Bicycle helmets save lives. They protect the head and brain during bicycle crashes where the cyclist’s head strikes the ground, a vehicle, or some other roadside object. But bicycle helmets are not perfect: there are times when even a properly worn helmet does not prevent a head injury.
How do helmets work?
The main part of the helmet that protects the head during an impact is the energy absorbing foam. This Styrofoam-like material crushes and/or cracks during an impact (Figure 1), and this crushing and cracking alters the head’s exposure in three important ways: it increases the duration of the impact, it decreases the peak force applied to the head, and it spreads this lower force over a larger area of the head. The first two effects reduce the head’s acceleration and thus reduce the potential for brain injury, and all three effects combine to reduce the potential for skull fracture.
So why might a helmet not protect against a head injury?
Most bicycle helmets are certified to standards that stipulate a minimum impact performance. This certification means that these helmets reduce the peak acceleration of a testing headform to below a specified level (typically 300 g) during a specific test impact (typically a drop test at an impact speed of 22 kilometers per hour). While all certified helmets should meet this minimum level of impact performance, not all certified helmets actually achieve this performance. Also, the standards do not address helmet performance for more severe impacts. Differences in helmet shape, coverage and foam design can affect the head protection capabilities of a helmet. These factors may render a helmet ineffective for a particular bicycle crash. Other factors, like helmet age, have little or no effect on head protection. Each of these factors is discussed in more detail below.
Helmet shape and coverage
Bicycle helmet shape and coverage vary. BMX-style helmets tend to be rounder, cover more of the head and have fewer vents than traditional helmets. Most bicycle helmets, other than certain mountain bike and BMX helmets, lack facial protection. The difference in coverage means that some locations, particularly on the front, back and side of the head, may not be covered by a particular helmet model, potentially leaving portions of the head exposed.
These coverage differences are allowed by current standards, which only require that helmets undergo impact tests on or above a prescribed test line (Figure 3). The test line sits relatively high on the forehead and well above the tops of the ears. If the helmet extends below the test line, this part of the helmet is not required to attenuate impacts and thus its design may be influenced more by other factors; e.g., style. For a helmet to be effective, it must first cover the impact site.
The foam used to construct bicycle helmets varies across manufacturers and models. Premium helmets are generally lighter, highly ventilated and more comfortable than basic helmets, but they are not necessarily better at protecting the head. For example, at impact speeds below 22 km/h, the premium helmet shown at the right of Figure 2 performed much the same as the basic helmet in the center of Figure 2. At impact severities above 22 km/h, however, the premium helmet provided less impact protection than the basic helmet (DeMarco et al., 2016).
Premium helmets are generally more optimized for weight and comfort while still meeting the standard, whereas basic helmets generally have more foam than they need to meet the standard. Thus, more expensive helmets do not necessarily provide better impact protection and may be less effective in more severe impacts.
Recent advances in our understanding of brain injury, and in particular concussion, have suggested that reducing head rotation during an impact reduces the risk of injury. Most current helmet standards do not address head rotation during impact and the ability of current helmets to attenuate head rotation has not been well studied.
Helmets could still leave the brain susceptible to rotationally induced injuries. Some newer helmet
models, on the other hand, include a thin inner liner designed to allow the helmet to rotate about the
head during an impact and thereby reduce the rotation experienced by the head during an impact. These devices show promise in the laboratory under some impact conditions, but their effectiveness at reducing real world head injuries in bicycle crashes is still being studied.
Most helmet manufacturers recommend periodically replacing an otherwise undamaged helmet, although this recommendation varies widely from two to 10 years. We recently tested over 700 bicycle helmets ranging in age from new to 26 years old and found that helmet age had little or no effect on foam properties and helmet impact performance (Kroeker et al., 2016: DeMarco, et al., 2017). Thus, helmet age alone does not appear to have a detrimental effect on a helmet’s protective capabilities, although other age-related degradation of the shell, straps or buckles may affect helmet performance. Moreover, impact damage can compromise a helmet’s impact performance regardless of age, thus all bicycle helmets should be replaced after an impact.
The benefits of bicycle helmet use are clear: helmets can attenuate impacts and reduce the forces
transmitted to the skull and brain during an impact. Helmets, however, do not guarantee protection in all impact conditions and some helmets provide better injury protection than others. A comparison of the location and severity of a head impact to the coverage and impact properties of a helmet can be performed to biomechanically evaluate helmet effectiveness in a particular crash. While bicycle helmets do save lives, they are not perfect.
The principles discussed in this article are not unique to bicycle helmets. They also apply to motorcycle helmets and other sport-related helmets, which have similar issues with helmet coverage and use energy absorbing foam liners.
*DeMarco AL and Elkin BS (2018). The Bicycle Helmet: A lifesaver with limitations. The Lawyer’s Daily, LexisNexis Canada.
DeMarco AL, Chimich DD, Gardiner JC, Nightingale RW, & Siegmund GP (2010). The impact response of motorcycle helmets at different impact severities. Accident Analysis & Prevention, 42(6), 1778-1784. DOI: 10.1016/j.aap.2010.04.019
DeMarco AL, Chimich DD, Gardiner JC, & Siegmund GP (2016). The impact response of traditional and BMX-style bicycle helmets at different impact severities. Accident Analysis & Prevention, 92, 175-183. DOI: 10.1016/j.aap.2016.03.027
DeMarco AL, Good CA, Chimich DD, Bakal JA, Siegmund GP (2017) Age has a Minimal Effect on the Impact Performance of Field-Used Bicycle Helmets. Annals of Biomedical Engineering, 45, 1974-1984. DOI: 10.1007/s10439-017-1842-4
Kroeker SG, Bonin SJ, DeMarco AL, Good CA, & Siegmund GP (2016). Age Does Not Affect the Material Properties of Expanded Polystyrene Liners in Field-Used Bicycle Helmets. Journal of Biomechanical Engineering, 138(4), 041005. DOI: 10.1115/1.4032804
U.S. Consumer Product Safety Commission, 1998. 16CFR Part 1203 Safety Standard for Bicycle Helmets; Final Rule, Vol. 63. U.S. Federal Register, Bethesda, MD (No. 46).