energy generated during the impact event
- Mechanical failure
- Suddenness of the stop
Snell limits the threshold in which it will certify a helmet to a peak value of 300 G’s. DOT’s standards require peak acceleration not to exceed 400 G’s. DOT also measures the duration of deceleration and requires that a helmet not exceed 200 G’s for more than 2 milliseconds and 150 G’s for more than 4 milliseconds.
The tests conducted by DOT and Snell are designed to test helmets on a linear impact rather than a rotational or oblique impact. However, a helmet which does not meet either of these standards, regardless of design will be inadequate and dangerous to wear. Tests of non-certified helmets show the energy transmitted to the brain in an impact to be greater by sometimes upwards of a factor of ten than those of certified helmets.
DOT and Snell focus largely on motorcycle helmets, however standards for bicycle helmets also exist. Snell has developed a separate test for bicycle helmets and offers a voluntary certification to those helmets that pass. The Consumer Product Safety Commission (CPSC) has adopted ANSI standards which have now been mandatory since 1999 for bicycle helmets. Unfortunately, there are still many helmets on the market, especially for small children, which do not meet CPSC or Snell standards for bicycle helmets.