ORIGINAL COMMUNICATION

Human Surrogates for InjuryBiomechanics Research

J.R. CRANDALL,1* D. BOSE,1 J. FORMAN,2 C.D. UNTAROIU,1 C. ARREGUI-DALMASES,1,2

C.G. SHAW,1 AND J.R. KERRIGAN1

1Center for Applied Biomechanics, University of Virginia, Charlottesville, Virginia2European Center for Injury Prevention, University of Navarra, Pamplona, Spain

This article reviews the attributes of the human surrogates most commonlyused in injury biomechanics research. In particular, the merits of human cadav-ers, human volunteers, animals, dummies, and computational models areassessed relative to their ability to characterize the living human response andinjury in an impact environment. Although data obtained from these surrogateshave enabled biomechanical engineers and designers to develop effectiveinjury countermeasures for occupants and pedestrians involved in crashes, themagnitude of the traffic safety problem necessitates expanded efforts inresearch and development. This article makes the case that while there are lim-itations and challenges associated with any particular surrogate, each providesa critical and necessary component in the continued quest to reduce crash-related injuries and fatalities. Clin. Anat. 24:362–371, 2011. VVC 2011 Wiley-Liss, Inc.

Key words: cadavers; dummies; human volunteers; models; crash; impact

INTRODUCTION

Injury biomechanics applies the principles ofmechanics to the analysis of human response andtrauma. Specific objectives for the field of injury bio-mechanics include quantifying the response of tis-sues, identifying the mechanisms of injury, anddefining injury tolerances at which the tissues fail torecover (Viano et al., 1989). Increased knowledge ofspecific injury mechanisms and associated injury tol-erances permits engineers to develop effective injurycountermeasures. Five surrogates, each with a dis-tinct set of inherent advantages and disadvantages,are commonly employed in these research anddevelopment efforts: cadavers, volunteers, anthro-pometric test devices (ATDs), animals, and computa-tional models. The term biofidelity pertains to theability of a given surrogate to approximate thebehavior of a living human under comparable loadingconditions. Evaluations of biofidelity are performedusing measured engineering variables that includekinetic parameters, such as forces and moments, aswell as kinematic parameters, such as displacementsand accelerations. Given the high speeds associatedwith modern modes of transportation, many of theinvestigations of human tolerance have used surro-gates to understand impact loading resulting from

automobile and aviation crashes. This article reviewsthe relative merits of different surrogates, each initself a necessary but insufficient component in theoverall scientific endeavor to enhance comprehen-sion of injury mechanisms and tolerances.

Human Cadavers

Although the field of anatomy has used cadaverssince at least the 4th century BC (Zhivadinoviket al., 2008), the use of cadavers for investigationsof injury tolerance first emerged in the latter half ofthe 19th century (Weber, 1859; Messerer, 1880).Since that time, cadavers, or post-mortem humansurrogates (PMHS), have been instrumental in the

Abbreviations used: ATD, anthropometric test devices; FE, finiteelement; PMHS, post-mortem human surrogates.

*Correspondence to: Jeff R. Crandall, Center for AppliedBiomechanics, University of Virginia, Charlottesville, Virginia.E-mail: jrc2h@virginia.edu

Received 18 October 2010; Revised 22 January 2011; Accepted24 January 2011

Published online 18 March 2011 in Wiley Online Library(wileyonlinelibrary.com). DOI 10.1002/ca.21152

VVC 2011 Wiley-Liss, Inc.

Clinical Anatomy 24:362–371 (2011)

development of injury countermeasures in the areasof occupational safety, athletics, transportation, andmilitary applications. Within the fields of automobileand aviation safety, they have aided in the develop-ment of improved occupant compartment padding,belt and air bag restrain systems, and windshielddesigns (King et al., 1995). In addition, they havebeen used to improve the external structures ofautomobiles for vulnerable road users includingpedestrians and cyclists (Crandall et al., 2002).

In terms of mimicking the living human, an obviousadvantage of cadavers relative to animal and dummysurrogates is the exact representation of anatomicalstructures. The ability to produce comparableresponse and injury, however, depends not only onthe geometry of the surrogate but also on the tissueproperties and, for certain injuries, on the physiologicresponse. Since the 1960s, the use of embalmedcadavers for injury testing has been largely aban-doned due to their excessive tissue rigidity, limitedranges of joint motion, and modified ultimatestrength of embalmed tissues (Crandall, 1994). Freshcadavers provide a more representative surrogate fortolerance and response investigations but offer a lim-ited window of time between rigor mortis and autoly-sis in which to perform the experiments. Althoughbone properties can retain a mechanically stabilizedstate for months (Yamada and Evans, 1970, Crandall,1994), post-mortem changes in soft tissue progressrapidly and may compromise the mechanical charac-teristics. In an effort to slow the decomposition pro-cess, fresh cadavers can be frozen at the time ofacquisition and then thawed immediately before test-ing. The use of long-term frozen storage of cadaversfacilitates increased complexity of experiments sinceit permits greater time to setup the experiments andpermits the testing in a single test series of severalanthropometrically matched cadavers that may takemonths to gather. Although freezing retains the ma-terial properties of hard tissue and, to a lesser extent,connective tissues, undesirable damage within manysoft tissues (e.g., internal organs) can arise when icecrystals formed during the freezing and thawing pro-cess disrupt the soft tissue cellular structure (Menz,1971; Lee and Pelker, 1985). The effects of this dam-age on tissue response and injury prediction are de-pendent on the freezing rates and temperatures aswell as the structure of the individual organs.

Independent of the storage technique, cadaverslack the basic physiologic functions associated withrespiratory activity and cardiac response. Obviously,realistic replication of injuries such as traumatic pneu-mothorax and hemothorax in cadaver must includean approximation of the in vivo conditions of the livinghuman. In an effort to replace fluids in solid organsfollowing post-mortem exsanguination and to recre-ate realistic circulatory pressures and flows, research-ers pressurize the cardiovascular systems of cadaverswith blood simulants before testing (Shaw et al.,2000; Petitjean et al., 2002; Lebarbe et al., 2005). Inaddition to alterations in bodily fluids, the volume anddistribution of gases within the body change post-mortem due to decomposition in the intestinal trackand, more importantly, in the lungs leading to lung

congestion, pleural fluid, as well as the collapse of thealveolar walls (McIlroy, 1952). Similar to the circula-tory pressurization process, researchers inflate thecadaver lungs in an effort to approximate at least thepre-death pressure and volumes (Shaw et al., 2000;Petitjean et al., 2002; Lebarbe et al., 2005).

Following the dissipation of rigor mortis, cadaversare essentially atonal and unable to simulate theresting muscle tone and contraction of livinghumans. This results in considerable challenges forpositioning of cadavers in the laboratory environ-ment and often necessitates the use masking tapeand other temporary supports to maintain a pre-impact position. Beyond the lack of cadaver muscleactivation for maintaining posture, field observationsindicate more than 50% of the occupants in frontalcrashes engage in avoidance maneuvers involvingmuscle bracing behavior (Ore et al., 1992; Petitet al., 1996). Reflexive tensing of the muscles altersthe load paths from the occupant to the deceleratingvehicle and restraints, thereby significantly influenc-ing the overall pre-crash kinematics (Ejima et al.,2008) and the dynamic loading of the occupant dur-ing the collision (Begeman et al., 1980; Manninget al., 1997). In general, the muscles in the extrem-ities provide propriotonic restraint that can distributethe restraining forces required of the vehicle’s seatbelt and air bag systems (Armstrong et al., 1970;Patrick and Trosien, 1971). In addition to changes inthe overall dynamics and positioning, muscle con-tractive forces may potentially influence the risk ofinjury. Although it is difficult to isolate the effects ofthe lack of muscular contractions relative to otherpost-mortem changes on the extent of trauma incadavers relative to humans, Foret-Bruno et al.,(1978) compared the injury patterns in field crasheswith laboratory experiments with cadavers and foundthe rib fracture tolerance of the living to be consis-tently higher than the dead for equivalent crashseverities. Although there have been efforts to mod-ify cadavers to approximate muscle tone either byadding external hardware such as springs (Begemanet al., 1980; Deng and Goldsmith, 1987; Klopp et al.,1995) or tailoring embalming techniques (Crandallet al., 1994), limitations of both approaches havediscouraged their subsequent development.

Another drawback of cadaver usage involves theavailability of specimens that meet selection criteriaappropriate for biomechanical studies. The pool ofapplicable cadavers is significantly diminished whenPMHS are screened for pre-existing pathologies(e.g., acute injuries, osteoporosis, metastatic can-cer), age, gender, and anthropometric requirements.Given the scarcity of cadaveric material, researcherssometimes resort to performing multiple tests on aPMHS by increasing the severity of loads and imple-ment checks of failure using diagnostic techniquessuch as radiographic images and palpation followingeach test. Although the diagnostic techniques canascertain gross pathologies, they cannot assess cu-mulative damage at the microstructural level. There-fore, the ultimate test to failure is often disregardedand the penultimate event assumed to be the finalnon-injury event. Alternatively, a failure-producing

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test that is preceded by a series of sub failureloads may be included or excluded in a data setbased on the presence, or lack thereof, changes instructural responses such as stiffness up to thepoint of failure.

It is well established that the injury tolerance ofhumans generally diminishes with advancing age.Although the median age of fatally injured crash vic-tims ranges between 35 and 44 years (NHTSA,2008), the population of cadavers available forresearch is biased toward elderly individuals.Although the cadavers used to define humanresponse and injury tolerance may represent themost vulnerable segment of the adult population,they do not necessarily characterize the broader tar-get population for injury countermeasure develop-ment. Differences in the types, combinations andseverities of injury that exist between elderly andyoung adult occupants complicate scaling of injuryrisk as function of age. The high degree of heteroge-neity inherent among the senescent population(Kent et al., 2005) produces overly large confidenceintervals on injury risk assessments in studies withelderly cadavers.

Finally, the cost and time required of cadavericexperimentation can be considerable and must beincluded in comparisons of the relative merits of sur-rogate types. Experiments with fresh cadavers in aimpact biomechanics facility require extensive proto-cols for handling and testing tissue that is potentiallybiohazardous (Duma et al., 1997). Incorporatingprinciples of Universal Precautions (CDC, 1987),these protocols provide procedural guidelines for thehandling that is consistent with the health andsafety of the personnel and establish a framework inwhich experiments involving biological material arecarried out with discretion, courtesy, and sensitivity.While an absolute necessity, these procedures sig-nificantly increase the complexity and cost ofcadaver testing.

Global cadaver usage for injury studies varieswidely depending on regional culture, habits, tradi-tions, and religions. Within the United States, con-cerns about societal acceptance of cadaver use ininjury biomechanics research has been documentedby mainstream media, archival journal publicationsand bestselling books (Roach, 2003). While high-lighting a perceived societal stigma associated withcadaver research has been the historical focus, thereappears to be a shift for increased public acceptancewith increased understanding and awareness of theresearch benefits and the continued acceptance ofimpact biomechanics as a research field (King et al.,1995). Along these lines, King et al. (1995) esti-mated that more than 60 lives are saved and count-less injuries prevented annually for each cadaverused in the development and validation of safetyimprovements related to car occupants. It is clearthat greater public acceptance could lead toincreased availability of cadavers. Improved accessto cadavers would markedly improve the quality ofthe injury tolerance studies and, in turn, acceleratethe pace of countermeasure development and livessaved.

Anthropometric Test Devices

ATDs, more commonly referred to as dummies,are mechanical analogs that aim to match theanthropometry, articulations, and structuralresponse of the living human. Dummies must exhibitboth internal biofidelity (i.e., comparable deforma-tions, accelerations, and articulations of the bodyregions) and external biofidelity (i.e., similar interac-tions of the dummy with the surrounding environ-ment including seats and restraints) to replicatehuman behavior under comparable loading condi-tions. To achieve the requisite levels of biofidelity,modern dummies are not simply anthropometricmanikins but, rather, complex engineering devicesmade of metal, foams, and polymer composites. Fur-thermore, the ATDs are instrumented with sensorsto record the accelerations, forces, and displace-ments of impact experienced by the body in a crash.Characterization of whole-body response requireshead-to-toe transducers with more than 135 chan-nels of instrumentation included in state-of-the-artdummies (Been et al., 2007). Given the degree ofdesign and complexity, the costs of an instrumenteddummy can be hundreds of thousands of dollars.

Since dummies are utilized in compliance and reg-ulatory testing in addition to research and develop-ment investigations, they have requirements forrepeatability and reproducibility beyond those forbiofidelity. Similarly, performance specificationsfor ATDs involve relatively high durability require-ments that involve sustaining multiple impacts atloading conditions beyond that normally intended fortesting evaluations. Given the elaborate pathome-chanics involved in human response during impact,these durability requirements can provide require-ments contradictory to those desirable for biofidelity.For example, the thoracic spine of the Hybrid IIIdummy is fabricated from a single steel componentthat cannot bend or extend, whereas the humanspine is multisegmented. This rigid steel constructionaffects spinal bending and overall torso motion dur-ing a crash test which subsequently influences thetransmission of restraint loads to various parts of thebody.

The kinematics of a dummy (Forman et al., 2008)subjected to a frontal-impact sled test at 48 km/h(Michaelson et al., 2008) can differently significantlyfrom those of the comparably sized male cadaversubjected to the same impact conditions (Fig. 1). Inparticular, the combined flexibility of the functionalvertebral units of the human spine allows thecadaver to flex forward in a continuous manner. Incontrast, the thoracic spine of the Hybrid III dummyis essentially rigid and does not permit bendingexcept in the cervical and lumbar spine regions.Thus, the forward excursion of the cadaver head andneck is the cumulative result of the forward flexionof the entire spine. With the dummy, the forwardmotion of the head results almost entirely from asharp change in angle at the lower neck interfacewith the rigid thoracic spine. The rigid Hybrid IIIspine also causes a greater mechanical couplingbetween adjacent body components and regions of

364 Crandall et al.

the dummy (Fildes et al., 2006). Collectively, the dif-ferences in dummy kinematics affect the forces andaccelerations that occur in the spine and thorax(Shaw et al., 2001), alter the restraint interactions,and confound the prediction of injury risk associatedwith the impact event. Although there are demon-strated differences between dummies and cadavers,it is essential to recall that a cadaver cannot replicatemany of the characteristics of a living human such asmuscle activation (Mertz and Patrick, 1967). Sincethe prediction of response and the development ofcountermeasures for a living human are the ultimateobjectives of efforts within the field of injury biome-chanics, the kinematic behaviors of cadavers anddummies must be evaluated not only relative to each

other but also relative to the inherent shortcomingsof each surrogate.

Since most ATDs lack frangibility of their compo-nents, they do not directly assess failure. Rather,dummies require the use of injury criteria to inter-pret the injury risk using recorded kinematic (e.g.,acceleration) and kinetic (e.g., force) parameters.These injury criteria are generally expressed asprobability curves, rather than definitive thresholdsor limits, since the likelihood of being injured variesby individual and is dependent on age, gender, phys-iologic condition, and other factors. The injury crite-ria have traditionally been derived from experimentswith cadaveric tissue or animals, although computa-tional models may be used in the future. Since most

Fig. 1. Differences in the spinal motion between a cadaver (left) and a HybridIII dummy (right) under equivalent impact conditions.

Fig. 2. Current state-of-the-art side impact dummy (left—WorldSID) and fron-tal impact dummy (right—THOR) (photos courtesy of Humanetics Innovative Solu-tions, Inc. and National Highway Traffic Administration).

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dummies lack internal structures representative ofsoft tissues and organs, structural parameters suchas force or acceleration in a body region must be uti-lized rather that specific loads or stresses within agiven organ. Given the sensitivity of tissue trauma tolocalized and directional factors, injury criteria basedon these general regional parameters are not overlyrobust for all loading environments.

The difficulty of trying to reproduce the behavior ofa living human under multiple loading conditions hasprevented the development of a truly omnidirectionaldummy. Rather, specific dummies are designed forparticular crash modes including frontal, side, andrear impacts (Fig. 2). Interpretation of dummyresponses for combined crash modes (e.g., obliqueloading) or for those which a dummy has not beenspecifically designed (e.g., rollover) are questionable.Similarly, ATD designs are tailored for different crashenvironments and restraint applications such as avia-tion or automobile conditions. Finally, politics andethnocentrism have resulted in dummies of markedlydifferent designs depending on the geographic regionof the world despite almost no variation in the under-lying human populations being represented.

The mid-size male or 50th percentile maledummy, nominally representing the average adultmale, has served as the standard for normalizationof response and evaluation of performance for manyof the current injury countermeasures. In an effortto produce the variation in size and gender thatreflect occupant population, families of dummieshave been developed, the most well known of whichis the Hybrid III family of dummies (Mertz, 1993).Since there has been insufficient cadaver testing tocharacterize the entire demographic range of thehuman population, scaling methods have beendeveloped to transform the geometric, inertial, andimpact response requirements of the mid-size maleto other dummies of specific size. Despite these

dummy families, current evaluations of vehicle andrestraint design through experimental testing covera relatively small percentage of the range of occu-pants observed in the field. Furthermore, it is clearthat characterization of human population by selectmass and stature variables neither captures a largepercentage of the underlying population (Fig. 3) nordescribes adequately the complexity of body shapeand composition inherent within the human body.Differences in body shape, characterized grossly bythe body mass index among other measures, havebeen shown to strongly influence the performance ofrestraints (Miller and Maripudi, 1996; Adomeit et al.,1997; Bose et al., 2008; Bose et al., 2010; Kentet al., 2010).

Attempts to produce crash dummies that yieldcrash responses approximating a tensed live humanhave been successful in some body regions (e.g.,neck) but not in others (e.g., abdomen). While dum-mies can provide stiffnesses of joints and body thatreflect the resistance to motion and deformation of aperson tensing their muscles, current ATD designscannot replicate the bracing behavior where theoccupant exerts forces on the vehicle environmentbefore an impending crash. Although researchershave preliminarily investigated active dummydesigns that could simulate bracing, it remains to beseen if a physical implementation of muscle tensingin dummies is feasible given the complexity and costor, alternatively, if implementation in computationalmodels of the occupant in crash simulation studiesmay be deemed more efficient for studying theseeffects.

Computational Models

The versatility of simulation permits engineers tosystematically analyze the complex array of intrinsicand extrinsic factors experienced by living humans in

Fig. 3. Stature and mass distributions based on the U.S. anthropometric referencedata reported in the study by McDowell et al. (2010). Anthropometric measurementsfrom Hybrid-III adult dummies (5th percentile female, 50th percentile male, and 95thpercentile male) have been included for comparison with the population estimates.

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actual crashes that is too large, too complex, and tooexpensive to tackle with conventional experimentalapproaches. Since their introduction within impactbiomechanics research in the early 1960s (McHenry,1963), mathematical models have played a support-ing but valuable role in understanding humanresponse and injury during an impact event. Recentimprovements in computational technology and inmodeling software, however, have expanded boththe prominence and potential of human models.Given envisioned future growth in the efficiency,power, memory, and storage capacity of computerhardware along with commensurate advances in soft-ware, the increased ability to create sophisticatedmodels of the human body and to simulate a widespectrum of conditions will invariably expand theirfuture utility within the field of injury biomechanics.

During model development, one of the most diffi-cult questions arises out of a need to balanceanswering the problems at hand relative to therequired complexity of the model. The balancebetween the predictive and descriptive capabilities ofthe model relative to the computational cost requiresinterpretative knowledge of modeling tools as well asjudgment and experience. Based on complexity,computational models can be broadly categorizedinto three main groups (Fig. 4; Yang et al., 2006):

1. Lumped-mass models consisting of concentratedmasses connected by springs and dashpots,which represent the basic dynamic response of ahuman during a certain crash event.

2. Rigid-body models consisting of rigid bodies con-nected by mechanical joints, which attempt tomodel the overall human structure, masses, massdistributions, articulations, and joint properties.

3. Finite element (FE) models consisting of ele-ments interconnected at points called nodes(model mesh). Constitutive material modelsare assigned to the elements correspondingto each tissue based on its mechanical prop-erties.

The simplicity of lumped-mass models limits theiruse to specific impact conditions that do not involvea large number of contacts or loading directions.When the environmental complexity prevents repre-sentation by lumped mass models, multibody model-ing may be sufficient if whole-body kinematics andbasic structural response values are the primary in-terest. Using a combination of inertial and geometricrepresentations of the body through ellipsoids andfacet surfaces, structural response characteristicsthrough prescribed joint properties and articulations,and nonlinear contact algorithms, multibody modelsare readily applicable to variational studies of theoccupant and crash populations. For these investiga-tions, multibody models provide an efficient balanceof accuracy relative to computational cost. The lackof failure descriptions at the tissue level, relativelysimple approximations of contact forces, and inabilityto model accurately the body deformation, however,limits the utility of the multibody models since theyhave to rely on interpretation of trauma using struc-tural injury criteria, many of which have been tai-lored to dummies rather than computational models.The efficiency and relative ease-of-use of multibodymodels, however, will ensure they remain an essen-tial component of current restraint and vehicledesign efforts in the near future, particularly in caseswhere optimization and parameter sensitivity analy-sis are involved.

Fig. 4. Mathematical human models. a: The lumped-mass model of thorax(Lobdell et al., 1973). b: Rigid-body model of pedestrian (Untaroiu et al., 2009).c: FE model of vehicle occupant (Choi et al., 2005).

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FE models are the most sophisticated and accu-rate human models but are also the most computa-tionally expensive. These models represent tissuelevel response and failure using governing equationsbased on the concepts of continuum mechanics. Inaddition, they facilitate digital expression of the com-plex three-dimensional geometry and topology of tis-sue structures. This combination of anatomical andmaterial detail allows the complex topology andboundary conditions to be modeled; the inhomoge-neous, anisotropic and nonlinear materials to besimulated, and the stress distributions in the varioustissue structures to be delineated. Thus, FE modelsare capable of predicting injuries based on local val-ues of stress and strain, offering considerable advan-tages over simpler multibody models, dummies, andin some circumstances even cadavers. However, theaccuracy of any computer simulation depends inher-ently on the quality of the human model in terms ofmodel geometry and material properties. In manyinstances, the underlying data required for FE modelrefinement and validation are not currently availablefrom human volunteer and human cadaver testing.Thus, continued evolution of increasingly complexmodels will require commensurate developmentsand research within the fields of experimental bio-mechanics, crash reconstruction, and epidemiology.

Although most human models are inherentlydeterministic (i.e., the models provide a unique out-put for a given set of initial conditions), probabilisticmodeling techniques provide a methodology allowingcomputer models to capture the uncertainties as wellas the natural variation present in human data(Thacker et al., 1997, 1998). The principle underly-ing stochastic methods is that the input parametersof the model are defined, not by a single value, butby a statistical distribution. Although probabilisticanalyses potentially present several challenges,including significant computational time due to themany trials required, they may hold the key to rep-resenting the inherent diversity of the human andcrash populations.

In matching utility with complexity, it is importantto remember that all modeling is an approximate so-lution technique used to solve a mathematical modelthat is itself an idealized representation of the actualphysical problem being studied. At best, our FE andmultibody models approximate the response of theactual human based on a set of simplifying assump-tions. At worst, they provide accurate representationof inaccurate data if proper procedures for verifica-tion and validation are not followed. The goal offuture developments in human models will be toenhance the model’s predictive capabilities byexpanding the data available for development andvalidation and minimizing controllable sources oferror.

Animals

Although animals have been used in biomechani-cal investigations since the times of the ancientGreeks, their use in scientific research in injury bio-mechanics did not develop until the mid-20th cen-

tury (Kent and Bass, 2007). The benefits of animaltesting are clear in that evaluation of the potentiallyinjurious events within a crash environment involv-ing a living human is best represented in a labora-tory setting by a living organism. Given the subinjurythreshold limitations on human volunteer testing,animals provide the only viable surrogate to studythe pathophysiological response to impact injury andcharacterize the sequelae following an injury. Inaddition, testing of living and dead animals (Lowen-hielm et al., 1977) can provide insight into control-ling for differences between living humans andcadavers when used to develop response and injurytargets.

Although injury tolerance of virtually every bodyregion has been studied using an animal model andanimals over a broad span of the phylogenic orderhave been used, differences in anatomy and physiol-ogy complicate the interpretation of animal data(Kent and Bass, 2007). For example, most animalsutilized in biomechanical investigations are quadru-peds, whereas humans are upright bipeds. The typeof ambulation obviously influences the developmentof an animal’s structural and tissue remodeling adap-tations to accommodate loads. These structural dif-ferences combined with anatomical variations amongthe species complicate the evaluation of injury coun-termeasures, such as seat belt restraints, that aredesigned to accommodate human-specific anthrop-ometry. Further complications with animal researchinclude difficulties interpreting functional outcomesin animals and complications related to scaling andextrapolating results from animals to humans giventhe differences in both physiology and anatomy.Although dimensional analysis techniques have beenused successfully to estimate human response fromanimal data, it is important to note that these scalinglaws are based on assumptions of geometric similar-ity and loading of analogous points of the body (e.g.,if a certain region of the animal is loaded in anexperiment that the equivalent region of the humanwould be loaded) that are frequently violated whenextrapolating results across species. In terms ofphysical appearance, non-human great apes are thespecies most anatomically similar to humans withessentially identical skeletal components and internalorgan arrangements. Due to the highly developedcognitive and behavioral characteristics of greatapes, however, considerable ethical concerns existover their use in experiments and have led toresearch restrictions and bans in a number of Euro-pean countries.

The United States has a well-developed systemgoverning the use of animals in experimentalresearch with the legal centerpiece being the AnimalWelfare Act of 1966 (US Code 2006). This legislationrequires that institutions performing animal experi-mentation create an Institutional Animal Care andUse Committee to oversee conformance to the Act.Furthermore, most animals used for biomechanicalresearch are purpose-bred in a carefully controlledenvironment. Although these legislative and institu-tional measures increase the cost and complexity ofthe research, they ensure that the testing on animals

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is performed in a humane and ethical manner.Despite these measures, animal testing remains acontroversial issue and is often intensely scruti-nized and heavily criticized by animal-rightsgroups. Although the research community strivesto improve non-animal surrogates, animals cur-rently provide the only viable means of assessingthe physiologic progression of injury in a controlled,laboratory environment.

Human Volunteers

Since they are identical to the population of inter-est, human volunteers are an obvious experimentalmodel for studying the response of living humans.Although investigations of human response can beperformed either through epidemiologic studies ofpeople involved in actual crashes or through labora-tory studies of human volunteers, both of theseapproaches have severe practical limitations (Kentand Bass, 2007). Human volunteer experiments inthe laboratory have the obvious shortcoming thattesting must be performed at subinjurious levels ofexposure [Nuremberg code of 1947, Helsinki Decla-ration of 1964 (revised in WMA 2008)]. Comparablewith animal testing, there are extensive protocolsthat need to be developed and be reviewed by anInstitutional Review Board to assure that the rightsand welfare of the human subjects are protected andto verify that procedures are scientifically sound andmeet all legal and ethical standards.

The laboratory testing environment with humanvolunteer permits prospective studies with experi-mental design aimed at hypothesis testing with con-trolled boundary and initial conditions. Below theinjury thresholds, volunteer testing has been instru-mental in developing an improved understanding ofhuman response with variations in the levels of mus-cle activation. In particular, low-speed testing withvolunteers in rear-impact scenarios have examinedhead neck motions in an effort to improve the mech-anisms associated with whiplash associated disor-ders (Kaneoka et al., 1999). Although muscletensing effects can be assessed, there are limitationsto the extent of direct measurement since mostinstrumentation must be non-invasive. Thus, loadsinternal to a volunteer’s body must be inferred frommeasured external parameters rather than meas-ured directly.

Although the ultimate goal is to eliminate thecrashes on the roads that happen every moment,these crashes in the meantime provide a potential

living laboratory with which to improve our knowl-edge of the causes and effects of injury. Unlike atraditional laboratory experiment which includes ex-perimental design and hypothesis testing, field stud-ies of actual car crashes are inherently retrospective.The retrospective analysis of these crash eventsmakes it impossible to control for the environment orto collect sufficiently detailed information about theloading environment, the initial positions of the occu-pants, or the magnitudes of the applied loads (Kentand Bass, 2007). Given the limited informationavailable to researchers, these field investigationscannot directly assess the injury mechanisms or tol-erances. The primary utility of these types of studiesis in identifying injury trends that can be used to es-tablish research priorities and in assessing the effi-cacy of countermeasures such as seatbelts orairbags (Kent and Bass, 2007). Often, they have lim-ited utility for defining the relationship betweenapplied loads and injury risk since too little is knownabout the nature of the loading.

CONCLUSIONS

Human response and injury during crash eventsresult from the interplay of a vast array of intrinsicand extrinsic factors. The list of human parametersalone is extensive and includes, but is not limited to,variations in size, obesity, gender, age, biomechani-cal tolerance, and pre-existing medical conditions.Expanding the complexity of the problem, this diver-sity may vary temporally within individuals (e.g.,changes in muscle tensing during a crash event) andacross populations (e.g., societal increases in elderlyand obese populations). Using a combination of sur-rogates, engineers strive to characterize theresponse and injury and then to develop effectivecountermeasures. Although each surrogate in and ofitself has inherent advantages and disadvantages(Table 1), the collective knowledge garnered fromexperiments with these surrogates have resulted indramatic reductions in the lives lost to crashes. Mov-ing forward, simulation technology and computa-tional human models appear to offer some of themost promising opportunities for extrapolation andenhancement of conventional injury and responsedata obtained in laboratory studies. These futureadvances in modeling, however, are inextricablybound to the continued use and evolution of experi-mental testing with other human surrogates includ-ing animals, cadavers, and dummies.

TABLE 1. Relative Merits of Human Surrogates for Injury Biomechanics

ATD Animal ModelsHuman

volunteers Cadavers

Human anthropometry Yes No Yes Yes YesHuman anatomy Partial No Yes Yes YesPhysiologic response No Yes Potential Yes NoTesting to injurious Levels Yes Yes Yes Not prospective YesDirect Observation of Injury No Yes Potential No Yes

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