Traffic Injury Prevention
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Factors Affecting Motorcycle Helmet Use: Size Selection, Stability, and Position Kim T. Thai, Andrew S. McIntosh & Toh Yen Pang To cite this article: Kim T. Thai, Andrew S. McIntosh & Toh Yen Pang (2015) Factors Affecting Motorcycle Helmet Use: Size Selection, Stability, and Position, Traffic Injury Prevention, 16:3, 276-282, DOI: 10.1080/15389588.2014.934366 To link to this article: http://dx.doi.org/10.1080/15389588.2014.934366
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Traffic Injury Prevention (2015) 16, 276–282 C Taylor & Francis Group, LLC Copyright ISSN: 1538-9588 print / 1538-957X online DOI: 10.1080/15389588.2014.934366
Factors Affecting Motorcycle Helmet Use: Size Selection, Stability, and Position KIM T. THAI1, ANDREW S. MCINTOSH2,3, and TOH YEN PANG4 1
School of Aviation, University of New South Wales, Sydney, Australia Australian Centre for Research Into Injury in Sport and Its Prevention (ACRISP), Federation University Australia, Ballarat, Australia 3 Monash Injury Research Institute, Monash University, Melbourne, Australia 4 School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Australia
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2
Received 23 January 2014, Accepted 10 June 2014
Objectives: One of the main requirements of a protective helmet is to provide and maintain appropriate and adequate coverage to the head. A helmet that is poorly fitted or fastened may become displaced during normal use or even ejected during a crash. Methods: Observations and measurements of head dimensions, helmet position, adjustment, and stability were made on 216 motorcyclists. Helmet details were recorded. Participants completed a questionnaire on helmet usability and their riding history. Helmet stability was assessed quasistatically. Results: Differences between the dimensions of ISO headforms and equivalent sized motorcyclists’ heads were observed, especially head width. Almost all (94%) of the helmets were labeled to be compliant with AS/NZS 1698 (2006). The majority of riders were satisfied with the comfort, fit, and usability aspects of their helmets. The majority of helmets were deemed to have been worn correctly. Using quasistatic pull tests, it was found that helmet type (open-face or full-face) and the wearing correctness were among factors that affected the loads at which helmets became displaced. The forces required to displace the helmet were low, around 25 N. Conclusions: The size of the in-use motorcycle helmets did not correspond well to the predicted size based on head dimensions, although motorcyclists were generally satisfied with comfort and fit. The in vivo stability tests appear to overpredict that helmets will come off in a crash, based on the measured forces, tangential forces measured in the oblique impact tests, and the actual rate of helmet ejection. Keywords: motorcycle helmets, head anthropometry, stability testing
Introduction Motorcycle helmets reduce the frequency and severity of crash-related head injury (Chiu et al. 2000; Kanitpong et al. 2008; Servadei et al. 2003), including the mortality risk from head injury (Auman et al. 2002; Deutermann 2004; Norvell and Cummings 2002; Sass and Zimmerman 2000). A Cochrane review of motorcycle helmet effectiveness by Liu et al. (2008) estimated that helmets provide a 69% reduction in the risk of head injury and a 42% reduction in the risk of death for motorcyclists in crashes. A study of motorcyclists presenting to a major metropolitan trauma center in Sydney Australia over an 18-month period found that helmeted mo-
Managing Editor David Viano oversaw the review of this article. Address correspondence to Dr. Andrew S. McIntosh, Australian Centre for Research Into Injury in Sport and Its Prevention (ACRISP), Federation University Australia, P.O. Box 668, Ballarat, Victoria 3353, Australia. E-mail: as.mcintosh@ bigpond.com
torcyclists demonstrated a reduced likelihood of intracranial injury of 66% compared to nonwearers (McIntosh et al. 2013). A critical element of helmet performance is helmet stability, which ensures that the helmet maintains adequate head coverage and remains in place during normal riding and during a crash. Loose adjustment of helmet retention straps and poor fitment have been attributed to helmet roll-off during crashes (Hurt et al. 1998; Mills and Ward 1985). Richards (1984) reported 4 cases in which a correctly fastened full-face motorcycle helmet became detached during a crash. The mechanism of detachment was reportedly the tendency of the helmet to pivot forward on the chin strap due to the low and forward location of the fixing bolts. It was also found that an apparently well-fitting, comfortable, and properly fastened helmet could be removed on 4 out of 14 wearers by simply pushing from behind. Though it was found that this was more likely to happen on a rider with a flattened occiput, no data on head dimensions of riders were given. Oblique impact tests were conducted on motorcycle helmets with the restraint system tightly adjusted 2-fingers tight and loose (McIntosh and Lai 2013). Although peak head and neck loads in these oblique tests were similar, all
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Factors Affecting Motorcycle Helmet Use helmets with the loosely adjusted restraint were ejected during the single impact. The Motorcycle Accidents In Depth Study (Motorcycle Industry in Europe 2004) and COST 327 Motorcycle Safety Helmets study (Chinn et al. 2001) reported helmet ejection rates between 7 and 14%. Chinn et al. (2001) reported that 91% of lost helmets came off during the impact phase. If McIntosh and Lai’s (2013) results are considered, the motorcyclist may have derived some benefit from the helmet during the first impact but none in any subsequent head impacts. In current motorcycle helmet test standards (e.g., AS/NZS 1698 2006; ECE 22.05 2002; JIS T 8133 2007; Snell M2010), the positional stability of a helmet is assessed by fitting the helmet to a rigid headform and applying a dynamic tensile load to the front and/or rear rim of the helmet. The helmet fails if it rotates beyond the prescribed limit or comes off the headform completely. Most of these standards have adopted International Organisation for Standardisation (ISO) headforms, whose dimensions are based on those in ISO/DIS 6220:1983, Headforms for Use in the Testing of Protective Helmets (European Committee for Standardization 2006). These dimensions were based on those of headforms developed by the UK Transport and Road Research Laboratory in the 1950s (European Committee for Standardization 2006) and it is unknown whether they are representative of Australian motorcyclists. DOT FMVSS 218 (2011) does not have a stability test. The Australian/New Zealand standard for motorcycle helmets (AS/NZS 1698 2006) includes a dynamic stability test that is performed on an A, E, J, M, or O sized ISO headform without a helmet comfort insert (AS/NZS 2512.1). The headform size is judged to be an appropriate fit, with the retention system adjusted to achieve optimum fit after a static preload is applied at the apex. A prescribed amount of slack is introduced in the adjustment via an incompressible spacer with a width of 50 mm and 2-mm-thick cross section. Helmet size, fit, and the retention system may be important considerations for future standards in improving helmet effectiveness. The main objectives of this study were to (1) identify factors that influence the size of helmet worn; (2) identify factors that influence helmet position and adjustment; and (3) examine the effects of helmet size worn and adjustment on helmet stability in a user population. In addressing these objectives, some questions on the appropriateness of helmet test protocols will be considered.
Methods Participants Recreational motorcyclists were recruited at 2 popular recreational riding sites, which were motorcycle club events, and at the Sydney Motorcycle Show (November 21–23, 2008) on 7 separate weekend occasions over a 9-month period in 2008 and 2009. Commuter riders were recruited through advertisement at the University of New South Wales and were surveyed by appointment. The target sample size was 300. The inclusion criteria were that the participant owned and wore a motorcycle helmet. As compensation for their inconvenience, participants were offered a chance to enter a draw to win motorcycling
277 equipment. The study methods were approved by the University of New South Wales Human Research Ethics Committee. Informed consent was obtained from the participants and/or parent/guardian. Helmet test methods have been described in detail in a companion paper (Thai et al. 2014). The following is a summary.
Measurements and Assessments Participants completed a short structured interview in which anthropometric, demographic, and helmet use data were collected. The head’s anterior–posterior and medio-lateral dimensions were measured, as was the head’s circumference (Figure 1). A wearer’s helmet was considered to be the correct size if the wearer’s head circumference fell within the specified size range; for example, a helmet with a specified range of 55–56 cm was considered the correct size for a person with a 56-cm head circumference. Head dimensions were compared to ISO headform dimensions. Dimensions within ±5 mm of headform dimensions were considered similar (Meunier et al. 2000). Head shape was defined in terms of the cephalic index (CIx), which is the ratio of head width to head length expressed as a percentage (Garson 1887). Head shapes were categorized into 3 groups: dolichocephalic (CIx < 75%), mesocephalic (75% ≤ CIx < 80%), and brachycephalic (CIx ≥ 80%). ISO headforms (A, E, J, M, and O) are in the mesocephalic range (75.6–80.2%). The size, age, standard compliance, and condition of each helmet were recorded. Each participant was asked to rate his or her helmet in terms of comfort, fit, and usability on a 5point Likert scale (very poor, poor, average, good, excellent). Helmet position (i.e., angular orientation or attitude) and adjustment were judged from visual and physical assessments. A helmet was considered correctly positioned and adjusted if it sat squarely on the head with the front rim (above the face aperture) no more than 2 finger-widths (≈50 mm) above the brow and with no more than 2 finger-widths’ worth of slack in the strap present under the chin. To assess helmet stability a test analogous to the Static Stability Test in the Australian Standard (AS/NZS 2512.7.1 2006) was undertaken. AS/NZS 1698 (2006) refers to a dynamic stability test (AS/NZS 2512.7.2 2009), not a static test. However, it was considered that conducting a dynamic stability test in the field would be too complex for the survey environments and, possibly, not ethical because of the potential risk to participants. In this context, a static stability test similar to the requirements of the bicycle helmet standard AS/NZS 2063 (2008) was considered appropriate. The interior comfort liner of each helmet was fitted with a hook-and-loop fastener strap before the participant was asked to fasten the helmet as he or she would normally. The end of the strap was attached to a Mecmesin AFG digital force gauge (Mecmesin Ltd., Sinfold, England), and a horizontal force was applied quasistatically through the strap in the forward and rearward directions from the rear and front rims, respectively (Figure 2). The force was applied by hand at as close to a constant rate as possible. The force required to displace the helmet was recorded. Each test
278
Thai et al. (length and width) and shapes of the ISO headforms were assessed. • Helmet-specific: The following associations were assessed: Size correctness by helmet age and the user ratings of comfort and fit; helmet adjustment correctness by helmet condition, retention system design, and usability ratings; and helmet stability by size correctness, adjustment correctness, and retention system design were assessed.
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Fig. 1. Anthropometrical measurement of the head using a sliding caliper: width (left) and anterior–posterior length (right).
was stopped either when the helmet ceased rotating on the wearer’s head or when it became too uncomfortable for the wearer. The position of the helmet was subsequently assessed and was considered unstable if it had been displaced on the head beyond the scalp movement. Statistical Analyses Pearson’s chi-square (χ 2) test for independence was used to compare between-group distributions for helmet size and adjustment by stability (stable and unstable). Comparisons of forces were conducted using Student’s t-tests and one-way analysis of variance. Helmet size and adjustment, in addition to stability in the 2 test directions, were treated as dichotomous dependent variables with the categories as defined in the previous section. Relationships between the independent variables (behavioral, physical, and helmet-specific factors) and the dependent variables were assessed using binary logistic regression analysis (Hosmer and Lemeshow 2000). The following factors and associations were assessed: • Behavioral factors: Associations between helmet size and adjustment by age, gender, main riding type, riding habit (e.g., experience, frequency, etc.), and prior crash involvement were assessed. • Physical factors: Motorcyclists’ head shapes and dimensions were tested as potential influences on size wearing and adjustment. Associations between helmet stability and the differences between the dimensions (length and width) and shapes of the motorcyclists’ heads and the dimensions
Fig. 2. Static stability test orientations: rearward (left) and forward (right).
All statistical analyses were conducted using PASW 18 for Windows software (SPSS Inc., Chicago, IL). The statistical significance level α was set at .05.
Results Demographics A total of 216 motorcycle riders were recruited for the study. Forty-four percent of participants were recruited at the recreational sites, 28.7% at the Sydney Motorcycle Show event, 18.5% at club meetings, and 8.8% through advertising at the University of New South Wales. The sample included 172 males (79.6%) and 44 females (20.4%) between the ages of 8 and 72 years (mean = 43.3 years, median = 45, SD = 11.8). The sample included 4 participants (1.9%) under 18 years who were pillion riders. With regards to ethnicity, 90.2% were European (Caucasian), 3.7% East Asian, 3.2% Middle-Eastern, 1.4% North American, 0.9% South Asian, and 0.5% Indigenous Australians. The majority, 78.7%, rode largely for recreation, 15.7% commuted, and 5.6% rode only as pillions. One hundred and twenty-seven riders (58.8%), including 61.2% of recreational riders, 61.8% of commuters, and 16.7% of pillion passengers, had previously been involved in at least one motorcycle crash or fall. All but one rider stated that they always wore a helmet when riding. Head Anthropometrics The range of head sizes measured was slightly larger than the ISO headforms used in helmet testing (A, E, J, M, and O) and there was a much greater range in head CIx (Table 1). Head measurements for 2 riders could not be completed due to time constraints. Head length ranged from 2.1 cm longer to 2.9 cm shorter than matching headform length (mean dif = 0.38 cm, SD = 0.93 cm, P < .001), with 40% of head lengths within ±5 mm of headform length. Head width ranged from 2.2 cm wider to 2.0 cm narrower than headform width (mean dif = 0.26 cm, SD = 0.79 cm, P < .001). Approximately 43% of heads were similar in width to the matching headforms. These differences resulted in cephalic indices that ranged from 13.8% higher to 9.7% lower than headform CIx (mean dif = −0.36%, SD = 4.46%, P > .05). The head CIx of greater than half the sample fell outside the mesocephalic definition, with 12.1% dolichocephalic and 40.9% brachycephalic. One-way analysis of variance showed significant differences between mean the CIx for the various ethnic groups (P < .01) but not for age, gender, or body mass
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Table 1. Comparison of riders’ head measurements with those of ISO headforms A, E, J, M, and O (n = 214)
Table 3. Helmet size and stability
Measure
ISO range (mean, SD)
Sample range (mean, SD)
Helmet size worn
Rearward
Forward
Both directions
Circumference (cm) Length (cm) Width (cm) Cephalic index (%)
50.0–62.0 (56.6, 4.77) 17.6–21.7 (19.9, 1.68) 13.4–17.4 (15.6, 1.63) 76.1–80.2 (78.3, 0.02)
52.0–65.5 (57.7, 2.10)∗ 17.2–21.8 (19.7, 0.92)∗ 13.3–18.0 (15.5, 0.73)∗ 67.5–92.5 (78.8, 4.39)
Correct size Smaller Larger Unknown All
11 (15.1) 13 (27.7) 23 (26.4) 0 (0) 47 (21.9)
13 (17.8) 13 (27.7) 22 (25.6) 0 (0) 48 (22.4)
8 (11.0) 11 (23.4) 16 (18.6) 0 (0) 35 (16.4)
∗P
< .001.
No. of helmets stable, n (%)
p > 0.05 (X 2 = 6.202, 4.439, 5.143).
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index. Post hoc tests revealed that people of Asian (northeast, southeast, south, and central) ethnicity had a significantly higher CIx (P < .01) than those of European ethnicity, with a mean difference of 4.9% (SD = 1.39%).
Helmet Characteristics Ninety-four percent of the 216 helmets displayed a label indicating compliance with AS/NZS 1698 (2006), 4.6% displayed a label indicating compliance with another helmet standard, and 1% displayed no standard compliance label. Most helmets (67%) were manufactured within the last 5 years, 13% were older than 5 years, and the date of manufacture was illegible or absent in 20% of helmets. On visual inspection, most helmets (89%) were in new or good condition and 11% showed signs of normal use but none were obviously damaged. The majority of riders were satisfied with the comfort, fit, and usability aspects of their helmets. In terms of fit, the mean score out of 5 was 4.1 (SD = 0.72), with 56 and 26% of participants rating their helmets as “good” and “excellent,” respectively. Riders generally chose particular helmets for their comfort and fit (51%), styling or design (34%), price (23%), and brand (14%). It is important to note that 5% of helmets were not selected by the user themselves; that is, they were received as a gift, recommended to them, or came with the bike purchase. In 8 of the 216 helmets the labeled size was missing or illegible. One in 3 (33.8%) riders was wearing the correct helmet size, according to their head circumference. Most helmets worn were intended for larger (40.7%) or smaller (21.8%) head circumferences. None of the behavioral, physical, or helmetspecific factors examined had significant associations with incorrect size wearing (Table A1, see online supplement). It was found that all 4 participants under 18 years were wearing helmets that were recommended for larger head circumferences (1.0 to 2.5 cm out of range).
Table 2. Mean forces (N) measured in rearward and forward directions by stability
Helmet Adjustment Ninety-three percent wore their helmet correctly or as recommended. Incorrect users included 5% who wore their helmet loosely and 2% who wore their helmet over other headwear such as knitted hats or bandanas (head scarves). Prior crash involvement was the only factor found to be significantly associated with correct helmet wearing (Table A1). Helmet Stability Rearward helmet static stability tests were conducted in 215 cases and forward in 214 cases. Two participants did not complete these tests due to time constraints. Thirty-five (16.4%) tested helmets were found to be stable in both the rearward and forward directions; that is, they were not displaced with the application of the maximum tolerable quasistatic force. Mean forces of 25.1 and 26.3 N were measured for rearward and forward directions, respectively (Table 2). No significant difference was found between the forces measured for stable and unstable helmets. Analyses were undertaken to determine the associations between stability, as determined in the physical tests, helmet size, and measured force. No significant associations were observed (Table 3 and Table A2, see online supplement). Helmet adjustment was found to have no significant association with helmet stability, even though 6.7% of incorrectly worn helmets were stable in all test directions compared to 17.1% of correctly worn helmets (Table 4). For unstable helmets, however, the effects of wearing correctness on displacement loads were statistically significant (Table 5). Incorrectly worn helmets were displaced by loads that were an average of 4.7 N lower than correctly worn helmets, in both the rearward and forward directions. No factors were found to be significantly associated with helmet instability, although commuter riders were least likely to wear unstable helmets (Table A3, see online supplement). Along with riders who wore their helmets incorrectly, it was Table 4. Rearward and forward stability by helmet adjustment Helmets stable when pulled, n (%)
Mean force (SD, n) Helmet stability Unstable Stable All
P > .05 (F = 0.458, 1.443).
Rearward
Forward
25.3 (7.08, 168) 24.4 (8.48, 47) 25.1 (7.39, 215)
26.6 (7.46, 166) 25.1 (8.07, 48) 26.3 (7.61, 214)
Helmet worn
Rearward
Forward
Both directions
Correctly Incorrectly All
45 (22.5) 2 (13.3) 47 (21.9)
47 (23.6) 1 (6.7) 48 (22.4)
34 (17.1) 1 (6.7) 35 (16.4)
P > .05 (χ 2 = 0.686, 2.304, 1.107).
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Thai et al.
Table 5. Helmet displacement forces (N) for unstable helmets by wearing correctness Mean force (SD, n) (N) Helmet worn Correctly Incorrectly All ∗P
Rearward
Forward
25.6 (7.02, 155) 20.9 (6.59, 13)∗ 25.3 (7.08, 168)
27.0 (7.46, 152) 22.3 (6.13, 14)∗ 26.6 (7.46, 166)
< .05 (F = 5.409, 5.332).
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found that pillion passengers, females, and riders wearing open-face helmets were wearing helmets that were displaced on their heads at significantly lower forces.
Discussion A sample of 216 riders was surveyed in order to gather data on helmet size, wearing, and stability. Stability testing found that helmets could be displaced on a wearer’s head with a mean quasistatic force of approximately 25 N in the rearward direction and 27 N in the forward direction. Almost all 94% of labeled helmets complied with AS/NZS 1698 (2006), which suggests that helmets may not fit users as well as they fit ISO headforms in certification and/or compliance tests. An alternative reason for this is that the interface between the headform and the helmet is different to the head and helmet interface. In practical terms, helmets rotated on the wearers’ heads at relatively low loads. Those loads are considerably less than those measured in the oblique impact tests referenced in the Introduction (McIntosh and Lai 2013). In those tests (n = 10, drop height 0.5 m, striker speed 35 km/h) the mean peak tangential force in one direction was 2.3 kN. No significant differences were observed between the tangential force components for the loose and 2-finger-tight adjusted helmets, although the tangential forces were slightly greater for the tighter helmets. The differences between the oblique impact tests, in vivo stability tests, and real-world crashes are unknown. In a crash there is also often a radial impact component that may increase the helmet stability; however, such a force is absent in the in vivo stability tests. The in vivo stability tests appear to overpredict that helmets will come off in a crash, based on the measured forces, tangential forces measured in the oblique impact tests, and the actual rate of helmet ejection; therefore, further research is required. It was found that two thirds of riders, for which helmet size was known, were wearing helmets too small or large according to the manufacturer’s sizing recommendation. Although no factors were associated with the correctness of helmet size wearing (Table A1), all 4 child pillion riders were wearing helmets that were intended for larger head circumferences. Consequently, the helmets were found to be unstable in these 4 cases and displaced at mean rearward and forward pull loads of 17.3 and 18.5 N, respectively. Helmet size selection was found not to be associated with helmet stability in our tests (Table 3). The results suggest that motorcycle helmet sizing recommendations may not be suitable for a large proportion of the
population. This finding agrees with findings from published anthropometric surveys (Gilchrist et al. 1988; Hurt et al. 1998). A broader range of head shapes was observed in the rider sample than is represented by the ISO headform range (Table 1). Ethnic differences identified in the results agreed with research that found that East Asian heads are rounder than Western or Caucasian heads (Ball et al. 2010; Gilchrist et al. 1988). Head shape, as defined by CIx, was not found to be associated with size correctness or helmet stability (Tables A1 and A3). Most riders (93%) surveyed wore their helmets correctly; that is, in the proper position and well adjusted. It was found that riders who had previously been involved in crashes were more vigilant regarding helmet adjustment and were 6.5 times more likely to correctly wear their helmets than non-crashexperienced riders. Other groups with a tendency not to wear the helmet correctly included females, pillion passengers (but not children), riders not wearing protective clothing, those wearing helmets with pinch–release buckle-type retention systems, and those with older helmets (Table A1). Adjustment was not associated with helmet stability in our tests; 93.3% of incorrectly worn helmets were unstable compared to 82.9% of correctly worn helmets, and displacement loads were significantly higher for correctly worn helmets (Table A3). Females, pillion passengers, and riders wearing openface helmets were observed to wear helmets that displaced at significantly lower loads. The gender effects were consistent with findings by Thom and Cann (1990), who found that males adjusted the retention system significantly more tightly than did females. The findings highlight 4 main areas for improvement in the current motorcycle helmet standards: • An “appropriate fit” on a headform does not approximate an appropriate fit on all users with the same head circumference. It may be necessary to evaluate helmet fit separately to helmet stability, perhaps by requiring helmets to be positioned on differently shaped headforms. This may result in better fitting helmets for a larger range of users. For this to be a requirement in a standard, a safety case is needed that can identify the relationship between fit and head protection. • The specific protocols employed in the adjustment of helmets in the current stability tests may not reflect motorcyclist practice. This might lead to situations where the helmet meets the stability requirements but displays poorer stability in vivo. A larger amount of slack prescribed in the adjustment for stability testing in the standard would better approximate the adjustment on wearers and give a more conservative measure of stability. This may lead to innovation in helmet design. • The addition of a rearward stability requirement should be considered in the standard. Because roll-off is more likely due to contact rather than inertial forces (Chinn et al. 2001), impact and sliding in the frontal region can cause rearward displacement. The results of this study show that roll-off propensity is similar in the forward and rearward directions. • A more comprehensive assessment of helmet stability could be conducted with the addition of an oblique impact test;
Factors Affecting Motorcycle Helmet Use
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such a test may have other benefits in terms of optimizing the helmet’s ability to manage angular head kinematics. As described, the radial impact force component in an impact may benefit helmet stability in a crash, and an appropriate test method may present a more valid assessment of stability. Helmet ejection prior to a head impact—that is, due to the sudden acceleration of the motorcyclist—has been documented in crash studies. Chinn et al. (2001) reported that only 3 helmets (1.3%) came off before the first impact compared to 23 helmets (9.9%) that came off after the first or second impact. The current dynamic stability test (AS/NZS 2512.7.2 2009) referenced in AS/NZS 1698 (2006), and similar tests, may assess characteristics of the helmet and its retention system that minimize preimpact helmet ejection. The current study had several limitations, in addition to those described already, that should be addressed in future research. It was not possible during the survey to measure the internal dimensions of the helmets worn. We have assumed that helmets compliant with the Australian standard have been manufactured to fit the A, E, J, M, and O sized ISO headforms used in standards compliance tests, but this may not be the case. Certainly the size ranges of various helmet brands and models differ. Future work should allow direct comparisons of helmet stability in vivo and in standards compliance tests on ISO headforms. Head shape was defined using the cephalic index. This might be limited because it is a measure of the ratio of head length and width. Other characteristics of the head, including the height of the head and face, may affect how a helmet sits on the head and its stability. How users were wearing helmets in the field was of concern; therefore, the current study was limited both by time and the availability of portable measuring tools to investigate better these characteristics. Therefore, other measurement methods such as that employed by Gilchrist et al. (1988) or 3-dimensional scanning methods such as those used by Meunier et al. (2000) may be required to improve helmet fit and stability for motorcyclists. The size of the in-use motorcycle helmets did not correspond well to the predicted size based on head dimensions, although motorcyclists were generally satisfied with comfort and fit. Correctly worn and adjusted helmets are more stable, in the context of the quasistatic in vivo test method applied in the study, but the difference in terms of applied forces is small compared to the potential forces applied in a crash. The relationships between in vivo static stability tests, dynamic laboratory stability tests, and helmet ejection in real crashes need further examination.
Acknowledgments The authors thank Guy Stanford and members of the Motorcycle Council of New South Wales, as well as Edgar Schilter, Declan Patton, and Hong Hua, for their assistance in carrying out the surveys. A preliminary analysis of some this data set was presented at the 17th IEA World Congress on Ergonomics in Beijing in August 2009.
281 Funding This work was conducted at the former School of Risk and Safety Sciences, the University of New South Wales (UNSW), under funding by an Australian Research Council (ARC) Linkage Grant LP0669480, “Pedal and Motor Cycle Helmet Performance Study.” The chief investigators were Andrew McIntosh, Paul McCrory, George Rechnitzer, and Caroline Finch. The project partners were the Commonwealth Department of Infrastructure and Transport, NSW Roads and Traffic Authority (now known as Transport for NSW), Transport Accident Commission Victoria, NRMA Motoring and Services, NRMA-ACT Road Safety Trust, and DVExperts International. This article does not represent the views of any of these organizations. The Australian Centre for Research Into Injury in Sport and Its Prevention (ACRISP) is one of the International Research Centres for Prevention of Injury and Protection of Athlete Health supported by the International Olympic Committee (IOC).
Supplemental Materials Supplemental data for this article can be accessed on the publisher’s website.
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282 Kanitpong K, Boontob N, Tanaboriboon Y. Helmet use and effectiveness in reducing the severity of head injuries in Thailand. J Transp Res Board. 2008;2048:66–76. Liu BC, Ivers R, Norton R, et al. Helmets for preventing injury in motorcycle riders. Cochrane Database Syst Rev. 2008;(1):CD004333. McIntosh AS, Curtis K, Rankin T, et al. Associations between helmet use and brain injuries amongst injured pedal- and motor-cyclists: a case series analysis of trauma centre presentations. Australian College of Road Safety Journal. 2013;24(2):11–20. McIntosh AS, Lai A. Motorcycle helmets: head and neck dynamics in helmeted and unhelmeted oblique impacts. Traffic Inj Prev. 2013;14:835–844. Method 7.1: Determination of Stability of protective Helmets—Static Stability. Methods of Testing Protective Helmets. Sydney/Wellington, New Zealand: Standards Australia/Standards New Zealand; 2006. AS/NZS 2512.7.1. Method 7.2: Determination of Stability of Protective Helmets—Dynamic Stability. Methods of Testing Protective Helmets. Sydney/Wellington, New Zealand: Standards Australia/Standards New Zealand; 2009. AS/NZS 2512.7.2. Meunier P, Tack D, Ricci A, Bossi L, Angel H. Helmet accommodation analysis using 3D laser scanning. Appl Ergon. 2000;31:361–369. Mills NJ, Ward RF. The biomechanics of motorcycle helmet retention. Paper presented at: 1985 International IRCOBI/AAAM Conference ¨ on the Biomechanics of Impacts; June 24–26; Goteborg, Sweden. Motorcycle Industry in Europe (MAIDS). MAIDS Final Report 2.0: In-depth Investigations of Accidents Involving Powered Two-Wheelers. Belgium: The European Commission; 2004.
Thai et al. Norvell DC, Cummings P. Association of helmet use with death in motorcycle crashes: a matched-pair cohort study. Am J Epidemiol. 2002;156:483–487. Part 1: Definitions and Headforms. Methods of Testing Protective Helmets. Sydney/Wellington, New Zealand: Standards Australia/Standards New Zealand; 2009. AS/NZS 2512.1. Protective Helmets for Motor Vehicle Users. Tokyo, Japan: Japanese Standards Association; 2007. JIS T 8133. Protective Helmets for Vehicle Users. Sydney/Wellington, New Zealand: Standards Australia/Standards New Zealand; 2006. AS/NZS 1698. Richards PG. Detachment of crash helmets during motorcycle accidents. Br Med J. 1984;288:758. Sass TR, Zimmerman PR. Motorcycle helmet laws and motorcyclist fatalities. J Regul Econ. 2000;18(3):195–215. Servadei F, Begliomini C, Gardini E, et al. Effect of Italy’s motorcycle helmet law on traumatic brain injuries. Inj Prev. 2003;9(3):257–260. Snell M. Standard for Protective Headgear for Use With Motorcycles and Other Motorized Vehicles. North Highlands: Snell Memorial Foundation; 2010. Thai KT, McIntosh AS, Pang TY. Bicycle helmet size, adjustment and stability. Traffic Inj Prev. 2014 (in press). Thom DR, Cann M. Motorcycle helmet retention devices: convenience and comfort. Paper presented at: 34th Human Factors and Ergonomics Society Annual Meeting; 1990; Orlando, FL. Uniform Provisions Concerning the Approval of Protective Helmets and Their Visors for Drivers and Passengers of Motor Cycles and Mopeds. Addendum 21: Regulation No. 22, Revision 4. Geneva, Switzerland: United Nations. 2002; ECE 22.05.
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Factors Affecting Motorcycle Helmet Use: Size Selection, Stability, and Position Kim T. Thai, Andrew S. McIntosh & Toh Yen Pang To cite this article: Kim T. Thai, Andrew S. McIntosh & Toh Yen Pang (2015) Factors Affecting Motorcycle Helmet Use: Size Selection, Stability, and Position, Traffic Injury Prevention, 16:3, 276-282, DOI: 10.1080/15389588.2014.934366 To link to this article: http://dx.doi.org/10.1080/15389588.2014.934366
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Traffic Injury Prevention (2015) 16, 276–282 C Taylor & Francis Group, LLC Copyright ISSN: 1538-9588 print / 1538-957X online DOI: 10.1080/15389588.2014.934366
Factors Affecting Motorcycle Helmet Use: Size Selection, Stability, and Position KIM T. THAI1, ANDREW S. MCINTOSH2,3, and TOH YEN PANG4 1
School of Aviation, University of New South Wales, Sydney, Australia Australian Centre for Research Into Injury in Sport and Its Prevention (ACRISP), Federation University Australia, Ballarat, Australia 3 Monash Injury Research Institute, Monash University, Melbourne, Australia 4 School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Australia
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2
Received 23 January 2014, Accepted 10 June 2014
Objectives: One of the main requirements of a protective helmet is to provide and maintain appropriate and adequate coverage to the head. A helmet that is poorly fitted or fastened may become displaced during normal use or even ejected during a crash. Methods: Observations and measurements of head dimensions, helmet position, adjustment, and stability were made on 216 motorcyclists. Helmet details were recorded. Participants completed a questionnaire on helmet usability and their riding history. Helmet stability was assessed quasistatically. Results: Differences between the dimensions of ISO headforms and equivalent sized motorcyclists’ heads were observed, especially head width. Almost all (94%) of the helmets were labeled to be compliant with AS/NZS 1698 (2006). The majority of riders were satisfied with the comfort, fit, and usability aspects of their helmets. The majority of helmets were deemed to have been worn correctly. Using quasistatic pull tests, it was found that helmet type (open-face or full-face) and the wearing correctness were among factors that affected the loads at which helmets became displaced. The forces required to displace the helmet were low, around 25 N. Conclusions: The size of the in-use motorcycle helmets did not correspond well to the predicted size based on head dimensions, although motorcyclists were generally satisfied with comfort and fit. The in vivo stability tests appear to overpredict that helmets will come off in a crash, based on the measured forces, tangential forces measured in the oblique impact tests, and the actual rate of helmet ejection. Keywords: motorcycle helmets, head anthropometry, stability testing
Introduction Motorcycle helmets reduce the frequency and severity of crash-related head injury (Chiu et al. 2000; Kanitpong et al. 2008; Servadei et al. 2003), including the mortality risk from head injury (Auman et al. 2002; Deutermann 2004; Norvell and Cummings 2002; Sass and Zimmerman 2000). A Cochrane review of motorcycle helmet effectiveness by Liu et al. (2008) estimated that helmets provide a 69% reduction in the risk of head injury and a 42% reduction in the risk of death for motorcyclists in crashes. A study of motorcyclists presenting to a major metropolitan trauma center in Sydney Australia over an 18-month period found that helmeted mo-
Managing Editor David Viano oversaw the review of this article. Address correspondence to Dr. Andrew S. McIntosh, Australian Centre for Research Into Injury in Sport and Its Prevention (ACRISP), Federation University Australia, P.O. Box 668, Ballarat, Victoria 3353, Australia. E-mail: as.mcintosh@ bigpond.com
torcyclists demonstrated a reduced likelihood of intracranial injury of 66% compared to nonwearers (McIntosh et al. 2013). A critical element of helmet performance is helmet stability, which ensures that the helmet maintains adequate head coverage and remains in place during normal riding and during a crash. Loose adjustment of helmet retention straps and poor fitment have been attributed to helmet roll-off during crashes (Hurt et al. 1998; Mills and Ward 1985). Richards (1984) reported 4 cases in which a correctly fastened full-face motorcycle helmet became detached during a crash. The mechanism of detachment was reportedly the tendency of the helmet to pivot forward on the chin strap due to the low and forward location of the fixing bolts. It was also found that an apparently well-fitting, comfortable, and properly fastened helmet could be removed on 4 out of 14 wearers by simply pushing from behind. Though it was found that this was more likely to happen on a rider with a flattened occiput, no data on head dimensions of riders were given. Oblique impact tests were conducted on motorcycle helmets with the restraint system tightly adjusted 2-fingers tight and loose (McIntosh and Lai 2013). Although peak head and neck loads in these oblique tests were similar, all
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Factors Affecting Motorcycle Helmet Use helmets with the loosely adjusted restraint were ejected during the single impact. The Motorcycle Accidents In Depth Study (Motorcycle Industry in Europe 2004) and COST 327 Motorcycle Safety Helmets study (Chinn et al. 2001) reported helmet ejection rates between 7 and 14%. Chinn et al. (2001) reported that 91% of lost helmets came off during the impact phase. If McIntosh and Lai’s (2013) results are considered, the motorcyclist may have derived some benefit from the helmet during the first impact but none in any subsequent head impacts. In current motorcycle helmet test standards (e.g., AS/NZS 1698 2006; ECE 22.05 2002; JIS T 8133 2007; Snell M2010), the positional stability of a helmet is assessed by fitting the helmet to a rigid headform and applying a dynamic tensile load to the front and/or rear rim of the helmet. The helmet fails if it rotates beyond the prescribed limit or comes off the headform completely. Most of these standards have adopted International Organisation for Standardisation (ISO) headforms, whose dimensions are based on those in ISO/DIS 6220:1983, Headforms for Use in the Testing of Protective Helmets (European Committee for Standardization 2006). These dimensions were based on those of headforms developed by the UK Transport and Road Research Laboratory in the 1950s (European Committee for Standardization 2006) and it is unknown whether they are representative of Australian motorcyclists. DOT FMVSS 218 (2011) does not have a stability test. The Australian/New Zealand standard for motorcycle helmets (AS/NZS 1698 2006) includes a dynamic stability test that is performed on an A, E, J, M, or O sized ISO headform without a helmet comfort insert (AS/NZS 2512.1). The headform size is judged to be an appropriate fit, with the retention system adjusted to achieve optimum fit after a static preload is applied at the apex. A prescribed amount of slack is introduced in the adjustment via an incompressible spacer with a width of 50 mm and 2-mm-thick cross section. Helmet size, fit, and the retention system may be important considerations for future standards in improving helmet effectiveness. The main objectives of this study were to (1) identify factors that influence the size of helmet worn; (2) identify factors that influence helmet position and adjustment; and (3) examine the effects of helmet size worn and adjustment on helmet stability in a user population. In addressing these objectives, some questions on the appropriateness of helmet test protocols will be considered.
Methods Participants Recreational motorcyclists were recruited at 2 popular recreational riding sites, which were motorcycle club events, and at the Sydney Motorcycle Show (November 21–23, 2008) on 7 separate weekend occasions over a 9-month period in 2008 and 2009. Commuter riders were recruited through advertisement at the University of New South Wales and were surveyed by appointment. The target sample size was 300. The inclusion criteria were that the participant owned and wore a motorcycle helmet. As compensation for their inconvenience, participants were offered a chance to enter a draw to win motorcycling
277 equipment. The study methods were approved by the University of New South Wales Human Research Ethics Committee. Informed consent was obtained from the participants and/or parent/guardian. Helmet test methods have been described in detail in a companion paper (Thai et al. 2014). The following is a summary.
Measurements and Assessments Participants completed a short structured interview in which anthropometric, demographic, and helmet use data were collected. The head’s anterior–posterior and medio-lateral dimensions were measured, as was the head’s circumference (Figure 1). A wearer’s helmet was considered to be the correct size if the wearer’s head circumference fell within the specified size range; for example, a helmet with a specified range of 55–56 cm was considered the correct size for a person with a 56-cm head circumference. Head dimensions were compared to ISO headform dimensions. Dimensions within ±5 mm of headform dimensions were considered similar (Meunier et al. 2000). Head shape was defined in terms of the cephalic index (CIx), which is the ratio of head width to head length expressed as a percentage (Garson 1887). Head shapes were categorized into 3 groups: dolichocephalic (CIx < 75%), mesocephalic (75% ≤ CIx < 80%), and brachycephalic (CIx ≥ 80%). ISO headforms (A, E, J, M, and O) are in the mesocephalic range (75.6–80.2%). The size, age, standard compliance, and condition of each helmet were recorded. Each participant was asked to rate his or her helmet in terms of comfort, fit, and usability on a 5point Likert scale (very poor, poor, average, good, excellent). Helmet position (i.e., angular orientation or attitude) and adjustment were judged from visual and physical assessments. A helmet was considered correctly positioned and adjusted if it sat squarely on the head with the front rim (above the face aperture) no more than 2 finger-widths (≈50 mm) above the brow and with no more than 2 finger-widths’ worth of slack in the strap present under the chin. To assess helmet stability a test analogous to the Static Stability Test in the Australian Standard (AS/NZS 2512.7.1 2006) was undertaken. AS/NZS 1698 (2006) refers to a dynamic stability test (AS/NZS 2512.7.2 2009), not a static test. However, it was considered that conducting a dynamic stability test in the field would be too complex for the survey environments and, possibly, not ethical because of the potential risk to participants. In this context, a static stability test similar to the requirements of the bicycle helmet standard AS/NZS 2063 (2008) was considered appropriate. The interior comfort liner of each helmet was fitted with a hook-and-loop fastener strap before the participant was asked to fasten the helmet as he or she would normally. The end of the strap was attached to a Mecmesin AFG digital force gauge (Mecmesin Ltd., Sinfold, England), and a horizontal force was applied quasistatically through the strap in the forward and rearward directions from the rear and front rims, respectively (Figure 2). The force was applied by hand at as close to a constant rate as possible. The force required to displace the helmet was recorded. Each test
278
Thai et al. (length and width) and shapes of the ISO headforms were assessed. • Helmet-specific: The following associations were assessed: Size correctness by helmet age and the user ratings of comfort and fit; helmet adjustment correctness by helmet condition, retention system design, and usability ratings; and helmet stability by size correctness, adjustment correctness, and retention system design were assessed.
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Fig. 1. Anthropometrical measurement of the head using a sliding caliper: width (left) and anterior–posterior length (right).
was stopped either when the helmet ceased rotating on the wearer’s head or when it became too uncomfortable for the wearer. The position of the helmet was subsequently assessed and was considered unstable if it had been displaced on the head beyond the scalp movement. Statistical Analyses Pearson’s chi-square (χ 2) test for independence was used to compare between-group distributions for helmet size and adjustment by stability (stable and unstable). Comparisons of forces were conducted using Student’s t-tests and one-way analysis of variance. Helmet size and adjustment, in addition to stability in the 2 test directions, were treated as dichotomous dependent variables with the categories as defined in the previous section. Relationships between the independent variables (behavioral, physical, and helmet-specific factors) and the dependent variables were assessed using binary logistic regression analysis (Hosmer and Lemeshow 2000). The following factors and associations were assessed: • Behavioral factors: Associations between helmet size and adjustment by age, gender, main riding type, riding habit (e.g., experience, frequency, etc.), and prior crash involvement were assessed. • Physical factors: Motorcyclists’ head shapes and dimensions were tested as potential influences on size wearing and adjustment. Associations between helmet stability and the differences between the dimensions (length and width) and shapes of the motorcyclists’ heads and the dimensions
Fig. 2. Static stability test orientations: rearward (left) and forward (right).
All statistical analyses were conducted using PASW 18 for Windows software (SPSS Inc., Chicago, IL). The statistical significance level α was set at .05.
Results Demographics A total of 216 motorcycle riders were recruited for the study. Forty-four percent of participants were recruited at the recreational sites, 28.7% at the Sydney Motorcycle Show event, 18.5% at club meetings, and 8.8% through advertising at the University of New South Wales. The sample included 172 males (79.6%) and 44 females (20.4%) between the ages of 8 and 72 years (mean = 43.3 years, median = 45, SD = 11.8). The sample included 4 participants (1.9%) under 18 years who were pillion riders. With regards to ethnicity, 90.2% were European (Caucasian), 3.7% East Asian, 3.2% Middle-Eastern, 1.4% North American, 0.9% South Asian, and 0.5% Indigenous Australians. The majority, 78.7%, rode largely for recreation, 15.7% commuted, and 5.6% rode only as pillions. One hundred and twenty-seven riders (58.8%), including 61.2% of recreational riders, 61.8% of commuters, and 16.7% of pillion passengers, had previously been involved in at least one motorcycle crash or fall. All but one rider stated that they always wore a helmet when riding. Head Anthropometrics The range of head sizes measured was slightly larger than the ISO headforms used in helmet testing (A, E, J, M, and O) and there was a much greater range in head CIx (Table 1). Head measurements for 2 riders could not be completed due to time constraints. Head length ranged from 2.1 cm longer to 2.9 cm shorter than matching headform length (mean dif = 0.38 cm, SD = 0.93 cm, P < .001), with 40% of head lengths within ±5 mm of headform length. Head width ranged from 2.2 cm wider to 2.0 cm narrower than headform width (mean dif = 0.26 cm, SD = 0.79 cm, P < .001). Approximately 43% of heads were similar in width to the matching headforms. These differences resulted in cephalic indices that ranged from 13.8% higher to 9.7% lower than headform CIx (mean dif = −0.36%, SD = 4.46%, P > .05). The head CIx of greater than half the sample fell outside the mesocephalic definition, with 12.1% dolichocephalic and 40.9% brachycephalic. One-way analysis of variance showed significant differences between mean the CIx for the various ethnic groups (P < .01) but not for age, gender, or body mass
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279
Table 1. Comparison of riders’ head measurements with those of ISO headforms A, E, J, M, and O (n = 214)
Table 3. Helmet size and stability
Measure
ISO range (mean, SD)
Sample range (mean, SD)
Helmet size worn
Rearward
Forward
Both directions
Circumference (cm) Length (cm) Width (cm) Cephalic index (%)
50.0–62.0 (56.6, 4.77) 17.6–21.7 (19.9, 1.68) 13.4–17.4 (15.6, 1.63) 76.1–80.2 (78.3, 0.02)
52.0–65.5 (57.7, 2.10)∗ 17.2–21.8 (19.7, 0.92)∗ 13.3–18.0 (15.5, 0.73)∗ 67.5–92.5 (78.8, 4.39)
Correct size Smaller Larger Unknown All
11 (15.1) 13 (27.7) 23 (26.4) 0 (0) 47 (21.9)
13 (17.8) 13 (27.7) 22 (25.6) 0 (0) 48 (22.4)
8 (11.0) 11 (23.4) 16 (18.6) 0 (0) 35 (16.4)
∗P
< .001.
No. of helmets stable, n (%)
p > 0.05 (X 2 = 6.202, 4.439, 5.143).
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index. Post hoc tests revealed that people of Asian (northeast, southeast, south, and central) ethnicity had a significantly higher CIx (P < .01) than those of European ethnicity, with a mean difference of 4.9% (SD = 1.39%).
Helmet Characteristics Ninety-four percent of the 216 helmets displayed a label indicating compliance with AS/NZS 1698 (2006), 4.6% displayed a label indicating compliance with another helmet standard, and 1% displayed no standard compliance label. Most helmets (67%) were manufactured within the last 5 years, 13% were older than 5 years, and the date of manufacture was illegible or absent in 20% of helmets. On visual inspection, most helmets (89%) were in new or good condition and 11% showed signs of normal use but none were obviously damaged. The majority of riders were satisfied with the comfort, fit, and usability aspects of their helmets. In terms of fit, the mean score out of 5 was 4.1 (SD = 0.72), with 56 and 26% of participants rating their helmets as “good” and “excellent,” respectively. Riders generally chose particular helmets for their comfort and fit (51%), styling or design (34%), price (23%), and brand (14%). It is important to note that 5% of helmets were not selected by the user themselves; that is, they were received as a gift, recommended to them, or came with the bike purchase. In 8 of the 216 helmets the labeled size was missing or illegible. One in 3 (33.8%) riders was wearing the correct helmet size, according to their head circumference. Most helmets worn were intended for larger (40.7%) or smaller (21.8%) head circumferences. None of the behavioral, physical, or helmetspecific factors examined had significant associations with incorrect size wearing (Table A1, see online supplement). It was found that all 4 participants under 18 years were wearing helmets that were recommended for larger head circumferences (1.0 to 2.5 cm out of range).
Table 2. Mean forces (N) measured in rearward and forward directions by stability
Helmet Adjustment Ninety-three percent wore their helmet correctly or as recommended. Incorrect users included 5% who wore their helmet loosely and 2% who wore their helmet over other headwear such as knitted hats or bandanas (head scarves). Prior crash involvement was the only factor found to be significantly associated with correct helmet wearing (Table A1). Helmet Stability Rearward helmet static stability tests were conducted in 215 cases and forward in 214 cases. Two participants did not complete these tests due to time constraints. Thirty-five (16.4%) tested helmets were found to be stable in both the rearward and forward directions; that is, they were not displaced with the application of the maximum tolerable quasistatic force. Mean forces of 25.1 and 26.3 N were measured for rearward and forward directions, respectively (Table 2). No significant difference was found between the forces measured for stable and unstable helmets. Analyses were undertaken to determine the associations between stability, as determined in the physical tests, helmet size, and measured force. No significant associations were observed (Table 3 and Table A2, see online supplement). Helmet adjustment was found to have no significant association with helmet stability, even though 6.7% of incorrectly worn helmets were stable in all test directions compared to 17.1% of correctly worn helmets (Table 4). For unstable helmets, however, the effects of wearing correctness on displacement loads were statistically significant (Table 5). Incorrectly worn helmets were displaced by loads that were an average of 4.7 N lower than correctly worn helmets, in both the rearward and forward directions. No factors were found to be significantly associated with helmet instability, although commuter riders were least likely to wear unstable helmets (Table A3, see online supplement). Along with riders who wore their helmets incorrectly, it was Table 4. Rearward and forward stability by helmet adjustment Helmets stable when pulled, n (%)
Mean force (SD, n) Helmet stability Unstable Stable All
P > .05 (F = 0.458, 1.443).
Rearward
Forward
25.3 (7.08, 168) 24.4 (8.48, 47) 25.1 (7.39, 215)
26.6 (7.46, 166) 25.1 (8.07, 48) 26.3 (7.61, 214)
Helmet worn
Rearward
Forward
Both directions
Correctly Incorrectly All
45 (22.5) 2 (13.3) 47 (21.9)
47 (23.6) 1 (6.7) 48 (22.4)
34 (17.1) 1 (6.7) 35 (16.4)
P > .05 (χ 2 = 0.686, 2.304, 1.107).
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Thai et al.
Table 5. Helmet displacement forces (N) for unstable helmets by wearing correctness Mean force (SD, n) (N) Helmet worn Correctly Incorrectly All ∗P
Rearward
Forward
25.6 (7.02, 155) 20.9 (6.59, 13)∗ 25.3 (7.08, 168)
27.0 (7.46, 152) 22.3 (6.13, 14)∗ 26.6 (7.46, 166)
< .05 (F = 5.409, 5.332).
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found that pillion passengers, females, and riders wearing open-face helmets were wearing helmets that were displaced on their heads at significantly lower forces.
Discussion A sample of 216 riders was surveyed in order to gather data on helmet size, wearing, and stability. Stability testing found that helmets could be displaced on a wearer’s head with a mean quasistatic force of approximately 25 N in the rearward direction and 27 N in the forward direction. Almost all 94% of labeled helmets complied with AS/NZS 1698 (2006), which suggests that helmets may not fit users as well as they fit ISO headforms in certification and/or compliance tests. An alternative reason for this is that the interface between the headform and the helmet is different to the head and helmet interface. In practical terms, helmets rotated on the wearers’ heads at relatively low loads. Those loads are considerably less than those measured in the oblique impact tests referenced in the Introduction (McIntosh and Lai 2013). In those tests (n = 10, drop height 0.5 m, striker speed 35 km/h) the mean peak tangential force in one direction was 2.3 kN. No significant differences were observed between the tangential force components for the loose and 2-finger-tight adjusted helmets, although the tangential forces were slightly greater for the tighter helmets. The differences between the oblique impact tests, in vivo stability tests, and real-world crashes are unknown. In a crash there is also often a radial impact component that may increase the helmet stability; however, such a force is absent in the in vivo stability tests. The in vivo stability tests appear to overpredict that helmets will come off in a crash, based on the measured forces, tangential forces measured in the oblique impact tests, and the actual rate of helmet ejection; therefore, further research is required. It was found that two thirds of riders, for which helmet size was known, were wearing helmets too small or large according to the manufacturer’s sizing recommendation. Although no factors were associated with the correctness of helmet size wearing (Table A1), all 4 child pillion riders were wearing helmets that were intended for larger head circumferences. Consequently, the helmets were found to be unstable in these 4 cases and displaced at mean rearward and forward pull loads of 17.3 and 18.5 N, respectively. Helmet size selection was found not to be associated with helmet stability in our tests (Table 3). The results suggest that motorcycle helmet sizing recommendations may not be suitable for a large proportion of the
population. This finding agrees with findings from published anthropometric surveys (Gilchrist et al. 1988; Hurt et al. 1998). A broader range of head shapes was observed in the rider sample than is represented by the ISO headform range (Table 1). Ethnic differences identified in the results agreed with research that found that East Asian heads are rounder than Western or Caucasian heads (Ball et al. 2010; Gilchrist et al. 1988). Head shape, as defined by CIx, was not found to be associated with size correctness or helmet stability (Tables A1 and A3). Most riders (93%) surveyed wore their helmets correctly; that is, in the proper position and well adjusted. It was found that riders who had previously been involved in crashes were more vigilant regarding helmet adjustment and were 6.5 times more likely to correctly wear their helmets than non-crashexperienced riders. Other groups with a tendency not to wear the helmet correctly included females, pillion passengers (but not children), riders not wearing protective clothing, those wearing helmets with pinch–release buckle-type retention systems, and those with older helmets (Table A1). Adjustment was not associated with helmet stability in our tests; 93.3% of incorrectly worn helmets were unstable compared to 82.9% of correctly worn helmets, and displacement loads were significantly higher for correctly worn helmets (Table A3). Females, pillion passengers, and riders wearing openface helmets were observed to wear helmets that displaced at significantly lower loads. The gender effects were consistent with findings by Thom and Cann (1990), who found that males adjusted the retention system significantly more tightly than did females. The findings highlight 4 main areas for improvement in the current motorcycle helmet standards: • An “appropriate fit” on a headform does not approximate an appropriate fit on all users with the same head circumference. It may be necessary to evaluate helmet fit separately to helmet stability, perhaps by requiring helmets to be positioned on differently shaped headforms. This may result in better fitting helmets for a larger range of users. For this to be a requirement in a standard, a safety case is needed that can identify the relationship between fit and head protection. • The specific protocols employed in the adjustment of helmets in the current stability tests may not reflect motorcyclist practice. This might lead to situations where the helmet meets the stability requirements but displays poorer stability in vivo. A larger amount of slack prescribed in the adjustment for stability testing in the standard would better approximate the adjustment on wearers and give a more conservative measure of stability. This may lead to innovation in helmet design. • The addition of a rearward stability requirement should be considered in the standard. Because roll-off is more likely due to contact rather than inertial forces (Chinn et al. 2001), impact and sliding in the frontal region can cause rearward displacement. The results of this study show that roll-off propensity is similar in the forward and rearward directions. • A more comprehensive assessment of helmet stability could be conducted with the addition of an oblique impact test;
Factors Affecting Motorcycle Helmet Use
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such a test may have other benefits in terms of optimizing the helmet’s ability to manage angular head kinematics. As described, the radial impact force component in an impact may benefit helmet stability in a crash, and an appropriate test method may present a more valid assessment of stability. Helmet ejection prior to a head impact—that is, due to the sudden acceleration of the motorcyclist—has been documented in crash studies. Chinn et al. (2001) reported that only 3 helmets (1.3%) came off before the first impact compared to 23 helmets (9.9%) that came off after the first or second impact. The current dynamic stability test (AS/NZS 2512.7.2 2009) referenced in AS/NZS 1698 (2006), and similar tests, may assess characteristics of the helmet and its retention system that minimize preimpact helmet ejection. The current study had several limitations, in addition to those described already, that should be addressed in future research. It was not possible during the survey to measure the internal dimensions of the helmets worn. We have assumed that helmets compliant with the Australian standard have been manufactured to fit the A, E, J, M, and O sized ISO headforms used in standards compliance tests, but this may not be the case. Certainly the size ranges of various helmet brands and models differ. Future work should allow direct comparisons of helmet stability in vivo and in standards compliance tests on ISO headforms. Head shape was defined using the cephalic index. This might be limited because it is a measure of the ratio of head length and width. Other characteristics of the head, including the height of the head and face, may affect how a helmet sits on the head and its stability. How users were wearing helmets in the field was of concern; therefore, the current study was limited both by time and the availability of portable measuring tools to investigate better these characteristics. Therefore, other measurement methods such as that employed by Gilchrist et al. (1988) or 3-dimensional scanning methods such as those used by Meunier et al. (2000) may be required to improve helmet fit and stability for motorcyclists. The size of the in-use motorcycle helmets did not correspond well to the predicted size based on head dimensions, although motorcyclists were generally satisfied with comfort and fit. Correctly worn and adjusted helmets are more stable, in the context of the quasistatic in vivo test method applied in the study, but the difference in terms of applied forces is small compared to the potential forces applied in a crash. The relationships between in vivo static stability tests, dynamic laboratory stability tests, and helmet ejection in real crashes need further examination.
Acknowledgments The authors thank Guy Stanford and members of the Motorcycle Council of New South Wales, as well as Edgar Schilter, Declan Patton, and Hong Hua, for their assistance in carrying out the surveys. A preliminary analysis of some this data set was presented at the 17th IEA World Congress on Ergonomics in Beijing in August 2009.
281 Funding This work was conducted at the former School of Risk and Safety Sciences, the University of New South Wales (UNSW), under funding by an Australian Research Council (ARC) Linkage Grant LP0669480, “Pedal and Motor Cycle Helmet Performance Study.” The chief investigators were Andrew McIntosh, Paul McCrory, George Rechnitzer, and Caroline Finch. The project partners were the Commonwealth Department of Infrastructure and Transport, NSW Roads and Traffic Authority (now known as Transport for NSW), Transport Accident Commission Victoria, NRMA Motoring and Services, NRMA-ACT Road Safety Trust, and DVExperts International. This article does not represent the views of any of these organizations. The Australian Centre for Research Into Injury in Sport and Its Prevention (ACRISP) is one of the International Research Centres for Prevention of Injury and Protection of Athlete Health supported by the International Olympic Committee (IOC).
Supplemental Materials Supplemental data for this article can be accessed on the publisher’s website.
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