Documentation:FIB book/Injury Biomechanics of the Head with Relation to Roller Coasters

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Introduction

Roller coasters can reach high accelerations posing potential hazards to users

Roller Coasters are often the flagship of amusement parks, and the main attraction for many customers.[1] With roller coaster rides often reaching an averaged sustained acceleration of 4g’s, serious safety considerations must go into the design process.[2] The biomechanics involved in roller coasters can be analyzed and measured through use of various injury criteria to assess the potential for injury. This assessment in roller coaster injury biomechanics has long been researched through literature and therefore also has its share in controversies.   


General Head Injury

Vehicle design often goes through rigorous testing to understand the biomechanics of the human passengers during extreme acceleration and breaking scenarios. Similarly, to ensure that all ride-goers exit with is an adrenaline rush, a comparable testing process is also conducted to thoroughly assess roller coasters. Often in roller coasters the chest and body is well restrained through the use of lap bars and over the shoulder restraints, but this still leaves the head free to move.[3] This is why further examination into head injury and the biomechanics of the head are required.

El Toro Test Seat Restraints
Ga-Deja Vu Seating

Similar to car crash biomechanics, head injury can be predicted through analysis techniques such as Head Impact Power (HIP) and Head Injury Criterion (HIC). These calculations both calculate empirical values which correlate a head injury metrics based on the input conditions.[4][5] The HIP value correlates how severe a head injury could be based on the mass, principal moments of head inertia, and acceleration from all 3 directions.[4]

Similarly, the HIC value measures the likelihood of a head injury occurring based on acceleration data plotted over time.[5]

The rider’s neck and head are often free to translate and rotate in response to the accelerations and motions experienced during a roller coaster ride due to the restraint mechanisms.[3] It is with this motion that the potential for injury can occur, either stemming from muscle strain or an internal brain injury. If excessive brain motion does occur it could lead to subdural hemorrhaging, also described as bleeding between the skull and brain.[6][3][7] The values of angular velocity, angular acceleration, and linear accelerations sustained during a roller coaster ride were measured for assessment. Comparing the sustained head values to other activities and injury limits it is clear that the magnitudes sustained during roller coasters are fairly minimal, despite what it feels like during the ride.[6][3]

While the limits for forces and injury criteria are not defined for roller coasters, existing limits can be taken from other applications and used as criteria for injury biomechanics research. From automotive testing and the AIS, the threshold for sustaining an AIS 1 injury is 300 [8], which would be an unacceptable result for a roller coaster. Additionally, tolerances of accelerations are known for humans, and these tolerances depend on the duration of the acceleration. For sustained accelerations in the seconds, 6g is the limit which will not cause injury or loss of consciousness.[8] However, for a short acceleration in which HIC is calculated, accelerations of 50g can be tolerated before injury.[8]

Even with the low potential for head injury when riding roller coasters, it does still have impacts on the overall head health and quality of life of riders. In some cases, symptoms such as nausea and or headaches can occur post roller coaster ride. The reason for these symptoms not being uniform across all riders, is due to the fact that forces experienced during rides varied between participants.[6] This further illustrates how complex the biomechanics of the head and brain truly are. If symptoms post roller coaster ride persist, it is likely that the rider had a preexisting condition which was further affected by the sustained motion and acceleration. From the results of the testing, this showed to worsen these underlying health conditions.[7]

Injury Biomechanics involving Roller Coasters

The measurement of the accelerations in the head are taken using equipment instruments with linear accelerometers and angular rate sensors to determine movement in all six degrees of freedom.[6] The Arndt study used an instrumented wearable helmet with only linear accelerometers [9], while more contemporary studies use instrumented mouthpieces, to measure the linear and angular accelerations closer to the center of gravity of the head.[6][10][3]

Mouth guards similar to those used in sports can be fitted with sensors to provide data about the head during motion

In all the studies, volunteer participants were instrumented and measured while riding various currently operating roller coasters. As these are all rides that are currently running, all the data collected was in the sub-injurious range. High-thrill roller coasters tested in these studies were advertised as having g forces of 4g sustained over the duration of the ride.[10] One key difference in the Burman study is that the measurements were taken post-injury, in an attempt to recreate and quantify the accelerations which caused injury.

The Arndt, Pfister and Kuo studies are summarized in the Table 1 below. The peak values from any of the participants on any of the roller coasters measured in each study are reported. HIC15 and HIP are calculated from these peak values.

Table 1: Biomechanical Measurements on Roller Coasters [6][9][10]
Study Linear Acceleration [g] Angular Acceleration [rad/s2] HIC15 HIP
Arndt 20 - 9.4 -
Pfister 10.2 1547 4.1 0.36
Pfister 6.9 1847 1.6 0.36
Kuo 8.0 290 2.7 -

The Arndt study measured the acceleration of the head over 71 different roller coasters. The highest instantaneous acceleration measured on any roller coaster in this study was around 20 g, with a resulting HIC15 value of 9.4. Interestingly, the high HIC15 value was calculated on a large, isolated acceleration peak, similar to those measured in head impacts.[9] This acceleration spike is not explained in the study; however, it only appears once on 1 out of the 71 roller coasters, which may be caused by an unusual roller coaster feature such as a sharp turn or a launch from a launch coaster.

The Pfister and Kuo studies measured the linear and rotational velocity and acceleration of participants over three roller coasters. The highest linear acceleration experienced by any of the participants was 10.2 g, with a resulting HIC15 of 4.1 on that ride.[6] Angular acceleration was also measured in this study, and the highest acceleration being 1847 rad/s2. Interestingly, the ride with the highest linear acceleration and the ride with the highest angular acceleration both resulted in a HIP of 0.36.

The Pfister and Kuo studies then compared their findings from roller coasters to the accelerations experienced by sports and normal day activities.[6][10] The results are summarized in Table 2 below.

Table 2 - Biomechanical Measurements Comparison to Roller Coasters [9][10]
Activity Linear Acceleration [g] Angular Acceleration [rad/s2] HIC15 HIP
Roller Coaster (Pfister) 10.2 1547 4.1 0.36
Pillow Fight 10.5 2054 1.3 0.21
Car Crash (5 MPH) 5.2 555 0.8 0.2
Car Crash (18 MPH) 29.3 1261 28.1 3.41
Soccer Header 10 1100 4.74 -
Football Hit 22 930 34.05 -

Overall, these studies show that riding roller coasters have a low chance of causing head injury, with the most comparable HIC15 value from the more modern roller coaster studies being that of taking a header in soccer. These studies also show that there is little variability in head accelerations between different passengers and between roller coasters.

Additional work has been done through simulation of the accelerations on a roller coaster, where a multi-body dynamic tool was developed to measure roller coasters in development.[8] This model is based off of the SOMLA model, which has the anthropometric data of the 50th percentile male. Two roller coasters were then simulated, the Looping Star, which is an extreme roller coaster, where riders are still in the sub-injurious range, and Gate Keeper, which is close to a standard roller coaster operating at any amusement park.

Table 3 - Simulated Biomechanical Measurements on Roller Coasters [6][10][8]
Ride Linear Acceleration [g] Angular Acceleration [rad/s2] HIC15
Looping Star 21.27 - 11.34
3 7.4 - 4.9
Kuo Study 8.0 290 2.7
Arndt Study 20 - 9.4

This shows that the results and measurements for the simulation are all reasonable, as the simulated data matches up with real data gathered in injury biomechanics studies. Concerns about biofidelity arise from this study, as the model would have to be biofidelic in all motions due to the numerous motions and forces that riders experience. While this study shows that the forces and injury criteria on a rider can be fairly accurately simulated, traditional injury biomechanics studies are still needed to quantify the accelerations on the head. Looking at all the studies and simulations, it can be seen that none of these measured accelerations or HIC values are near the injury criteria limits. The highest HIC was from the simulation, which was higher than anything measured on an operating ride, but still had a safety factor of 26 under an injurious level. Similarly, all the peak accelerations were only measured on instantaneous peaks, which on average were 5 times lower than the accelerations needed to cause injury.

Looking at all the studies and simulations, it can be seen that none of these measured accelerations or HIC values are near the injury criteria limits. The highest HIC was from the simulation, which was higher than anything measured on an operating ride, but still had a safety factor of 26 under an injurious level. Similarly, all the peak accelerations were only measured on instantaneous peaks, which on average were 5 times lower than the accelerations needed to cause injury.

Strength of Literature in the Field

The quality of literature mentioning head injury in roller coasters is generally quite weak. A majority of the research done regarding roller coasters is post-injury medical studies, with only a few injury biomechanics related studies which measure the forces and accelerations in the head. While post-injury medical studies can be useful in understanding the severity and location of injuries that occurred, they do not tell us much about the biomechanics of how an injury happened or lead to any standards that can help engineers design safer roller coasters. One such study mentions that 55% of people who experienced head injury after riding roller coasters already had a previous history of headache, which makes it difficult to draw any conclusions about roller coasters being the cause of any of these injuries.[11] As injury biomechanics studies have all shown that the forces and accelerations sustained are not significant to cause head injuries alone.[10][2]

However, from an injury biomechanics standpoint, a strength of the literature is that there seems to be an improvement in the methods of measuring the forces. While the Arndt study only had patients wear an instrumented helmet [9], the Pfister and Kuo studies improved on the methods of Arndt by using instrumented mouthpieces.[6][10] These improvements allow the acceleration measurements to be taken closer to the center of gravity of the head, better estimating the real forces on the head. Additionally, the inclusion of angular acceleration sensors allow for the use of more injury criteria that may better account for the correlation between rotational acceleration and brain injury.


A flaw of all these studies is that they all are taking biomechanical measurements on rides that are already standing and operational, meaning that they have been proven safe to a safety institution prior to these studies taking place. These studies all confirm what was already known on these roller coasters, that they do not cause head injury. The testing that is done on rides that are not yet operational is done by agencies who do not publish the results to the public. Research studies do not have the ability to test on these prototype roller coasters, and instead must rely on simulation to try to calculate the possibility of head injury.

Controversies

While reviewing literature pertaining to injury biomechanics and roller coasters, identifying potential controversies is crucial. The Pfister study examining kinematics on roller coasters and their risks on the human body during rides at Six Flags Great Adventure has provided valuable results. However, these results may be ethically controversial due to funding by Six Flags, introducing a potential conflict of interest that should be further examined.

Medusa (Six Flags Great Adventure)

Since Six Flags could be interested in the outcome of the research, there is a possibility that their funding could influence the study’s methodology, analysis, or conclusion in a way that would benefit the company. The research asserts that the rides are safe because “the accelerations and jerk experienced on these rides are well below the injury thresholds established for TBI”. [6]

The Pfister study cites key foundational studies in head injury biomechanics, including studies by Holbourn [11] and Gennarelli [12]. Nonetheless, the potential for bias arises when considering the selectivity in their literature review. Pfister’s assertion that “previous research has not conclusively linked roller coaster rides with TBI” [6] may be in part due the small literature base and lack of public database available on roller coaster forces and accelerations, as well as the absence of a national organization in charge of overseeing roller coasters.


This concern for bias is further increased by a later study by Scranton [13], which reports a case of subdural hematoma (SDH) following a motion simulator ride, contrasting with Pfister's findings. Importantly, Scranton references earlier studies on head injury biomechanics which predate the Pfister study. Scranton cites studies such as Wepfer’s 17th-century work on intracranial hemorrhage [14], Hulke’s 1883 case report on subdural hematoma [15], and German et al.'s 1964 remarks on subdural hematoma and aphasia [16], which are not mentioned in the Pfister study​​​​. Moreover, Scranton notes that Pfister “concluded that there appears to be a low risk of traumatic brain injury (TBI) due to the head motions induced by roller coaster rides, but they did not specifically consider SDH.” [13]

This discrepancy in citations and focus between the Pfister and Scranton studies suggests a potential underemphasis or oversight in Pfister's review of literature, possibly influenced by Six Flags' funding. While Pfister acknowledges some foundational studies, their review might lack a comprehensive approach that considers contrasting views and nuances in different types of TBI, such as subdural hematoma. It is important to note that there is likely no intentional bias by Six Flags or the researchers conducting the study, but rather the Pfister study, and the studies that are based off of it, are not standalone studies about the biomechanics of roller coasters, but have a link to the largest amusement park companies, which may cause bias. However, although the results may be beneficial to Six Flags in reinforcing the safety of their roller coasters to the public, the same outcomes likely would have occurred had this been a publicly funded study.

Furthermore, noteworthy flaws may be found in Pfister’s analysis. The study states that “riders experience the highest stresses during the initial drops and during turns”.[6] Although these stresses are within safety limits, the researchers do not discuss any long-term impacts of these stresses or how repeated exposure may alter the results. This lack of comprehensive risk analysis may suggest a minimization of findings that should invite further investigation into ride safety. When examining the reported data, the study reports peak acceleration and jerk values, without discussing the entire range of data.


While injury biomechanics research for roller coasters is fundamental in preventing injuries, careful consideration must be made and corporations like Six Flags should be examined closely as funders for this research. The potential conflict of interest demands independent parties to rigorously test and validate the results to ensure scientific and ethical robustness.

Further Research and Work Needed in the Field

Roller coaster engineering is a highly regulated industry and generally as much engineering design as possible is applied to any new creation for maximal safety. Often much of the research found and cited included metrics such as linear acceleration and angular acceleration. This was researched for its effect on the heads of riders in the roller coasters. Additionally, Burman suggests that there may be a correlation between both the weight and height of a rider and the range of motion that the rider’s head would experience during the ride [3], opening the door for research opportunities to explore the relationship between rider anthropometrics and head accelerations. US Government agencies have long spoken about the need for quantifying g-force exposures on these rides.[17] Further research could result in the implementation of a maximum allowable HIC and HIP required for all rides to be operational. This would result in a more biomechanical data oriented approach to safety regulation in these amusement park attractions.

Little research has been done surrounding the risks of cervicocephalic arterial dissections in young people who have ridden roller coasters. This is an important cause of stroke in young people and is often not recognized due to non-specific symptoms of headache and neck pain.[18] It was found this can have a delay of symptom onset for up to 11 days making diagnosis very difficult. The same researchers warn that the exposure to sudden hyperextension, hyperflexion, and rotation of the neck such as that in roller coasters could make individuals with underlying connective tissue disorders more susceptible to arterial dissection.[18]

A further area needing additional research is around neck injuries sustained from riding roller coasters. Studies conducted have shown that 1 in 1600 rides yielded spinal injury.[19] In fifty six percent of these cases a surgery was the required form of treatment, speaking directly to the severity of these injuries.[19] The safety of roller coasters for spinal injuries has evolved from the time of the study. Nevertheless, there has not been clear quantifications of biomechanical data to suggest the safety protocols are optimized fully. This is a clear area requiring further study from biomechanics specialists.

The difficulty behind quantifying biomechanical limits to roller coaster rides is that the injury threshold is undefined due to the diversity of individuals riding roller coasters. There is simply no way of knowing the health backgrounds that may affect how people react to the forces experienced on roller coasters. This clearly makes regulation difficult. The only possibility of overcoming this fact is developing a foundation of biomechanical injury criterion and data.

References

  1. ↑ Backus, Fred (August 14, 2021). "Rollercoaster is America's favorite amusement park ride - CBS News poll".
  2. ↑ 2.0 2.1 Pendrill, Ann-Marie; Eager, David (2 September 2020). "Velocity, acceleration, jerk, snap and vibration: forces in our bodies during a roller coaster ride". Physics Education. 55. doi:10.1088/1361-6552/aba732 – via IOPscience.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Burman, Chirag (Fall 1-31-2013). "Quantification of human motion on roller coaster rides". New Jersey Institute of Technology – via New Jersey Institute of Technology Digital Commons. Check date values in: |date= (help)
  4. ↑ 4.0 4.1 Zhan, Xianghao; Li, Yiheng; Liu, Yuzhe; Domel, August G.; Alizadeh, Hossein Vahid; Raymond, Samuel J.; Ruan, Jesse; Barbat, Saeed; Tiernan, Stephen (2 June 2021). "The relationship between brain injury criteria and brain strain across different types of head impacts can be different". Journal of the Royal Society. 18. doi:10.1098/rsif.2021.0260 – via Journal of the Royal Society Interface.
  5. ↑ 5.0 5.1 Nellippallil, A.B.; Berthelson, P.R.; Peterson, L. (Raj K.). "Chapter 10 - Robust concept exploration of driver's side vehicular impacts for human-centric crashworthiness". Multiscale Biomechanical Modeling of the Brain. Prabhu. pp. 153–176. ISBN 978-0-12-818144-7. Check date values in: |year= (help)
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 6.12 Pfister, Bryan J. PhD; Chickola, Larry MS; Smith, Douglas H. MD (December 2009). "Head Motions While Riding Roller Coasters: Implications for Brain Injury". American Journal of Forensic Medicine & Pathology. 30: 339–345. doi:10.1097/PAF.0B013E318187E0C9. eISSN 1533-404X. ISSN 0195-7910 – via Ovid.
  7. ↑ 7.0 7.1 Tseng, Peggy MD; Kearl, Yvette Liza MD; Ansari, Ashkon MD (April 2019). "Roller Coaster–Induced Subdural Hematoma in a Previously Healthy Teenager". Pediatric Emergency Care. 35: 76–78. doi:10.1097/PEC.0000000000001802 – via Lippincott Journals.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 Viegas, Mário, "Development of Computational Models of Roller Coaster Vehicles and Occupants for Injury Analysis" (2016). Theses. https://fenix.tecnico.ulisboa.pt/downloadFile/1689244997255858/dissertacao.pdf
  9. ↑ 9.0 9.1 9.2 9.3 9.4 Arndt, Steven R. Ph.D; Cargill, Robert S. Ph.D; Hammoud, P.E. Selim Ph.D (September 1, 2004). "Head Accelerations Experienced during Everyday Activities and While Riding Roller Coasters". Proceedings of the Human Factors and Ergonomics Society Annual Meeting. 48: 1973–1977. doi:10.1177/15419312040480 – via Sage Journals.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 10.5 10.6 10.7 Kuo, Calvin; Wu, Lyndia C.; Ye, Patrick P.; Laksari, Kaveh; Camarillo, David B.; Kuhl, Ellen (15 November 2017). "Pilot Findings of Brain Displacements and Deformations during Roller Coaster Rides". Journal of Neurotrauma. 34: 3198–3205. doi:10.1089/neu.2016.4893 – via Mary Ann Liebert, Inc. Publishers.
  11. ↑ 11.0 11.1 Holbourn, A.H.S. (01 June 1945). "THE MECHANICS OF BRAIN INJURIES". British Medical Bulletin. 3: 147–149. doi:10.1093/oxfordjournals.bmb.a071895 – via Oxford Academic. Check date values in: |date= (help)
  12. ↑ Gennarelli, T.A. (November 1993). "Mechanisms of brain injury". Journal of Emergency Medicine. 1: 5–11. PMID 8445204 – via PubMed.
  13. ↑ 13.0 13.1 Scranton, Robert A.; Evans, Randolph W.; Baskin, David S. (19 November 2015). "A Motion Simulator Ride Associated With Headache and Subdural Hematoma: First Case Report". Headache: The Journal of Head and Face Pain. 56: 372–378. doi:10.1111/head.12717 – via Headache Journal.
  14. ↑ Hoessly, Gian-Fortunat (February 1966). "Intracranial Hemorrhage in the Seventeenth Century". Journal of Neurosurgery. 24: 493–496. doi:10.3171/jns.1966.24.2.0493 – via Journal of Neurosurgery.
  15. ↑ Hulke, J.W. (10 November 1883). "Middlesex Hospital: Severe blow on the right temple, followed by right hemiplegia and coma, and then by spastic rigidity of the left arm; trephining; evacuation of inflammatory fluid by incision through dura mater; quick disappearance of cerebral symptoms; complete recovery". A MIRROR OF HOSPITAL PRACTICE, BRITISH AND FOREIGN. 122: 814–815. doi:10.1016/S0140-6736(02)23988-4 – via The Lancet.
  16. ↑ German, William J.; Flanigan, Stevenson; Davey, L.M. (01 January 1966). "Remarks on Subdural Hematoma and Aphasia". Neurosurgery. 12: 344–350. doi:10.1093/neurosurgery/12.CN_suppl_1.344 – via Lippincott. Check date values in: |date= (help)
  17. ↑ Smith, Douglas H.; Meaney, David F. (8 July 2004). "Roller Coasters, G Forces, and Brain Trauma: On the Wrong Track?". Journal of Neurotrauma. 19: 1117–1120. doi:10.1089/08977150260337921 – via Mary Ann Liebert, Inc. Publishers.
  18. ↑ 18.0 18.1 Lascelles, K; Hewes, D; Ganesan, V. "An unexpected consequence of a roller coaster ride". Neurology, Neurosurgery, & Psychiatry. 71. doi:10.1136/jnnp.71.5.704 – via BMJ Journals.
  19. ↑ 19.0 19.1 Freeman, Michael D.; Croft, Arthur C.; Nicodemus, Clarence N.; Centeno, Christopher J.; Elkins, Whitney L. (November 2005). "Significant Spinal Injury Resulting From Low-Level Accelerations: A Case Series of Roller Coaster Injuries". Archives of Physical Medicine and Rehabilitation. 86: 2126–2130. doi:10.1016/j.apmr.2005.05.017 – via Science Direct.


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