The evaluation of speed skating helmet performance through peak linear and rotational accelerations
- 1Neurotrauma Impact Science Laboratory, University of Ottawa, School of Human Kinetics, Ottawa, Ontario, Canada
- 2Division of Neurosurgery, Children's Hospital of Eastern Ontario (CHEO), Ottawa, Canada
- Correspondence to Clara Karton, Neurotrauma Impact Science Laboratory, University of Ottawa, School of Human Kinetics, A106-200 Lees Avenue, Ottawa, ON K1S 5S9, Canada;
- Received 27 July 2012
- Revised 3 December 2012
- Accepted 11 December 2012
- Published Online First 11 January 2013
Objective Like many sports involving high speeds and body contact, head injuries are a concern for short track speed skating athletes and coaches. While the mandatory use of helmets has managed to nearly eliminate catastrophic head injuries such as skull fractures and cerebral haemorrhages, they may not be as effective at reducing the risk of a concussion. The purpose of this study was to evaluate the performance characteristics of speed skating helmets with respect to managing peak linear and peak rotational acceleration, and to compare their performance against other types of helmets commonly worn within the speed skating sport.
Materials and methods Commercially available speed skating, bicycle and ice hockey helmets were evaluated using a three-impact condition test protocol at an impact velocity of 4 m/s.
Results and discussion Two speed skating helmet models yielded mean peak linear accelerations at a low-estimated probability range for sustaining a concussion for all three impact conditions. Conversely, the resulting mean peak rotational acceleration values were all found close to the high end of a probability range for sustaining a concussion. A similar tendency was observed for the bicycle and ice hockey helmets under the same impact conditions.
Conclusion Speed skating helmets may not be as effective at managing rotational acceleration and therefore may not successfully protect the user against risks associated with concussion injuries.
Background and significance
Categorised by a mass start, short track speed skating (SS) is a fast-paced, competitive sport, where competition races typically involve four to eight skaters on the ice at one time. The oval racing track only measures 111.12 m consequently creating tight corners adding difficulty for skaters to maintain control at high skating speeds in excess of 50 km/h.1 ,2 Due to the high speeds and body contact inherent to the sport, head injuries are a concern for both SS athletes and their coaches.2 In the 2010 Winter Olympic Games held in Vancouver, short track was among the highest ranked sports for reported head injuries in relation to the number of athletes registered (18%) and one of the only four sports with reported cases of concussions.3 It was also identified as the sport with the highest risk for male athletes, where 27.8% of registered male athletes sustained a head injury.3 Overall, head and cervical spine injuries were the most frequent injury locations reported for both male and female athletes in these Winter Games.3 While recent international discussions regarding concussions in sport has dedicated attention to the diagnosis, treatment and return-to-play guidelines,4 a better understanding of injury prevention is important.
As of 1984 the use of helmets in SS to mitigate head and brain injury has been a mandatory practice in Canada.1 Helmet type and specifications, however, vary depending on the ability and competition level within individual skating clubs across the country.1 ,5 ,6 While certified speed skate helmets are most often required at the competitive level, it is not uncommon for novice skaters to be permitted to use a certified hockey or bicycle helmets as long as the openings on the helmet are not large enough for a skate blade to pass through.1 ,5 ,6
Most types of helmets have been successful at what they were originally designed to accomplish which is to protect against catastrophic injuries such as skull fractures and intracranial bleeds. Yet mild traumatic brain injury (mTBI), commonly referred to as concussion, is still frequent in sport7 ,8 and is among the most common injury diagnosed during SS competitions.2 Due to the growing concern regarding this injury, the capacity of helmets to protect from concussions should be assessed.
Current standards, including the one that governs SS helmets, interpret safety based exclusively on a linear acceleration threshold thus resulting in protection against the mechanisms commonly associated with traumatic brain injuries.9–11 This remains despite research indicating that rotational acceleration may be a better indicator of brain injury and should be included when evaluating head protection and risk of concussion.12–14 In addition, head injury tolerance research attempts to link the biomechanical response to the mechanisms of head injury, including mTBI. Proposed kinematic mTBI thresholds which describe the head injury event through both the peak resultant linear and rotational accelerations resulting from impact have been used to predict injury risks.10 ,12 ,15
The purpose of this study was to evaluate the capacity of helmets commonly worn by short-track speed skaters to reduce peak linear and peak rotational acceleration of a hybrid III head form. This may provide awareness concerning the protective abilities of SS helmets against mTBI.
Materials and methods
Monorail-guided drop carriage
A monorail-guided drop carriage is often used for helmet safety standards to simulate a falling mechanism (figure 1).16–18 The system consists of a guiding rail, a carriage assembly and an impact surface. The monorail was used to accurately guide the fall of a head form and the carriage assembly was designed to ensure that the head form remained perpendicular to the floor (figure 2). The impact surface consisted of a hemispherical nylon pad (diameter 12.6 cm) covering a 2.5 cm-thick modular elastomer programmer 60 Shore Type A disc secured to a large metal anvil (figure 1). The velocity of the carriage was measured just prior to impact via an electronic time gate and recorded using National Instruments VI-Logger software.
Hybrid III head and neck form
A 50th percentile male hybrid III head form (mass 4.54 kg), and neck form (mass 1.54 kg) were used for this study (figure 3). These surrogates were engineered to approximate the response to a human head and neck under impact.19 ,20 The head form was equipped with nine single-axis Endevco 7264C-2KTZ-2-300 accelerometers mounted in a 3-2-2-2 orthogonal array, which allowed for the measurement of its three-dimensional motion.21
Three helmet models were composed of expanded polystyrene liners, a material designed to withstand a single impact, and were impacted only once at each location. One helmet model was composed of vinyl nitrile, a material designed to withstand multiple impact, and was impacted three times at each location. Thus, nine SS model I helmets, nine SS model II helmets, nine bicycle helmets and one ice hockey helmet were tested for a total of 28 helmets. The characteristics of the various helmets are described in table 1.
The test protocol was designed to elicit high linear and rotational accelerations and consisted of three impact sites: Front Boss Positive Azimuth (FBPA), Side Centre of Gravity (SCG) and Rear Boss Negative Azimuth (RBNA) (table 2 and figure 4). One centric (SCG) and two non-centric (FBPA, RBNA) impact conditions were chosen, as the direction of the applied force was shown to have an effect on the dynamic impact response22 and risks to injury.23 When the direction of the impact vector is through the centre of gravity of the head form it is defined as a centric impact, whereas an impact vector applied away from the centre of gravity is considered a non-centric impact. All three impact sites were to the side of the head which is a common impact site in sport and presents high risk.24–26 The helmeted head form was impacted three consecutive times at each of the three impact sites at a velocity of 4 m/s. To create the non-centric impact conditions, the anvil extension was moved 6.5 cm horizontally from its centre. The accelerometers were sampled at 20 kHz and filtered using a 1000 Hz low pass Butterworth filter, consistent with the SAE J211 automotive industry standard,27 and for hybrid III head form impact research.28 ,29 A comparison between the class 1000 and the class 180 filter was shown to result in peak acceleration differences of less than 2.2%.30 Data were collected for 60 ms duration. Processing of the data was performed using a DTS TDAS module (TDAS Pro Lab system, Calabasas, California, USA). A series of one-way analysis of variance (ANOVA) were performed for each dependent variable and impact site comparing the four helmets. Therefore, a total of six AVOVAs were performed using SPSS V.18.0 software (SPSS Inc, Chicago Illinois, USA). A significance level of α=0.05 was set, and a post hoc Tukey test was performed when significance was found. This was done to establish significant differences between the helmets.
The four helmet models examined in this study were impacted upon under three conditions and the resulting mean peak linear and rotational accelerations are presented in table 3.
A comparison between the four different helmets revealed that the bicycle helmet and the SS helmet I elicited the lowest peak linear accelerations consistent across all impact conditions.
At the FBPA impact site the bicycle helmet resulted in significantly lower peak linear acceleration than the SS helmet II (F3,8=14.7, p<0.05), and the ice hockey helmet (F3,8=14.7, p<0.05). The SS helmet I produced linear accelerations lower than the hockey helmet (F3,8=14.7, p<0.05). The SS helmet I and the bicycle helmet performed significantly better for linear accelerations at the SCG impact site than the SS helmet II (F3,8=18.5, p<0.05), and the hockey helmet (F3,8=18.5, p<0.05). Statistical analysis from impacts to the RBNA impact site showed that linear acceleration from the SS helmet I was significantly lower than SS helmet II (F3,8=5.3, p<0.05).
Both SS helmet I and SS helmet II peak rotational accelerations were higher than the bicycle and ice hockey helmets for all impacts, where the bicycle helmet maintained the lowest peak rotations for two of three impact sites. At the FBPA impact site the bicycle helmet performed significantly better than the SS helmet I (F3,8=33.2, p<0.05) and SS helmet II (F3,8=33.2, p<0.05) for peak rotational acceleration. At the same site, the hockey helmet also performed better rotationally than both SS helmet I (F3,8=33.2, p<0.05), and SS helmet II (F3,8=33.2, p<0.05). In terms of peak rotational accelerations elicited from the SCG site, the hockey helmet performed better than SS helmet I and SS helmet II (F3,8=40.1, p<0.05). In addition, both SS helmet I and the bicycle helmet performed better than the SS helmet II (F3,8=40.1, p<0.05) at the SCG impact site. Finally, the SS helmet I, the bicycle and the hockey helmet all performed better than SS helmet II (F3,8=13.3, p<0.05) for peak rotational accelerations from the RBNA impacts.
The main findings of this study indicate that there are significant performance differences between the four tested helmets in terms of peak head accelerations resulting from impact. Both dependent variables were chosen based on their association with head and brain injuries.9–14 Peak linear and rotational accelerations resulting from each impact were compared to brain injury tolerance thresholds associated with probability risks for sustaining mTBI established by Zhang et al.12 The study estimated injury based on event reconstructions and determined linear and rotational probability thresholds. It was proposed that a 25%, 50% and 80% probability of sustaining an mTBI for peak resultant linear accelerations, and peak resultant rotational accelerations, were 66, 82 and 106 g, and 4600, 5900 and 7900 rad/s2, respectively.12
The bicycle helmet was the only one which maintained peak linear accelerations below a 25% risk of mTBI for all three impact sites (figure 5). The SS helmet I went above the 25% threshold twice while the SS helmet II and ice hockey helmet went above the 50% risk twice. All four helmets did not manage to perform as well against rotational accelerations and were unable to reduce peak rotational acceleration below the 50% risk (figure 6). Moreover, the SS helmet II consistently yielded rotational acceleration values above the 80% injury risk, and two of three impact to the SS Helmet I were above the 80% threshold (figure 6).
While the bicycle and ice hockey helmets performed better than both SS helmets, all four were less apt at reducing peak rotational accelerations. These results are consistent with the literature where sports helmets successful in managing peak linear acceleration resulting from impact have not always provided corresponding lower risks of injury from rotational acceleration.11 ,37 ,38 As rotational acceleration may play a more important role in contributing to the cause of mTBI,9 ,13 ,14 this suggests that SS helmets may not be optimised for protection against this type of head injury.
SS helmet II consistently yielded higher acceleration of the head form than SS helmet I for all impact conditions. Similar results were found in previous studies where performance among different helmet models varied within the same impact conditions, in spite of meeting equal safety standards and certifications. Furthermore, the individual model responses change depending on the impact site.29 ,38 The observed differences among the individual SS helmet models was not fully understood, though previous research has suggested that variations in shell geometry and liner density could influence helmet performance.38 ,39
Current speed skate helmets are designed to pass a safety standard created to protect the user against catastrophic injury through impact attenuation based solely on linear acceleration. Short-track SS helmets are tested using the ASTM standard F1849-07 which requires a helmet to test under a 300 g peak linear threshold. Research has indicated that protocols that elicit both linear and rotational acceleration are of importance when evaluating the performance characteristics of helmets, and that the monorail drop system used with the Hybrid III head and neck is a viable methodology to establish such protocols.40 ,41
The results from the present study also highlight the importance of measuring rotational acceleration to assess the capacity of helmets to protect against mTBI. Safety standards that recognise peak linear acceleration as having the only influence may be a limited when dealing with the risk of concussion and its prevention.
Although widely used, there is an inherent limitation when using rigid physical models to represent human head response. The 50th percentile adult male hybrid III head and neck form has been designed and validated for head impacts in the anteroposterior direction.19 ,42 The hybrid III head form is made of steel, and is therefore not biofidelic and can only approximate the dynamic properties and impact response of a real human head.20 ,43 The hybrid III neck form is also made of stiff materials and was validated against inertial loading rather than direct impact. This may have an influence on the dynamic response of the head form, dependent on impact condition.29 Finally, the methodology used for this investigation required that the head and neck form not have an attached torso and be guided in a controlled drop impact and this may have an influence on the resulting dynamic response. For these reasons, this study was not meant to replicate human head injury, but rather create a controlled environment in which the comparison of the dynamic response dependent variables could be made. This should be considered when results from the present study are being interpreted.
This research indicated that the tested helmets commonly used in SS were effective at maintaining peak linear accelerations within a lower boundary threshold for sustaining an mTBI at 4 m/s. However, these helmets were less successful at managing rotational acceleration, where all impact scenarios produced values at a higher risk of suffering from an mTBI. This suggests that the helmets worn by short track speed skaters may be more effective at mitigating risks of severe brain injury than those associated with concussion.
What are the new findings
Speed skating helmets display a lower ability to manage peak rotational acceleration than peak linear acceleration under the tested impact conditions.
The two tested speed skating helmet models did not reduce peak linear and rotational accelerations as well as the bicycle and ice hockey helmets.
How might it impact on clinical practice in the near future
This manuscript highlights the effectiveness of helmets in speed skating, which have been designed to protect from a skull fracture and structural brain injury, but not necessarily from a functional head injury such as a concussion. Clinicians need to know this information when managing patients who have normal diagnostic imaging of the brain, yet may have symptoms of concussion or postconcussion syndrome. It is also important to know that although any type of helmet may not prevent a concussion, there are helmets that are more protective than others and thus the potential exists for developing a better helmet to protect young athletes, which will have a direct effect on clinical practice by reducing the number of clinic and hospital visits. In turn, this will not only reduce the economic burden of the disease, but also the more important priceless socioemotional component. Involvement in sports and a healthy lifestyle is always encouraged by physicians, and this can be made safer by helmet manufactures providing the best protective helmets for such high-speed sports such as speed skating.
We would like to thank ThinkFirst, Cheryl Marinsky and the Gloucester Concordes for funding this research.
Contributors In accordance with the ICMJE on authorship and contribution in the conduct and reporting of research, all the authors have substantially contributed to the conception and design, acquisition of data or analysis and interpretation of data. In addition, all authors were participants in drafting the article, or in critically revising it for important intellectual content. The final version of this article was approved for publication by all contributing authors.
Competing interests None.
Provenance and peer review Not commissioned; externally peer reviewed.