Impact Energy Attenuation Performance of Football Headgear
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Br J Sports Med 2000;34:337–341 337 Impact energy attenuation performance of football headgear Andrew S McIntosh, Paul McCrory Abstract headgear are widely available and often used in Objectives—Commercially available foot- football, little is known about their actual per- ball head protectors were tested to deter- formance during impact.3 The degree of mine their impact energy attenuation per- protection oVered by headgear is contentious, formance and ability to reduce the with some people advocating that their func- likelihood of concussion. tion should be to prevent abrasions and Methods—Prospective study using stand- lacerations34 and others advocating that their ardised impact test methods with both role should be to reduce concussion. rigid (magnesium) and Hybrid III head- Headgear wearing rates or controlled studies forms. of headgear eVectiveness have not been well Results—Eight commercially available documented. Gerrard et al5 reported that 20% head protectors from six manufacturers of rugby union players used headgear in 1992. were tested. The magnitude of the head- Our experience in studying headgear use in form accelerations increased as the drop under 15 rugby union in 1999 indicates that height was increased, ranging from a the voluntary wearing rate in this age group is minimum of 64 g from a height of 0.2 m to about 60%. Headgear wearing rates probably a maximum of 1132 g from a height of 0.6 diVer between codes, age groups, and skill lev- m. The head injury criterion and maxi- els. The potential for headgear to reduce rates mum headform acceleration values fol- of concussion relies not only on players wearing lowed a similar trend. A steep increase it, but also on the ability of the headgear to was noted in the magnitude of maximum attenuate impact energy.2 headform acceleration and head injury In this study, a selection of commercially criterion when the drop height was in- available football headgear was tested to deter- creased from 0.4 to 0.5 m. This indicates mine impact energy attenuation performance that the foam material was completely with regard to the ability to reduce likelihood of compressed at an impact energy above concussion and more severe head injury. about 20 J and therefore oVers little protection against impacts of greater Methods severity. Repeated tests using a drop Eight soft helmets from six manufacturers were height of 0.3 m showed that some helmets tested using a standardised methodology. exhibit a “memory” eVect, whereby im- Owing to the lack of availability of two models pact performance is reduced by up to 50% at the time of testing, a limited number of tests with repeated impacts. were performed on these two models, whereas Conclusions—Laboratory tests indicate all tests were performed on the remaining six that current commercially available foot- medium sizes. Tests were conducted on five ball headgear performance will not reduce small sizes selected from the best performing the likelihood of concussion. The absence models in the medium size tests. Tests were School of Safety of internationally recognised standards conducted using two separate headforms, a Science, University of for soft headgear designed to ameliorate rigid magnesium alloy headform and a Hybrid New South Wales, concussion is a major deficiency in sports III headform. Sydney, Australia 2052 injury prevention. A S McIntosh (Br J Sports Med 2000;34:337–341) RIGID HEADFORM TEST University of A standard test methodology was used as per Keywords: football; headgear; helmet; head; concussion Melbourne, the 1999 Australian standard AS/NZS Department of 2512.1.1.6 This is similar to many international Neurology, Austin and Head injury is a common problem to all codes 1 helmet testing standards such as the SNELL Repatriation Medical of football throughout the world. One of the and ANSI standards. The impact energy Centre, Heidelberg, head injury prevention issues that is often attenuation test is similar to the 1998 standard Victoria, Australia raised is the role of helmets or headgear in this 3084 and Olympic Park for headgear of the International Rugby Board setting. In order for a helmet to function eVec- 7 Sports Medicine (IRB), except that the latter does not specify Centre, Swan Street, tively, it must attenuate impact energy, thereby Melbourne, Australia decreasing the magnitude of the impact force Table 1 Velocities and impact energies of test matrix protocol 3004 to the head. This in turn leads to a decrease in P McCrory head acceleration and less potentially injurious Drop Impact Impact energy Impact energy 50th height velocity rigid headform size percentile Hybrid III stress on the brain and cerebrovascular tissues. (m) (m/s) C(J) headform (J) Correspondence to: The means by which helmets attenuate energy Dr A McIntosh 0.2 2.0 9.8 — email: is through liner deformation, so that the 0.3 2.4 14.7 13.8 [email protected] magnitude of the impact force is inversely pro- 0.4 2.8 19.6 — portional to the magnitude of the liner 0.5 3.1 24.5 — Accepted for publication 0.6 3.4 29.4 27.7 10 March 2000 deformation.2 Although “soft” helmets or www.bjsportmed.com 338 McIntosh, McCrory Table 2 Summary of rigid headform test results 1200 Maximum headform Head injury 1000 Drop No of acceleration (g) criterion height models Size Impact site Condition (m) tested Min Max Min Max 800 Medium Fronto-lateral Ambient 0.2 6 64 483 73 1192 600 0.3 8 106 569 183 2106 0.4 6 141 832 279 4132 0.5 6 482 1073 1451 6000 400 0.6 8 524 1132 2225 7253 Hot 0.3 7 109 689 184 2850 0.6 7 547 1032 2191 7089 200 Cold 0.3 7 108 672 183 2523 0.6 7 443 1085 1458 6620 0 Centre-front Ambient 0.4 6 819 1026 4584 6118 Maximum headform acceleration (g) 0.2 0.3 0.4 0.5 0.6 Small Fronto-lateral Ambient 0.2 5 68 105 84 148 0.3 5 97 298 186 713 Drop height (m) Figure 1 Maximum headform acceleration v drop height for eight models of rigid headform. Complete results are the drop mass, the filter characteristics for given for six models, and results for the 0.3 and 0.6 m tests headform acceleration, or sampling rate. These only for the other two. are critical parameters in impact testing.8 In addition, the drop height is limited to 300 mm. 7000 All testing was performed at the New South 6000 Wales Roads and TraYc Authority’s “Crash- lab” in Sydney, which is certified by the 5000 National Association for Testing Authorities and routinely conducts test to AS/NZS 4000 2512.1.1. HIC 3000 A size C magnesium alloy headform was used to test the medium size headgear. The 2000 total drop mass was 5 kg. A size B magnesium alloy headform was used to test small size 1000 headgear. The total drop mass was 4.1 kg. A 0 single impact was delivered to one site as 0.2 0.3 0.4 0.5 discussed below. Headform acceleration was Drop height (m) sampled at 10 kHz with an Endevco 7265 Figure 2 Head injury criterion (HIC) v drop height for accelerometer. Acceleration data were filtered six models of rigid headform. The results for two models using a Channel Class 1000 filter, and impact were omitted to keep the scale in proportion with an HIC of velocity was measured using a light gate. Tests 1000 as the standard pass/fail criterion. were conducted on headgear conditioned to the rigid and deformable headforms were cold (−10°C), ambient (20°C), and hot (50°C) performed using pieces of polymeric foam cut environmental conditions according to AS/ from the same samples. NZS 2512.2 (1998).9 As well as the single impact testing above, SELECTION OF ANATOMICAL TEST IMPACT SITES further testing to determine impact perform- On the basis of previously published studies in ance under repeated frontal/lateral impacts was which video analysis of head impacts in rugby performed, with impacts at three minute inter- league, rugby union, and Australian football vals to a total of five times. For single impacts, were performed, the temporal-parietal area was only one impact per test site was delivered. identified as the most frequently struck area and thus selected for testing purposes.10 11 In addition, because published experimental ani- HYBRID III HEADFORM TEST mal research has suggested that fronto-lateral A second series of tests were conducted on the impacts may also result in brain shearing same selection of headgear to determine the injury, this impact site was also tested.12 13 impact energy attenuation performance using a Hybrid III headform. This headform is used in SELECTION OF IMPACT ENERGY AND VELOCITY motor vehicle crashworthiness testing and has a Tests were conducted over a range of impact deformable skin that more closely resembles a speeds and energies. These were selected on human head than the rigid headform. All tests the basis of initial performance of the headgear were conducted at the University of New South and the standard test protocols. It was found Wales, School of Safety Science, Biomechanics that performance was poor from drop heights and Gait Laboratory using a standard drop test of greater than 0.6 m, so further increments in rig. The Hybrid III 50th percentile headform drop height were not performed. Table 1 gives and instrumentation had a total drop mass of the impact velocities and energies for the vari- 4.7 kg. Three orthogonal Endevco 7264 acceler- ous drop heights. ometers were used to measure headform accel- eration at the headform’s centre of mass.