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Blast Injury: Translating Research Into Operational Medicine

Chapter 10

BLAST INJURY: TRANSLATING 

RESEARCH INTO OPERATIONAL 

MEDICINE

JAMES H. STUHMILLER, PhD*

INTRODUCTION 

FIRST-GENERATION INJURY MODELING

SECOND-GENERATION INJURY MODELING

THIRD-GENERATION INJURY MODELING

FOURTH-GENERATION INJURY MODELING

SUMMARY

*Senior Vice President and General Manager, Simulation, Engineering, and Testing, L-3 Communications, 10770 Wateridge Circle, San Diego, California 92121
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

INTRODUCTION

the Military Operational Medicine Research Program 

(MOMRP) of the US Army Medical Research and Materiel Command (USAMRMC; Fort Detrick, Md) has 

been a leading world research organization in the study 

of blast-related injury. The MOMRP program has

	 •	 conducted extensive animal tests to establish

injury patterns, elucidate injury mechanisms, 

and provide critical data for establishing injury standards; 

	 •	 developed mathematical models of physiological response in both animals and humans 

to extrapolate test data to situations of military

relevance;

	 •	 developed injury criteria that are used

throughout the military and civilian communities to estimate injury potential and set safe 

limits of exposure; and

	 •	 developed instrumentation to standardize the

measurement of blast environments forinjury

assessment.

This research covers the traditional blast-effect

categories:

	 •	 primary effects—those resulting from the crushing effects of blast overpressure,

	 •	 secondary effects—those resulting from the 

impact of debris accelerated by the blast wave

and following winds, and

	 •	 tertiary effects—those resulting from impacts 

with walls and the ground caused by accelerating the body itself. 

In addition, other effects have been studied—for

example, incapacitation, injury, and lethality from

the inhalation of gases generated by explosives and

effects on sensory systems, including auditory injury. 

Research findings have assisted in human protection

in areas other than blast, including the development

of better body armor, automobile safety systems, and 

less injurious nonlethal weapons.

Many of the key findings of the MOMRP blast research program have been published in peer-reviewed

literature. Hardware and software for the characterization and analysis of blast pressure have been widely

distributed. Nonetheless, other results appear only in 

technical reports and are less accessible. The purpose 

of this publication is to provide an overview of the

USAMRMC blast research program, to show how 

quantitative physiology has provided useful solutions

to operational medicine, and to indicate future directions of research.

In 1864, during the American Civil War, the 48th

Pennsylvania regiment was made up primarily of men

who had been coal miners in civilian life. As the siege

of Petersburg, Virginia, dragged on, the idea arose to 

dig a tunnel under the Confederate lines. It was dug, 

filled with gunpowder, and the fuse lit on July 30, 1864. 

The initial explosion killed 280 Confederate soldiers.

But, ironically, the subsequent Battle of the Crater 

led to casualties totaling 1,500 Confederate and 4,000 

Union soldiers, not counting any who were wounded 

or missing.1

In 1968, fuel-air explosives were introduced in the

Vietnam War to clear densely forested areas, minefields, and enemy soldiers. The tremendous blasts 

from dispersed clouds of kerosene literally crushed 

everything below. The Russians further perfected these

thermobaric weapons and used them in Afghanistan 

and the Chechen Republic. 

In 1979, during developmental testing of a new

M198 howitzer (Rock Island Arsenal, Rock Island, 

Ill), firing of the most energetic rounds generated 

blast pressures in the crew locations, which slightly 

exceeded the limits of Military Standard 1474 (“Noise

Limits for Military Materiel”). By regulation, soldiers 

could not be exposed to such intense noise, so further

testing was stopped. Because the capability to fire the 

energetic rounds was a key element in the mission of 

the howitzer, fielding of a critical weapon system was 

effectively blocked.

On October 23, 1983, the First Battalion, 8th Marines Headquarters building in Beirut, Lebanon, was 

destroyed by a terrorist truck laden with compressed 

gas-enhanced explosives.2 The resulting explosion and

the collapse of the building killed 241 marines, sailors, 

and soldiers. Use of massive amounts of low-tech

explosives to produce mass casualties has become an

increasingly common tactic in modern, asymmetric 

warfare and in civilian terrorism.

As of October 2005, improvised explosive devices

(IEDs) have accounted for one third of all American

deaths in Iraq. Roadside bombs were used in World

War II by Belorussian guerillas to derail Nazi trains as

part of the “rail war,”3 in Northern Ireland against the 

British Army, in Afghanistan against the Russian Army, 

in Lebanon against the Israelis, and now against the 

United States throughout the Middle East. In 2005, the 

US military invested $3.3 billion in IED countermeasures, primarily through improved armor and other

technologies.4

As these examples show, for hundreds of years,

blasts from explosions have been a threat on the battlefield, a threat to civilians from acts of terrorism, and a

threat to soldiers in training. For more than 25 years, 
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Blast Injury: Translating Research Into Operational Medicine

Blast Injury Taxonomy

The violent consequences of being near an explosion can produce a wide range of injuries that can 

be organized into a taxonomy (Table 10-1). The high

pressures of the blast can crush the body and cause 

internal injury. These injuries are called primary blast 

injuries. Strong winds behind the blast front can hurl 

fragments and debris against the body and cause the 

same blunt trauma or penetration injuries that would 

occur if the material were propelled by other means. 

This class of injuries is called secondary injuries. The 

strong winds behind the blast front and the pressure 

gradient in the wave can exert significant forces that

can accelerate the body and cause the same blunt 

trauma that would occur in a fall or a car crash. 

This class of injuries is called tertiary injuries. The 

extreme heat and light released by the explosion can

cause burning and blindness, whereas inhaling the 

toxic fire gases can lead to immediate incapacitation

or delayed lethality. This class of injuries is called 

quaternary injuries. Finally, any of these traumas can 

lead to subsequent effects caused by disruption of 

the body’s biochemical or neurological system. This 

class of injuries is called collateral injuries. Although 

blast provides a unique process by which projectiles

are propelled, bodies are accelerated, and trauma is 

caused, the resulting injuries and sequelae can also 

result from other traumatic events.

Injuries that result from crushing overpressure,

however, are truly unique to blast. These primary

blast injuries occur because the body is not a solid, 

incompressible mass; it has air-containing organs that 

will crush under the external load. Crushing allows

the outer shell of the body in those regions to move

rapidly inward, thus distorting the air-containing 

tissue and producing local, large stresses. Injuries 

to the lung, gastrointestinal (GI) tract, and upper 

respiratory tract (URT) are common manifestations 

of this effect. Rapid distortion of the air-containing 

organs can transmit stress to neighboring solid organs 

as well. Contusions to the heart, for example, arise in

part from the strong stress waves that develop in the

lung. Large deformations of the body can also lead 

to stresses in solid organs that result in damage (eg, 

liver and spleen lacerations). Finally,rapid volumetric

changes outside the normal physiological range can 

disrupt systemic processes. Creation of air emboli 

by forcing gas across the air–blood barrier of the 

lung and creation of large pressure transients in the 

vascular system may be responsible for brain injury

and cell death.

The study of blast injury can be roughly divided

into two objectives: (1) to characterize primary blast

injuries, processes that are not investigated in other

trauma research; and (2) to relate blast to projectile impact characteristics, whole-body motion, and extreme

heat, light, or atmospheric environmental conditions

that, in turn, lead to nonprimary injuries. The first objective has required new research, whereas the second

objective has required adaptation of previous research.

Together, a complete, quantitative physiological understanding of blast injury has emerged.

Post-World War II Research

Following development of nuclear weapons,

research into blast injury centered on the effects of 

extremely large blasts—in particular, establishment

of lethality criteria for a wide range of conditions. 

Biological injuries from blast were categorized as 

primary, secondary, and tertiary, but there was little 

attempt to understand the mechanism of these injuries. During this period, empirical models of lethality 

were used.

In the United States, the majority of blast biology 

research was conducted at the Blast Test Site at Kirtland Air Force Base (Albuquerque, NM). The Lovelace Foundation for Medical Education and Research 

(Albuquerque, NM) operated this site from the 1950s 

until 1980, under the sponsorship of various defense

agencies concerned with nuclear weapon effects or, in 

the 1970s, with fuel-air explosions. Animals ranging

from mice to steers, as well as test dummies, were used 

to estimate lethality from blast in the open, in buildings, in foxholes, and with combinations of blast and

thermal effects. (See the comprehensive history of the

Blast Test Site.5) 

Lethality correlations developed during this time

have become part of nuclear weapons assessment and

Army field manuals. The so-called Bowen curves relate

lethality to the strength of the blast, as measured by 

peak pressure and duration.6,7 An empirical scaling 

based on body mass allows the same correlations to 

be used for all large animals and humans.

Occupational Limits of Blast Exposure

Whereas the Bowen curves provide a criterion for

lethality, Military Standard 1474 provides a criterion

for hearing protection during occupational exposure

to blast.8 The standard is based on a 1965 report of the 

National Research Council Committee on Hearing, 

Bioacoustics, and Biomechanics (CHABA; Washington, DC) that sets noise limits for occupational environments.9,10 The Military Standard, using a relation 

adopted from CHABA that depends on amplitude 

and duration of the overpressure, limits the number of

impulse noise (blast) exposures that can be received in

a day. If hearing protection (plugs or muffs) is worn, a 
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

TABLE 10-1

TAXONOMY OF BLAST INJURY 

Unique to high-order 

explosives,results

from the envelopment of the body in 

the overpressurization wave.

Body surface and 

internal organs are 

rapidly distorted 

because the body 

contains highly compressible tissues (aircontaining organs) 

that undergo rapid 

volume changes.

Impact on the body 

from flying debris 

and bomb fragments. 

Whole-body acceleration caused by the 

blast wind.

Uneven forces on the

body caused by the 

blast winds.

All explosion-mediated injuries not 

associated with pressure or wind effects.

High temperatures.

Toxic gases.

Secondary consequences of trauma.

Exacerbation or complications of existing

conditions.

Primary

Secondary

Tertiary

Quaternary

Collateral

Gas-filled structures are 

most susceptible because 

they suffer the greatest distortion—upper 

airways, lungs, gastrointestinal tract, and middle 

ear.

Internal distortions of 

air-containing organs 

cause distortion of 

neighboring solid 

organs—heart, liver,

spleen, and kidneys.

Differential loadings 

within the body, especially the vascular system, can cause upset that 

can be transmitted to 

other parts of the body.

Any body part may be 

affected. 

Depends on the speed, 

mass, and shape of the 

impacting object.

Any body part may be 

affected. 

Depends on the surface 

condition that the body 

impacts.

Primarily head/neck 

and extremities that can

be accelerated relative to

the torso.

Any body part may be 

affected.

Body surfaces, eyes.

Respiratory system.

Systemic responses from 

massive trauma.

Blast lung (pulmonary barotrauma). 

Tympanic membrane rupture and middle ear damage.

Abdominal hemorrhage and perforation; globe (eye) 

rupture. 

Concussion (traumatic brain injury without physical 

signs of head injury).

Laceration of the liver, spleen, and kidneys.

Contusion to the heart. 

Distortion and rupture of the great vessels.

Air emboli introduced across the air–blood boundary of 

the lung. 

Surges in blood flow and pressure that may lead to tissue injury in the brain.

Any injury associated with impact of high-speed objects. 

These modes are not unique to blast; however, blast

provides a different way of propelling the objects.

Penetrating ballistic (fragmentation) or blunt injuries.

Eye penetration (can be occult), skull fracture, etc.

Any injury associated with whole-body motion and 

impact. These modes are not unique to blast; however,

blast provides a different way of accelerating the body.

Typical injuries that would occur in falls or car crashes. 

Fractures, contusion, and closed- and open-head injuries, etc.

Traumatic amputation; muscle tears.

Burns (flash, partial, and full thickness).

Asphyxia.

Injury or incapacitation from inhaled toxic fire gases.

Not unique to blast.

Angina, hyperglycemia, and hypertension.

Asthma, chronic obstructive pulmonary disease, or other

breathing problems from dust, smoke, or toxic fumes.

Category Characteristics Body Part Affected Types of Injuries
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Blast Injury: Translating Research Into Operational Medicine

greater number of exposures is allowed. The standard

contains a special limit, called the Z-line, corresponding to the auditory limit of 5 shots/day when wearing 

protection. Exposure above the Z-line is prohibited because of the potential for nonauditory injury. Although 

it is unlikely that any data to support this limit existed

when the CHABA originally met, it is understandable 

that, at some level—no matter how effective hearing

protection is—there will be a risk of injuries to other 

parts of the body from blast. In 1965, no occupational 

noise approached the Z-line limit. 

As noted previously, the M198 howitzer, a critical new weapon system, exceeded blast noise limits

and could not be fielded. It fell on USAMRMC and 

MOMRP to address this issue. It had always been 

suspected that the Z-line was a conservative limit, but

it was not known at what level internal injury would

first appear or how it would manifest. 

To solve the immediate crisis of the M198 howitzer,

a human volunteer study was organized.11 Starting 

from distances known to be safe, the volunteers

moved closer to the howitzer until they were at the

crew positions. Medical examination showed that

hearing could be protected and that no internal injuries were sustained. The M198 testing proceeded, and 

the system was fielded, although a training restriction 

on the number of exposures above the Z-line was

made. Additionally, USAMRMC initiated a research 

program for blast injury to develop a replacement

standard for the Z-line. Until a new standard was 

developed, similar human-rated qualification of

weapons systems would be conducted whenever the

Z-line was exceeded.12

Military Operational Medicine Research Program 

(Blast Injury Research)

From the beginning, the MOMRP blast research program was model-based. The decision to use a modelbased system came from a balanced assessment of the 

immediate and long-term benefits of animal, physical 

surrogate, and mathematical model approaches. On 

one extreme, a test of the actual threat against a human

underthe exact conditions ofinterest is the least ambiguous way to determine an effect and one that requires 

no understanding of the internal mechanisms. This is 

ethically not possible for conditions that produce injury 

and is impractical to define the range of conditions. On 

the other extreme, a mathematical model that captured

every mechanical, chemical, and biological aspect of

the human body can provide a simulation basis for

studying any situation without risk to an individual.

Given the state of knowledge and computing, this goal

exceeds any reasonable endeavor. In between these

extremes are combinations of animal, physical surrogate, and mathematical models that describe some 

part or scale of the interaction (Figure 10-1). The goal 

of the MOMRP effort is to use the appropriate mix of

tools to derive the best possible immediate answer,

while continuing to develop incrementally better

mathematical models that will expand the generality

of our understanding. 

The first problem addressed was the interpretation 

of the pressure signal. Unlike the idealized waveforms

that had been encountered and characterized in largescale explosions, the howitzer waveform contained

two peaks. This form confounded the simple empirical 

Fig. 10-1. Animals, physical surrogates, and mathematical models. 

Response Injury

Human Target

Animal Model

Compliant Physical Target

Rigid Physical Target

Rigid Mathematical Model

Mathematical Model

Complete Mathematical Simulation

Reproducible Operational Biofidelic Validated Statistical

Biomechanical

Correlate

FACTORS FAVORING BIOLOGICAL TESTIN

G

FACTORS FAVORING SIMULATION

Fidelity

Understanding Required

Generality, Knowledge Gained

Risk, Cost

“Compliant” Mathematical Model

Impact
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

correlates that existed and, because they varied in

magnitude and time of separation from place to place 

around the howitzer, were suspected to be artifacts of 

the test geometry or the instrumentation. Jaycor, Inc 

(San Diego, Calif), developed a mathematical model

of the propagation of the blast from the muzzle of the 

weapon, accounting for the nonlinear reflection from 

the ground. The model proved conclusively that the

waveforms were from those interactions. Furthermore,

the model was able to provide a complete map of the

hazardous areas around the gun, for all gun elevations

and charge types, without requiring extensive physical measurements. With this model, analysis of other

weapon systems could also be made with data from 

only a few measurement locations for calibration.13

Using the results of the model as a guide, animal 

studies were formulated and conducted at the Blast 

Test Site. From 1980 until its closure in 1998, the focus of 

the research was injury, not lethality. Studies spanned 

blast conditions that produced no observable effects

to serious, life-threatening injury and death. Great 

care was taken to document pathologies observed in

all organs. Primarily sheep were used as test subjects 

to provide consistency across all studies. This 18-year

effort produced nearly 2,000 animal exposures that

represent the most extensive collection of blast-related

injury data in the world. Subsequently, a comprehensive pathology scoring system for blast injuries was

developed.

The earliest injury tests were focused on establishing a threshold level of blast overpressure, below

which injury would not occur. Unfortunately, such 

a level could not be determined definitively, because

small probabilities of injury required a large number 

of animals to estimate thresholds with confidence. 

These low-level tests did show, however, that the aircontaining organs (ie, lungs, GI tract, and trachea/

larynx) were the first organs to be injured.14

Next, tests were designed to determine if URT injury

occurred before lung and GI injuries, with the thought 

that a harmless precursor injury could be used as an 

indicator of impending, more serious, internal injury. 

This hypothesis was based on auditory experience in

which a temporary threshold shift (a minor, transient 

hearing loss) always preceded permanent hearing loss 

and, therefore, allowed human subjects to be used to 

establish safe boundaries. Unfortunately, blast-caused 

URT injury occurs at essentially the same levels that

cause lung and GI injuries.

Finally, tests were conducted to establish a universal

blast exposure dose that could predict injury from the

blast pressure signal alone, eliminating the need to 

conduct animal or human tests. The logical candidate, 

based on the trends seen in structural damage from 

blast, was impulse—the time integral of the pressure 

signal. Impulse, however, did not provide a universal

correlate with blast injury.

Failure of these attempts to identify simple patterns 

or markers in blast injury was amplified when blasts 

in enclosures were studied. Results of these studies 

underscored the need to understand actual anatomical dynamics and root causes of primary blast injury. 

Mathematical modeling helped guide research and 

establish criteria.

FIRST-GENERATION INJURY MODELING

First-generation models correlated injury to mechanical responses of the body, rather than to the 

characteristics of the blast pressure itself. This biomechanically based criterion was driven by failure

of pressure-based correlates to predict injury for 

simple waves in the free field or for complex waves in

enclosures.

The mechanism of injury was hypothesized to consist of three causal steps: (1) blast waves create temporal and spatial loading distributions on the external

body surface; (2) the body deforms under the loading 

and causes internal stresses and strains in the tissues; 

and (3) when these tissue stresses exceed material limits, injury occurs. Computational power, both in terms 

of computer speed and simulation software, was in 

its infancy in the early 1980s; however, mathematical

modeling made significant contributions. Four organ 

systems were considered in first-generation injury 

modeling: (1) the tympanic membrane, (2) the trachea, 

(3) the GI tract, and (4) the lung.

Observed Blast Injuries

Blast can cause a wide range of injuries. As the explosive levels grow, the magnitude of the forces acting

on the body grows, and the extent of the body response

increases. Occupational standards, however, focus on

identifying the organs most easily injured and on setting 

acceptable levels of lesser injuries.

From testing at the Blast Test Site, a clear pattern of 

blast injuries emerged. Over time, as more data have

been collected, these patterns have been reaffirmed;

and, using the animal tests currently available, the

patterns could be assigned statistical significance. 

To observe the pattern of injury from a range of tests

that used different explosive weights, ranges, heights

of burst, orientations, and number of exposures, it is

necessary to select a single characteristic of exposure.

One such parameter is the charge weight per characteristic volume. In the free field, this volume is a sphere

with a radius equal to the distance to the test subject; in 
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Blast Injury: Translating Research Into Operational Medicine

enclosures, this volume is the volume of the enclosure.

Surprisingly, and, perhaps fortuitously, this quantity 

provides a good correlation and similar numerical

values for the thresholds in all cases.

The average explosive weight per volume, at which

minor and severe injuries are seen for each organ

group, is shown in Figure 10-2 for tests in enclosures. 

A similar trend exists for the free-field tests. Organs

can be separated into three groups having similar characteristics. The air-containing organs (larynx, trachea,

lung, and GI tract [forming the first group]) show the 

first signs of injury at approximately the same blast

intensity, which is well below that seen in other groups. 

The liver and spleen (Figure 10-3) fall into a second

group, whereas the kidney, pancreas, and gallbladder 

form the third group. Clearly, the air-containing organs 

are the most vulnerable and, therefore, require the

greatest consideration for occupational exposure. All

organs are seriously injured at similar blast intensity. 

Lethality, which is primarily due to a multiple organ 

failure, is seen at this same level.

Exploratory Injury Models

Mathematical modeling of biological systems is 

particularly difficult, because every aspect of the system (geometry, material properties, and strengths) is 

complex and determined by ever smaller structures, all

the way down to the cells themselves. Therefore, each

aspect of the model must be guided and validated by

experimental data.

Tympanic Membrane Injury Modeling

Tympanic membrane rupture is a common blast 

injury and one that occurs at blast levels usually well

below any of the internal injuries. Although not considered a life-threatening injury, eardrum rupture can 

be extremely painful and may or may not be associated

with hearing loss. In the mid-1980s, eardrum rupture 

was a concern of blast exposure in training and in combat. Thus, an effort to develop a mathematical model of

the process was undertaken. Finite element modeling 

(FEM) was used to model the stapes and the membrane. 

The stresses at which the membrane tore were based 

on independent tissue properties measurements. The 

model was subsequently validated against in vitro data

to establish biomechanically based injury criterion.15

Tracheal-Laryngeal Injury Modeling

The interest in establishing URT injury as a precursor to more serious internal injury prompted the need 

for a model that could be used to extrapolate URT

injuries from animal to humans. Blast-induced tracheal 

injury is often striated in appearance, corresponding 

to the cartilage between the bony rings. In extreme

conditions, it can present as a confluence of hemorrhagic tissue (Figure 10-4). The trachea was selected 

for modeling because of its geometric simplicity and 

because material properties were available from the

literature. Blast loading on the neck was used as the 

external boundary condition, and injury was correlated

with peak stress that developed in the tissue. This

model predicts the mechanical response of the larynx,

and the general trends of the injury are correlated with 

stress in the tissues.16

Fig. 10-2. Observed blast injury. GI: gastrointestinal.

Enclosures

Threshold Severe

Larynx

Trachea

Lung

GI

Liver

Spleen

Kidney

Pancreas

Gallbladder

Deat

Normalized Charge 

h

Weight (kg/

m3

)

0.06

0.05

0.04

0.03

0.02

0.01

0.00

Fig. 10-3. Liver injury resulting from blast.
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

contents. For large overpressure, the stresses became

so great that rupture of the wall occurred. Once the 

mechanism was established, surrogate GI models were 

developed that allowed this bubble–wall interaction

phenomena to be further studied.17–20

With these surrogate observations as a guide, a

mathematical model of the bubble dynamics and wall 

response was developed.21 The model produces the 

same dynamic response and, when coupled with material properties and the strength of the tract wall, can 

provide a predictive model for GI injury from blast.

Because lung injury was eventually selected as

the primary injury to set blast exposure limits, the GI

model was not pursued. The model suggested, however, that GI injury may be greater in sheep than in

humans because the sheep is a ruminant with much 

more gas in the intestines. A summary of the early 

blast data analysis and model development is found in

the publication Modeling of the Non-Auditory Response 

to Blast Overpressure: Characterization and Modeling of 

Thoraco-Abdominal Response to Blast Waves.

22

Lung Injury

Even though all of the air-containing organs are

injured, Walter Reed Army Institute of Research

(WRAIR) physicians determined that injury to the lung

is of greatest concern and should be used to set exposure standards. As a result, modeling research efforts 

focused on understanding lung injury mechanisms 

and developing predictive lung models.

Most organs of the body are liquid-filled, some are 

air-filled, but the lung is a complex matrix of more

Fig. 10-4. Tracheal injury resulting from blast. than 500 million small air sacs (or alveoli), with tens

Fig. 10-5. Gastrointestinal injury resulting from blast. 

Gastrointestinal Injury Modeling

 GI injury from blast appears first as small petechia 

in the gut lining, grows in hemorrhagic area as blast 

intensity increases, and can reach frank rupture. In 

sheep, the injury most commonly appears in the cecum, although large blasts produce injury throughout 

the GI tract (Figure 10-5). GI injury—especially rupture 

of the tract wall—can lead to sepsis and death, and 

can complicate treatment of other injuries, especially 

those in the lung.

Rabbit experiments confirmed that GI injuries were

associated with sections of the tract containing air 

bubbles. The bubbles collapsed during the overpressure phase of the wave and then violently reexpanded.

If the bubble was close to the tract wall, this rapid 

volumetric change led to deformation of the wall. The

effect varied with bubble size, overpressure, and tract
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Blast Injury: Translating Research Into Operational Medicine

Fig. 10-6. Lung injury resulting from blast. (Left) Complex waves. (Right) Freefield waves.

of millions of connecting airways.23 Under normal 

physiological motion, this structure acts as a simple 

balloon that fills and empties, with a pressure drop 

from inside to outside that is modest and reflects 

the resistance of the many small airways. Under the 

rapid motion caused by blast, however, air cannot

flow fast enough through the airways, and the lung 

acts as a collection of isolated air bubbles—a foam. 

Foams have the interesting physical property that

their speed of sound (the speed at which a volumetric

disturbance in one part of the material can be spread 

to other parts) is only 30 to 40 m/s—one tenth that of 

air and one fiftieth that of water.24,25 This low speed of 

sound causes the lung parenchyma directly behind the 

moving chest wall to be crushed and severely injured,

if the chest wall velocity is great enough. The injury

produces a characteristic surface hemorrhage pattern 

predominantly on the blast side in the free field, but 

can involve the entire lung when the blast is enclosed

(Figure 10-6).

In the early 1980s, Jaycor began working with Professor YC Fung at the Bioengineering Department at 

the University of California, San Diego, to collect data

that would support the development of a lung injury

model. Professor Fung had already been studying the 

material properties of lung tissues.26 Over the subsequent several years, he and his colleagues established

the key dynamic properties of lung tissue under the 

rapid motion associated with blast.27 They confirmed 

the low speed of sound and determined its variation with transpulmonary pressure. This group also 

established the correlation of internal lung pressure 

with edema—the breakdown of the alveolar walls.28

They measured the properties of the whole lung and 

lung tissue for both small animals and humans, and 

they showed that the material properties were similar. 

Experiments were conducted to elucidate the injury

process, and mechanisms of injury at the tissue level

were proposed.28,29 Jaycor, in one of the first applications of finite element analysis of biological systems, 

developed a two-dimensional model of the thorax

under blast loading to quantify these tissue injury 

processes.30,31 That model reproduced the slow speed 

of the internal pressure waves in the lung, showing

that the highest values of these pressures were located

on the pleural surface of the blast side, the heart, and 

the spinal process. These locations are similar to lung 

contusion observed in animals exposed to blast.

INJURY Software, Versions 1.0 to 4.3

Finite element analysis, although insightful, was too 

problematic at that time to be used as a means to understand the hundreds of animal tests that were being 

collected or to be used as a replacement for the Z-line. 

Consequently, simpler models were used: modeling the 
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

inertia of the chest wall and the density and speed of 

sound of the lung to create the so-called pleural surface 

model (Figure 10-7).22 When blast pressure acts on the

chest wall, it accelerates, creating a pressure wave in

the lung tissue, which in turn pushes back against the 

chest, thus slowing it down.32 The mechanical model 

was also validated against closely spaced blast loads

that produce a double-peaked wave within the lung.33

The total work (integral of the product of force and 

velocity) done on the lung represents the magnitude 

of damage caused by crushing the alveoli. This irreversible work was the first biomechanical correlate of

lung injury. Normalized work, which is the irreversible

work divided by the product of the lung volume and

ambient pressure, has proven to be a reliable predictor

of blast lung contusion that can be applied across species, body weight, and altitude. The final version of the

pleural surface model was published in 1996.34

Work also proved to be a correlate of lethality. The Bowen curves, developed decades earlier as

empirical correlates of lethality observed in the free

field, were shown to correspond to a particular work 

value. This finding not only provides a rational basis

for the curves, but also provides a way to make lethality estimates in situations other than the free field. 

The lethality criteria based on work also allowed 

other observations, such as the reduction of lethality for slow-rising blast waves, to be explained with

a single theory.34

The first versions of the INJURY software (Jaycor, 

Inc)—versions 1.0 through 4.5—were developed to

solve the pleural surface model equations and provide

a prediction of the probability of injury. The input to 

this analysis is the so-called free field or side-on pressure measurement, which is converted to torso load

with formulas for wave reflection. Effects of body

orientation are accounted for by the angle of the chest 

to the blast direction. The medical staff at WRAIR used

these versions of INJURY software to make case-bycase health hazard assessments of weapon systems.

Simple Waves

Use of a biomechanical model to describe blast 

injury requires knowledge of torso loading. Loading 

distributions can be quite complex, even for simple,

free-field blast waves.Ablast wave in the free field has a

relatively simple description. The explosion produces a

high-pressure region that evolves into a shock frontthat

moves at a speed greater than the speed of sound. The

pressure decreases behind the shock and can even drop

below the ambient pressure. Tables of experimentally

determined values and elegant theories and correlations capture these data.35When a blast wave strikes the

ground, it creates a reflected wave with a second shock

front.The incident andreflected waves can interact with

one another and, if the blast is strong enough, produce a 

third shock wave, the so-called Mach wave, that moves

parallel to the ground. Above a certain height, called

the triple point, where the three shocks intersect, one 

observes the incident and reflected waves; below the

triple point, one observes only the Mach wave (Figure

10-8).As can be seen, even free-field explosions produce

complicated blast patterns. 

When a shock wave hits a body, another complicated

interaction occurs. The surface of the body facing the 

blast source acts as a reflector, producing its own reflected wave; but, because itis generally curved, notflat,

there can be many reflected shock waves. As the wave

passes aroundthe body,the angle ofthe incident wave to

the surface changes, altering the strength of the reflected 

waves; at the side, there is no reflected wave. Near this

location, the flow from the blast separates from the body 

and creates a region behind the body that has a pressure 

lowerthan the incident wave, but higherthan ambient.

Consequently, the loading pattern on a standing human 

in a simple blast wave is quite complex: (a) the chest, 

assuming facing the blast,receives a loading that can be

2 to 8 times greaterthan the incident wave; (b) the sides 

receive a loading comparable with the incident wave;

and (c) the back feels a loading that is a fraction of the 

incident wave.These interactions can be calculated with

computational fluid dynamics (CFD) simulations.36–38

If the torso is above the triple point, then the distribution is even more complicated because the body

Fig. 10-7. Pleural surface model of chest–lung interaction.

Fig. 10-8. Mach stem and triple point. HOB: height of burst.

Pload Pwave Pbulk dt

dv

m = Pload −Pwave −Pbulk dt

dv

m = P

load (t ) Pwave

V

Pbulk

Charge of Weight,W

Incident Wave

Mach Stem

Range,R

Path of

Triple

Point

HOB

Ground Reflection
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Blast Injury: Translating Research Into Operational Medicine

is hit by two waves: (1) the incident component and

(2) the reflected component.

For an animal standing on all fours, there is even

more complication. One part of the wave passes

over the back of the animal in the manner described

previously, but the other part of the wave passes

under the animal and creates additional reflections 

off the ground. The chest, which is normally facing 

downward, can receive a blast loading that consists

of multiple shock interactions.

Clearly, even in the “simple” geometry of the free

field, the blast loading on the body required to determine blast injury can be quite complex. Characterizing

the torso loading in terms of a single, free-field pressure 

measurement was not adequate. 

The Blast Test Device

Because the blast loading in the free field was not 

easily inferred from a free-field pressure gauge, a new 

instrument was developed. Eventually known as the

Blast Test Device (BTD), it consists of a metal cylinder,

supported from the ends, with the approximate diameter of the sheep chest and with four surface-mounted 

pressure gauges evenly spaced around the cylinder.

The latest BTD has an anthropomorphic shape and 

provides a host of attachments for ease of use in complex testing environments (Figure 10-9). One pressure

gauge is oriented facing the oncoming blast wave, with

the other gauges corresponding to the sides and back. 

The BTD is oriented in the same direction as the test 

subject, vertically for a standing human or horizontally

for a standing animal.39

Since its inception, the BTD has been the preferred 

instrument for providing loading data for blast injury

analysis. Because it measures the loading directly, there 

is no need to interpret the instrument output, correct 

for gauge orientation, account for being in or out of 

the Mach wave, or otherfactors that are required when

free-field gauges are used. Over the years, the BTD

has been placed directly on the ground to simulate 

a soldier firing from a prone position, propped up in 

a seat simulating a passenger in a vehicle, and hung

from vertical supports to simulate suspended animals

in an enclosure.

Blast in Enclosures

In the late 1980s, increased importance of Military 

Operations on Urban Terrain (MOUT) emphasized 

the need to train on weapons fired from buildings or 

enclosures. Initial animal studies showed that these 

situations are far more dangerous than their free-field 

counterparts and showed what appeared to be nonintuitive trends (eg, that animals farther away from

the explosive, but in a corner, are much more severely

injured than animals closer to the blast, but along a 

wall). These findings meant that the explosive charge

weight and the distance, which had been mainstays of 

characterizing free-field blast, were no longer sufficient 

factors to consider.

Blasts in enclosures, often called complex exposures, led to attempts to extend previous injury and

lethality correlations. Even though the Bowen curves

provided a correlation of lethality in the free field, it

could not be applied to complex waves. Although the

peak pressures were about the same as would occur 

at the same distance from the same charge in the free 

field, the durations were much longer because of the 

wave reflections from the walls. If the Bowen curves

were used, they would often predict lethality in benign 

conditions. Ad hoc changes were proposed to the definition of duration in an attempt to explain the data,

but the changes were not satisfactory.

The BTD proved to be the perfect instrument for

characterizing the effects of blast in an enclosure. BTD 

measurements in corners showed the strong blast loading on the back side of the instrument, the side away 

from the blast and facing the corner. In many pressure 

traces, clusters of very large peaks were observed in the

Fig. 10-9. Blast Test Device.
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

side or back gauges that had never been seen in the free

field. When the pressure measurements from the back

side of the BTD were used in INJURY software, the 

large work values calculated explained the occurrence

of injury in the lung lobes facing away from the blast. 

Although the BTD measurements offered a plausible explanation forthe injuries seen in enclosures, the

complexity and violence ofthe pressures measured had

not been anticipated. It seemed likely that these pressures were from a convergence of waves coming to the

corner from many reflective paths, but there was also

concern that some or all of the effects may be artifacts 

of the pressure gauges in such harsh environments.Although simple ray-tracing techniques had adequately 

described the howitzer blast in the open, they proved

inadequate to capture the complex reflections and

interactions of the blast waves in an enclosure.

Computational Fluid Dynamics 

CFD represents the numerical solution of the highly 

nonlinear, highly coupled, fluid dynamics equations. 

The science and art of CFD was first developed to support nuclear weapon design and effects analysis; but, 

by the late 1980s, the technique was used in all areas 

of engineering. 

The technique was adapted to blasts in enclosures 

to provide a rational understanding of “complex”

blast. Simulations using the Equation Independent 

Transient Analysis Computer Code (EITACC; Jaycor, 

Inc) were made to understand the data from the BTD. 

Simulations could reproduce, with remarkable fidelity, 

the details of pressure measurements.38 As computer 

power rapidly grew in the early 1990s, CFD was used 

routinely to analyze complex waves (eg, to determine

the effects of other objects in an enclosure on the blast 

loading).36

The first-generation models provided a way to predict injuries to the tympanic membrane, URT, GI, and, 

most importantly, the lung. These models, although 

simple, were based on biomechanical principles and 

had parameters that could be rationally determined 

from anatomy and tissue property measurements. 

The emerging importance of explosions in enclosures

prompted development of new blast instrumentation,

the BTD, and use of state-of-the-art CFD tools.

SECOND-GENERATION INJURY MODELING

The second generation of injury modeling extended

the single-degree-of-freedom models of primary blast 

injury to use the data directly from the BTD and to 

standardize the health hazard assessment process. 

Also, models for secondary and tertiary injuries were 

developed. The collaborative work between MOMRP

and the Joint Non-Lethal Weapon Program (JNLWP)

on projectile impact injury for nonlethal weapons led 

to the development of models to capture modes of

impact injury to all parts of the body that could be 

used to assess secondary blast injury. The collaborative

work between MOMRP and the National Highway 

Traffic Safety Administration (NHTSA) led to models 

of injury from whole-body impacts that could be used 

to evaluate tertiary blast injury. Finally, a cooperative

project between MOMRP and the Defense Nuclear 

Agency explored the combined overpressure and thermal injuries and the collateral effects of compromised 

immune systems.

INJURY Software, Versions 5.0 to 7.1

Previous versions of INJURY software that considered the loading from a single side of the body successfully predicted free-field blast injury. In the free 

field, the injury is primarily in the lobes on the blast 

side. Complex wave exposures, however, showed

the importance of loading from all sides of the body 

and, consequently, the pathology showed injury to all 

lung regions. The refinements of the INJURY software 

benefited from the growing number of animal data 

collected with BTDs at the Blast Test Site.40

Total Work on the Lung

The first change in INJURY software was to compute 

the normalized work from four calculations with the 

pleural surface model, one for each of the BTD measurements. The work was summed and correlated 

with overall observed pathology. This simple approach

explained the injury trends, especially the increased

hazard in corners of enclosures.

Whole-Body Loading

The importance of simultaneous loading from all 

sides of the body led researchers to consider more 

complete biomechanical descriptions of thoracic motion. The Lobdell model, developed at General Motors

Corporation (Detroit, Mich), is used in the automotive

industry to characterize chest impacts and has been 

calibrated by cadaver studies.41 The model accounts 

for the entire torso mass and, therefore, can be used 

to estimate whole-body acceleration from blast. It 

replicates the dynamic force–distance relationships 

of large chest deformations with a combination of 

springs and dampers. The Lobdell model, however,

is not anatomically based; it characterizes the chest 
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Blast Injury: Translating Research Into Operational Medicine

with an equivalent mass, but not an equivalent area.

For solid-body impacts, only the chest mass is needed; 

but, for blast waves that are described by a pressure

(force/area), a chest area must be assigned.

To determine the effective area of the chest acted on

by a blast wave, a finite element model of the thorax

based on the National Library of Medicine (Bethesda, 

Md) Visible Human Project data set was developed.42

The internal organs were resolved as separate anatomical regions and materials. Material properties of 

the lung were based on earlier, established measurements; the heart treated as a liquid-filled sac; and the 

ribs and chest wall muscles combined into a single 

material whose elastic properties were selected from 

data in the literature. The model was validated for

the same impact conditions that had been used to 

calibrate the Lobdell model, including both frontal 

and side impacts.

The validated finite element model was examined

using a series of tests for which blast loadings and lung 

pathology were known in detail. The resulting prediction of internal pressure distribution closely matched 

the observed pathology patterns. The total normalized

work was determined from the calculated chest wall 

motion, which also agreed with the simpler INJURY 

software estimates. Finally, the fully validated model

was used to determine the effective area that should

be associated with the Lobdell model. The frontal area 

presented to the blast could be divided into the partthat

is deformed by the blast and the part that moves with

the entire torso. The resulting modified Lobdell model 

(Figure 10-10) agrees with the more detailed finite element model with respect to chest compression, total 

normalized work, and whole-body motion. The pleural 

surface model was replaced by the modified Lobdell 

model in the next generations of INJURY software.43

Effects of Multiple Exposures

Based on animal test data available at the development of the first versions of INJURY software, it was

believed that there was no cumulative injury to the

lung from blast (ie, the lung was injured on first exposure, or it was never injured, no matter how many

times the blast was applied). As more data were collected, it became clear that there was a cumulative

effect in lung injury and that this effect was critical to 

determining the occupational exposure.

The animal data set was analyzed using a model 

of tissue fatigue to account for cumulative damage.

Multiple logistic regressions, incorporating the fatigue 

factor, were used to produce correlations forfourlevels

of lung contusion: (1) trivial (small petechia), (2) slight

(less than 5% of the lung surface area contused), (3) 

moderate (less than 30% of area contused), and (4) 

severe (more than 50% of area contused).

Probabilistic Prediction

Multioutcome logistic regression was used to generate correlations between normalized work and each 

level of lung injury severity. This advanced mathematical technique brings the greatest statistical power to 

the development of the correlation and allows INJURY

software to predict not only the expected injury, but

also the most likely distribution within a population 

(Figure 10-11).

Full Coupling to the Blast Test Device

The interface to INJURY software was changed to 

allow the software to directly use the pressure data 

files taken from the BTD. Data format standards were 

established, and standardized reports were generated 

with each analysis.44 A tightly coupled instrument 

and software analysis procedure was established that 

continues to be used.

Replacement of the Z-line in Military Standard 1474

From the release of the first version of INJURY

software, health hazard assessments for nonauditory 

blast have been made based on its analyses. As the

BTD gained acceptance, evaluation of occupational

exposure conditions in which nonauditory blast injury was suspected used the BTD–INJURY software 

combination to estimate the probability of lung inFig. 10-10. Modified Lobdell model. jury. Initially, the assessments were done by WRAIR

Ae + Af = Ab

Af

Af

Ae M3

M1

K12

C12

M2

K23i

K23

C23

Kve23 Cve23
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

physicians, and the upper limit on exposure was the

occurrence of 1% of the most trivial lung injuries. To

transition the technology to the US Army Center for 

Health Promotion and Preventive Medicine (CHPPM;

Aberdeen Proving Ground, Md—it is now known as

the US Public Health Command [PHC]), the Executive Agent for the Army’s Health Hazard Assessment

(HHA) Program, it was necessary to standardize the 

procedure and to interpret the predictions in terms of 

a risk assessment code (RAC). 

The RAC, used by CHPPM to assess the health 

hazards of military materiel, is a score from 1 to 5 that 

indicates the potential consequences of the hazard. A 

RAC of 5 represents a hazard that is negligible, whereas 

a RAC of 1 represents a hazard with catastrophic 

consequences. 

A second form of the blast injury assessment software called BOP-HHA (Jaycor, Inc)45 can analyze data 

from a complex test series in which multiple positions, explosive strengths, and repeated shot data are

collected. The software determines the probability of 

each of the four levels of primary blast injury, from

the trivial to the most severe, and incorporating both

biological and shot-to-shot variabilities. These probabilities are used along with the hazard matrix to

determine the RAC.

MOMRP sponsored a human volunteer study at the

Blast Test Site from 1989 through 1997.46–51 The test procedure exposed groups of individuals, each wearing

hearing protection, to an increasing sequence of blast 

noise conditions. (See Figure 10-12 for an example of

volunteers being exposed to blast from a simulated

mortar.) Pretest audiograms established baseline hearing profiles for each individual and ensured proper

fitting of the protective devices. Posttest audiograms

established the temporary threshold shift at a range 

of auditory frequencies. Temporary threshold shifts 

of 25 dB were taken as the recoverable failure limit

for exposure of an individual. Long-term tracking of

each individual showed that all hearing returned to

baseline levels.

The study produced about 2,000 subject-exposure

tests designed to determine the effects of blast intensity and number of exposures on recoverable failure.

As would be expected, the percentage of failures

increases monotonically with blast intensity and with 

number of exposures. The design of the experiment

envisioned groups of 64 subjects progressing through

the exposure matrix. That number of subjects would

allow an estimate of the probability of failure at the 95% 

confidence level.Invariably, some subjects dropped out

of each group, causing the significance of the results 

Fig. 10-11. Probabilistic prediction of lung injury. BOP: blast overpressure; NS: number of repeated exposures; Wsum: sum 

of normalized work from all sides of the body.

Trace Injury: BOP data

Trace Injury: Correlation

No Injury: BOP data

No Injury: Correlation

Moderate Injury: BOP data

Moderate Injury: Correlation

Severe Injury: BOP data

Trace Injury: Correlation

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

–14 –12 –10 –8 –6 –4 –2 0

Sj = 1.8772 In(Wsum) + 0.7752 In(NS)

Probability
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Blast Injury: Translating Research Into Operational Medicine

for a particular exposure condition to drop below the

target confidence level.52

An independent statistical analysis of the volunteer

data was conducted using logistic regression analyses.53

The subject-exposure tests were pooled into a pass-fail

outcome with blast intensity, duration, and number of 

exposures as independent variables. The large number

of data gave the analysis considerable statistical power

and resulted in an estimate of the failure criteria with a 

narrow confidence band. Results showed that the observed threshold for failure, a 25-dB temporary threshold shift, is nearly 10 dB higher than the current Military 

Standard limit. In addition, the analysis showed that the 

best correlation of blast injury decreases with duration, 

but is consistent with data on the risk of short duration 

exposure, such as rifle fire.

Taken together, the estimated human threshold 

tolerance to both auditory and nonauditory blast 

injuries are greater than that previously indicated in

Military Standard 1474. Formal revisions of the Military Standard to incorporate model-based criteria for 

both auditory and nonauditory injuries are undergoing 

consideration for adoption by the HHA Program. 

Secondary Blast Injury Modeling

In occupational settings, primary blast injury is the 

only concern. In combat, where blasts are much larger, 

debris can be propelled at the soldiers by the blast 

and lead to impact trauma or secondary blast injury. 

MOMRP has developed a biomechanical model of

the impact trauma that can be used to evaluate this

hazard.

In 1996, an interagency collaboration was formed—

involving USAMRMC; the Armament, Research,

Development, and Engineering Center (ARDEC); the

National Institute of Justice (Washington, DC); and the

US Department of Transportation (Washington, DC)—

to quantify projectile impact injuries in terms of the 

characteristics of the projectile (mass, velocity, shape,

and composition) and the region of the body impacted. 

The research collected and reviewed all previous studies and mathematical models on impact injury and 

selected the most credible set that would describe the 

widest range of blunt trauma and penetrating injuries. 

A software program, the Interim Total Body Model 

(ITBM; Jaycor, Inc), was developed and distributed to

the sponsoring organizations.54

The ITBM software depends on the characterization 

of the impact loads from the projectiles. Specialized instrumentation for measuring the magnitude, duration, 

and extent of the forces delivered by the projectiles was

developed.55,56 A specialized test apparatus was developed to accelerate various projectiles at controlled

speeds onto the instrumentation for determination of 

dynamic properties and other test protocols for static 

properties. The combination of the testing apparatus, 

protocol, and model estimation became the standard 

for nonlethal weapon assessment.

WRAIR conducted swine tests to determine internal

injuries from nonlethal projectile impacts. Test conditions covered an injury spectrum from mild skin bruising to through-and-through thorax penetration.

Subsequent to this initial work, the Joint Non-Lethal 

Weapon Directorate (JNLWD; Quantico, Va) was

formed, and in 2005, a Memorandum of Agreement 

between USAMRMC and JNLWP was formalized to

share technology and resources. Under this agreement, 

joint research continued on blunt trauma from projectiles that includes model development, and animal

and postmortem human subject testing. This research 

benefits both organizations by providing fundamental blunt trauma injury data to support USAMRMC 

soldier protection research programs and JNLWD

nonlethal weapons assessment programs. 

This partnership has produced new biomechanically based models for a wide range of secondary 

blast (impact) injuries. A finite element model of skin 

Fig. 10-12. Volunteers being exposed to simulated mortar

blasts.
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

response under impact has produced a model for 

penetration injury that takes into account the detailed 

anatomical structure and clothing.57 This model has 

been validated against animal studies at the Air Force

Research Laboratory (Brooks City Base, Tex) and historical data collected from the literature. Similarly, a 

biomechanically based correlate for rib fracture has 

been developed that accounts forthe detailed anatomical structure and material properties of bone.58 Similar 

cross-applications of head injury, skull fracture, and 

abdominal injury are being pursued. The ITBM, the 

Advanced Total Body Model (ATBM), and derivative

applications provide the best estimation of injury from

secondary blast effects.

Tertiary Blast Injury Modeling

The third mode of injury results from whole-body 

translation caused by the blast pressure differential 

and blast winds. These forces can hurl the body itself 

into surrounding objects and cause subsequent tertiary 

injuries. Although these injuries can result from blast, 

they are not unique to blast and have been studied in

many other circumstances. In 1993, MOMRP began a 

collaborative effort with NHTSA(Washington, DC) on

head and neck injuries—the most significant injuries 

that occur from body impacts. This collaboration has 

produced models of closed-head injury and head-neck 

injury that can be applied to tertiary blast injuries.

Combined Injury Modeling

Complications of exposure to nuclear explosions

include the following:

	 •	 physical trauma from the blast wave,

	 •	 burn trauma from thermal radiation, and

	 •	 disturbance to the immune system from the

ionizing radiation. 

Because of the possibility of sepsis and secondary infection, reduced immune function is especially 

dangerous when it occurs along with injury to the 

gut. In 1990, the Defense Nuclear Agency, now the 

Defense Threat Reduction Agency (Fort Belvoir, Va),

approached MOMRP for assistance in defining a relationship between blast and GI injury that could be 

used to assess battlefield consequences. As part of the 

joint work,Jaycor developed mathematical models and

a visual assessment of the nuclear battlefield showing

where combined injury effects could occur. In addition, 

a cellular-based model of the response of the immune 

Fig. 10-13. Example of blast injury data retrieved and displayed using IISYS software.
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Blast Injury: Translating Research Into Operational Medicine

system to ionizing radiation was developed.59 This 

model incorporated stem cell dynamics and the interaction of white cells with opportunistic infections. The 

results generalized the standard, empirical response 

curves previously used into a form that could study a

wider range of exposures and interventions.

Closing of the Blast Test Site

New residential communities around Kirtland Air 

Force Base objected to the noise from the blasts, making it increasingly difficult to schedule tests at the Blast 

Test Site. Furthermore, the success of biomechanical 

modeling in understanding and predicting blast injury 

greatly reduced the need for new animal studies. Consequently, in 1998, the Blast Test Site in Albuquerque, 

New Mexico, was closed.

Although the original questions about blast injury 

had been answered, data that had been collected for 

more than four decades of research could never be

reproduced. With this realization, Colonel Karl Friedl

(Director, Telemedicine and Advanced Technology

Research Center) identified materials of irreplaceable 

historical value (eg, the original investigator notes

from studies conducted at Hiroshima) and sent them 

to national libraries. Other remaining materials were 

shipped to Jaycor for safekeeping.

Under MOMRP support, these blast data are undergoing sorting, cataloging, and electronic preservation.

Figure 10-13 provides an example of the data that can

be retrieved from the database with IISYS software

(Jaycor, Inc). This process continually supports MOMRP’s ongoing blast injury programs, including the 

refinement and extension of INJURY software, the refinement of HHAsoftware for occupational exposure,

and studies of injury from thermobaric explosives.

THIRD-GENERATION INJURY MODELING

The third generation of injury modeling expands

the anatomical and physiological fidelities of the injury 

process. These models use finite element analyses to 

capture the mechanical response of the body to blast 

trauma, and high-fidelity, systemic simulations to 

capture the physiological response to inhaled toxic

gases. Finite element models allow predictions of 

regional injury patterns and the effects of protective

clothing. Physiological models extend the effects of

blast to include inhalation injury, incapacitation, and 

delayed lethality.

Finite Element Modeling 

FEM is a mathematical tool developed for analyzing engineering structures that is finding increasing 

application in biomechanics and trauma assessment. 

The approach breaks the object to be studied—in this 

case the human body—into small volumes or elements.

The elements correspond to parts of different tissues 

or organs, and they can be irregular in shape so that 

the anatomy is accurately represented. Because each 

element represents part of a single tissue, its mechanical properties are considered uniform. Each of the elements interacts with its neighboring elements through 

the common boundaries. As one element moves and

changes shape, these forces and motions (stresses and 

strains) are transmitted to neighboring elements. 

Asimulation begins with application of an external

loading, from the blast overpressure or a projectile impact, and the calculation advances in time to the motion

and distortion of the whole body and all of its internal 

elements. Every part of each tissue experiences stresses

and strains as the body responds to the external load;

when those quantities exceed the material strength of

the tissue or the organ, damage occurs. The power of 

the FEM approach is that, in principle, if the anatomy 

is adequately resolved by enough elements, if the material properties of each tissue are adequately known, 

and if the limiting stress/strain is known, then injury 

from any trauma can be predicted.

Each of these aspects (geometric features, material properties, and failure limits) are only imperfectly 

known, however, so FEM is still very much an art. First,

the human body is extremely complicated, with structures on every scale down to the cells; consequently,

it is not possible to generate a complete anatomical 

representation. Judgment must be used to resolve the

anatomical features that are most important for the 

process being studied. Second, almost no biological tissue is truly homogeneous; rather, it is made up of finer 

structural elements that make the apparent material 

properties dependent on the scale of elements being 

used. Again, judgment is needed to select material 

“constitutive” relations and their parameters that will

capture the mostrelevantresponses atthe scales ofinterest. Finally, most tissues, because of their complex composite nature, do not have simple failure stresses and

strains. The mechanism of damage can be partial, cumulative, and sometimes self-correcting. Furthermore,

the most important damage is often functional, rather 

than structural, so that simple mechanical failure is not 

the critical endpoint. For example, rupture of a blood

vessel can either lead to a small blood loss that is selfcorrecting or it can lead to a fatal hemorrhage, with only 

a slight difference in the extent of the material failure.
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

Despite the considerable challenges facing the application of these engineering concepts to biological 

systems, powerful results have been achieved and, in

time, many of the difficulties noted previously will be

overcome by the relentless increase in computational

power.

Finite Element Modeling in Blast Injury

One of the first applications of FEM to blast injury 

was the study of wave propagation in the lung under

blast loading.30,60 This work was conducted when FEM 

software was in its infancy, but it was able to demonstrate the critical nature played by the lung’s unique 

material properties on concentrating and distributing 

loading. The lung, because it has the compressibility of 

air but a density 100 times greater than air, has a very

low speed of sound—only about 30 to 40 m/s. When

the chest wall moves at a fraction of that speed (eg, 10

m/s), considerable pressures can build up at the inner 

surface of the thorax wall. When the velocities become

greaterthan 20 m/s, the equivalent of shock waves can

be produced that are very destructive. These insights

helped guide the early INJURY software models.

The next application was rupture of the tympanic

membrane.61 In this case, the geometry of the affected 

organ is far simpler than other parts of the body. The 

mechanical structure of the outer ear consists of the 

tympanic membrane, a tissue of well-known geometry 

and material properties; the ossicular chain, again with 

well-known geometry and properties; and the cochlea 

that, although it has a complex internal structure, acts

only as a flexible base for purposes of determining the

load on the ear drum. This model was able to provide

a biomechanical explanation and a predictive model

for tympanic membrane rupture. It even explained

some of the unique tearing patterns seen in this kind 

of injury.

The first application of FEM to the whole thorax was

used to calibrate the Lobdell model that was described 

previously. The Lobdell model is a nonphysiological

model of thorax response under impulsive loading.

The model specifies a mass of the sternum region, 

but not an area. Because blast waves apply a pressure

(force/area) to the body, it is necessary to determine 

the area that corresponds to the Lobdell model mass 

so that the model can be used to compute the chest 

motion. For this purpose, a FEM model of the human 

thorax was constructed from the Visible Human Project

data set. The anatomical detail was quite limited, with 

the model only resolving the chest wall, lung, heart,

and diaphragm. Because these are composite tissues, 

material properties were estimated from composites 

of the underlying tissue materials. The model was 

calibrated against human cadaver studies using impact

masses. From these simulations, the Lobdell model 

could be reinterpreted in terms of the true physiological elements and, in particular, the equivalent area of

loading determined. This model was then used to 

study blast loading, and a correlation was developed

to translate pressure loading into forces that could be 

applied to the masses in the Lobdell model. This key 

correlation forms the basis of the INJURY software 

versions 5.0 to 7.1.

The joint research program between MOMRP and 

NHTSA on blast and car crash injury produced a series of finite element models of the human skull that 

were used to set skull fracture injury criteria. Here, the 

geometry is relatively simple and can be accurately

determined by medical imaging. Jaycor developed

mathematical transformations to produce FEM models 

of the skull directly from imaging, including the resolution of the composite structure of the inner and outer 

tables and the diploe.62–64 These studies produced biomechanically based head-injury criteria that could be 

used in the automotive field for head impacts against

interior vehicle structures and for tertiary blast injury.

These same biomechanical models are used to predict 

focal skull fractures that arise from the head hitting 

very small protrusions or the secondary blast projectiles.65 These models are also being used to study the 

effects of head protection66 and the creation of a single 

skull fracture criterion for all insults. These models are 

used in the ITBM.

These four examples of FEM application were

developed to improve or validate other simpler

injury models and correlations. Beginning in 2000, 

MOMRP began a program with Jaycor to model the 

blunt trauma that occurs behind body armor when 

the armor stops a high-speed bullet with FEM as the 

final product. These models reproduce the anatomy 

in far greater detail than those used previously, and

required new medical imagery and new mathematical treatment to analyze, segment, and build robust 

elements. The models developed have been validated

with animal tests in which medical imaging is used 

to produce subject-specific models that are validated

against instrumented animal tests. 

Starting in 2003, MOMRP and JNLWD formulated

a collaborative research program to apply these highfidelity thoracic response models to the impact of 

projectiles. The result is ATBM, which uses finite element simulation to determine rib fracture and lung 

contusion based on projectile-specific, subject-specific, 

and location-specific conditions. JNLWP uses ATBM

to provide the most accurate estimate of nonlethal

weapon effects. The same model increases the fidelity 

of estimation of secondary blast injury. 
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Blast Injury: Translating Research Into Operational Medicine

In addition to better biomechanical response, ATBM 

computes the trajectory and impact locations of projectiles, including the determination of impact locations 

on individuals in a crowd. This capability, when applied to secondary blast injury, allows for computing 

the statistics of both injury locations and injuries under 

realistic combat conditions.

INJURY Software, Version 8

The latest version of INJURY software, released

in 2005 from the MOMRP Web site, introduces further anatomical refinement (Figure 10-14). Account 

is taken of the fundamental anatomical difference 

between sheep (narrow chest, wide sides—for which 

the majority of the injury data have been determined)

and humans (broad chest, narrow sides—for which 

the model is applied). In addition, body orientation is 

accounted for by distinguishing the front and back in 

the Lobdell model. This generation of the model has 

been validated against Blast Test Site data that have

been carefully quality checked, and the code offers a 

red-yellow-green characterization of the relative hazard of the blast exposure.

Toxic Gas Inhalation

In addition to the mechanical blast injuries, explosives produce toxic gases that, when inhaled, can lead

to disorientation, incapacitation, or death. Both health 

hazard and survivability assessments require a means

to estimate the probability of these endpoints from 

exposure to gases generated by blast or the resulting

fires.

The Live Fire Test Program (LFTP; Director of Operational Test and Evaluation) is a US Department of

Defense organization responsible for overseeing and

evaluating the survivability of US systems and vehicles

from enemy weapon systems.In 1994, the LFTPinvited

MOMRP to participate in the formulation of human 

effects from enemy weapon fire in the areas of blast 

and toxic gas. MOMRP entered into a series of projects

with Jaycor to evaluate existing toxic gas exposure

standards, identify shortcomings, and demonstrate 

improvements that could be made in the near term to

assist the LFTP. Small animal tests were conducted that 

highlighted the shortcomings of existing standards.67

These projects resulted in the airway blood chemistry 

model that was used to expand toxic gas survivability

assessments. This work identified that the shortcomings of existing standards arose from their lack of a

physiological basis, both to scale results from animals 

to humans and to account for critical physiological 

responses.

Following this initial evaluation, an effort to develop

a detailed physiological model of the body’s response 

to toxic gas inhalation was undertaken. This effort

produced a series of source books that reviewed and

summarized existing models and experimental data

from which the research effort had been planned. 

The first book of this series reviewed all previous

models, mechanisms, and assessments of biological 

effects caused by toxic gas inhalation.68 All existing

test data—from books, reports, and papers—were collected in a second book.69 These data were digitized 

and analyzed to evaluate current standards and to

identify missing data that would be needed to construct a physiologically based model. The third book 

of this series reviewed all control-of-breathing models,

because ventilation was the dominant physiological

factor missing from previous models.70

These reviews clearly identified the lack of knowledge of ventilation changes that occur during toxic gas

inhalation; thus, an extensive series of small animal

tests has been conducted atWRAIRand at the Lovelace

Respiratory Research Institute (Albuquerque, NM). 

These studies on ventilation effects, coupled with a

compilation of thousands of previous animal tests

on incapacitation and lethality, provide the basis for

Fig. 10-14. INJURY software, version 8.2, used to estimate

lung injury from blast exposure.
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

developing physiologically based models.

The first version of the Toxic Gas Assessment

Software (TGAS, Jaycor, Inc) provides estimates of

immediate incapacitation from any combination of 

seven toxic gases: (1) carbon monoxide, (2) hydrogen

cyanide,(3) hydrogen chloride,(4) nitrogen dioxide,(5)

acrolein, (6) low oxygen, and (7) carbon dioxide. The

gases can vary in concentration over time, and TGAS

accounts for animal species, mass, and activity. TGAS

estimates ventilation changes and tissue absorption

to compute a body-weight–normalized internal dose, 

accounting for all of the factors described previously.

The normalized internal dose is used to determine 

a dose-response curve that is species and exposure

independent. The model predicts the probability of 

immediate incapacitation that can provide a toxicological assessment of survivability for both military and

civilian applications.71

TGAS version 2.0 provides estimates of immediate

incapacitation and immediate and delayed lethalities 

from combinations of these same gases.72 Many irritant 

gases produce lung injury that can lead to death long 

after exposure. Because the exposure levels that can

cause these effects might be much smaller than those 

that cause incapacitation, it is necessary to set standards based on the most limiting condition.

Explosions often occurin enclosures or vehicles that

have fire suppression systems. These systems often

contain halide compounds, which can break down 

by pyrolytic processes and introduce acid gases into 

the enclosure atmosphere. These acid gases are toxic

themselves and have a strong effect on ventilation. Furthermore, the explosion can cause secondary fires that

release other complex toxic gases. The combination

of these primary and secondary effects of explosions

leads to an even more complex mixture of noxious

gases that must be considered to determine incapacitation and lethality.

In the past 30 years, physiologically based pharmacokinetic (PBPK) models have been developed to

describe the physiological effects of the inhalation of 

harmful organic gases. These models are used to set 

internal dose limits, based primarily on blood concentration levels, for each compound. This internal

dose approach allows animal data to be scaled to 

humans and allows effects due to time-varying gas

concentrations to be estimated from data collected 

under constant conditions. The primary use of PBPK 

models has been in setting short-term environmental

exposures, but these results are readily assimilated into

the TGAS framework.

The first step taken was to survey the current literature on PBPK modeling and the known mechanisms 

of interaction of halocarbon materials with the body. 

The development of PBPK models involves replicating with mathematics the transport, metabolism, 

and elimination of inhaled chemicals. Furthermore, 

the chemical and physiological parameters used by 

these models must be estimated from in vitro tissue

measurements or limited animal testing. These quantities generally have large variations in their values

and require a systematic means to come to a single, 

accepted value. The mathematical basis for PBPK

models, their variation in application, the techniques

for establishing parameter values, and the methods for

interpreting and using the results are summarized in a 

comprehensive source book.73 Although PBPK models 

are used in many aspects of physiology, considerable 

literature exists just for halocarbons. Not only are

these substances important to fire-extinguishing applications, but also many are ozone depleting and are 

being eliminated from global use for environmental

reasons. The sheer number of possible compounds 

that vary in importance—depending on their environmental impact, chemical composition, and/or 

pyrolytic effects— makes this area extremely complex.

These compounds and the PBPK models that have

been developed to understand their biological effect

are discussed in a second source book.74 Finally, not 

only are the chemical and biological impacts of these 

substances complex, but also so is the regulation that

controls their use and replacement. Because devising

appropriate fire protection schemes is a component 

of blast mitigation, the regulatory landscape of this 

complex area is analyzed and summarized in a third

source book.75

PBPK models include a respiratory component that 

determines the amount of chemical inhaled during the 

exposure. Because these models have been developed

at different times and by different researchers, there is 

little consistency in the selection of normal physiological parameters, especially ventilation. Furthermore,

all of the models assume that ventilation is unaffected

by the gases inhaled and ignore any interaction in 

ventilation caused by gas mixtures. A PBPK model

(which uses common physiological parameters) has 

been developed that has been recalibrated against all

halocarbon test data available in the literature. Furthermore, this model has been coupled to the TGAS 

version 2.0 model for describing ventilation changes

from chemical and exercise effects. The combined

model can analyze more than 30 gases in combination 

and is designated TGAS version 2.0P.76

TGAS versions 1.0 to 2.0P account for ventilation

changes with factors that vary with the external concentrations of the gases. The animal studies, however,

show that these ventilation changes vary with time in

a complex way. Many of the changes do not occur for
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Blast Injury: Translating Research Into Operational Medicine

several minutes in small animals, corresponding to the

time required for critical blood chemistry changes to 

occur. Because of large differences in the ventilation-tobody mass ratio between small animals and humans, 

these effects might take longer to occur. Because some 

toxic gases suppress ventilation, whereas others enhance it, ventilation is critical to properly account for

the temporal and species effects in extrapolating small

animal results.

To account for these complex ventilation changes,

a Dynamic Physiological Model (DPM) was developed.77 The DPM includes models of ventilation,

circulation, and metabolism and, most importantly, 

models of the neural control systems that govern these

processes. The neural controls are driven, in turn, by

blood chemistry changes sensed by chemoreceptors 

in the carotid arteries and the medulla oblongata of 

the brain. Consequently, the DPM contains extensive

blood chemistry models and models for the metabolism in the brain. This model reproduces most of the 

significant ventilatory changes seen in animal tests and

provides a physiologically based means to extrapolate

those critical changes to humans (Figure 10-15). 

Blast Effects Behind Body Armor

As the US military is engaged in urban and asymmetric warfare, soldiers are increasingly exposed to

blasts from nonmilitary explosives. In the Afghanistan and Iraq conflicts, nearly 65% of all casualties are 

from blast injury. Because of the concern that ballistic 

body armor may have some unexpected amplifying effect on blast injury, the Natick Soldier Center 

(Natick, Mass) has teamed with MOMRP to make a 

scientific determination of the effects of body armor 

on blast injury.

MOMRP is assisting this effort in two key areas. 

First, the considerable experience of WRAIR in animal

tests has been applied to assisting the Natick Soldier 

Center to conduct animal tests that will provide hard

evidence of the effect of blast on armor and the resulting injury. The experience of Jaycor is being used

to analyze test data, utilize mathematical models to 

interpret animal results, and extrapolate the findings

to humans. This effort is providing further validation

of the INJURY software model, especially in extreme

environments where lethality occurs.

Fig. 10-15. Ventilation response to acute carbon monoxide exposure.

Experimental Data

Upper Range

Lower Range

Dynamic Physiology Model

1.5

1.4

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Minute Ventilation/Body Mass (L/min/kg)

Time (min)

–5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

To support armor evaluation, a specialized BTD to

measure the force under clothing and armor is being 

developed. This new device, coupled with a new series

of INJURY software models, will provide the assessments needed to determine the vulnerability of current

protection systems.

Fig. 10-16. Integrated finite element model of the head, 

neck, thorax, and abdominal regions.

FOURTH-GENERATION INJURY MODELING

The fourth generation of injury modeling integrates

mathematical models of the mechanical response of 

the body with those describing the physiological and 

systemic responses. These integrated models will predict physical and cognitive performance consequences,

not just pathological outcomes, and will address 

neurocognitive injury that occurs through secondary

processes. These models are important steps toward 

a complete, quantitative understanding of biological

response to all traumas.

Performance Endpoints

Previous modeling of blast effects has focused on

predicting injury and lethality. Although these are 

important endpoints, they do not characterize the 

complete hazard caused by blast nor the full operational impact. At exposures far less than required to

produce immediate incapacitation and death, blast 

can degrade the physical and mental performance of 

warfighters, with the consequence that they may be 

unable to accomplish the mission or to protect and 

defend themselves against other lethal threats.

The goal of the Physical and Cognitive Performance

Modeling Project is to develop and validate mathematical models that predict these performance endpoints. 

The work will combine the finite element models used 

to characterize the physical damage of blast trauma 

with the DPM used to characterize the physiological 

consequences. The integrated finite element model 

consists of models that describe the head, neck, thorax,

and abdominal regions (Figure 10-16). The DPM is 

used to estimate oxygen delivery and incapacitation

in hypoxic and toxic gas exposures78–82 (Figure 10-

17). Integration of these two modeling systems will 

produce a complete description of the physical and 

physiological responses to blast trauma.

Central Nervous System Injury

The possibility that blast is responsible for the 

traumatic brain injury (TBI) observed in returning

warfighters has become an issue of national importance. Similar concerns exist in sports and automotive

safety. Blast-induced TBI involves a complex and not

fully understood mechanical pathway that is different 

from that encountered in civilian settings. The ability

to assess the magnitude of this risk, prevent its occurrence, and detect and diagnose it may be enhanced by a 

predictive methodology that can link traumatic events

to meaningful neurocognitive measures and clinical

outcomes. The Predictive Model of the TBI project will

use focused laboratory and clinical tests to strengthen 

the links in the causal path and mathematical model-
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Blast Injury: Translating Research Into Operational Medicine

ing to tie the links together into a usable predictive

methodology.

Epidemiological data from Operation Enduring 

Freedom/Operation Iraqi Freedom (OEF/OIF) are correlating mild TBI to blast. A recent study indicated that 

59% of blast-injured patients from OEF/OIF admitted 

to Walter ReedArmy Medical Center had at least mild

TBI.83 In that study, data were provided by Dr Deborah

Warden, Director of the Defense and Veterans Brain

Injury Center, based on patients seen at Walter Reed

Army Medical Center. Of the 450 patients with brain 

injuries treated between January 2003 and February 

2005, most brain injuries were caused by IEDs, and 

closed-brain injuries outnumbered penetrating ones. 

It is believed that cases are likely underreported because of lack of prompt diagnosis. Mild TBI adds a 

significant, long-term challenge to patient recovery

and rehabilitation, especially when other permanent 

disabilities are involved, such as loss of extremities

that requires the patient to learn new skills. It has also 

been observed that mild TBI from the current conflict

accounts for a larger proportion of casualties than in 

other recent US wars. 

Extreme head trauma from blunt impact leads to

massive damage and hemorrhaging that have been

studied extensively. Thresholds for high-mass, lowspeed impact head injury have been based on tissue

pathological observations in primates and postmortem human subjects. Although these thresholds are 

not based on biomechanics and neurological modeling, they have served the automotive safety needs.

Mild TBI is now being seen in returning soldiers, as 

well as in sports, and has a far more subtle injury 

mechanism, a more subtle clinical outcome, and a 

much lower threshold. Thresholds for these lower 

level injuries will not be developed readily from

epidemiological data alone and are not easily studied 

in animal models. A greater understanding of the 

underlying mechanical processes will be required. 

Furthermore, blast presents additional challenges 

because the primary mechanism of action is not blunt 

impact to the head, may involve other mechanical

pathways, and is far more rapid than almost all previous head injury causes.

Fig. 10-17. Schematic diagram of circulatory and respiratory systems in the Dynamic Physiology Model. Temp: temperature; Vol: volume.

External Temp

Clothing

Wind Velocity

External 

O2, CO2

Altitude Activity

Body

Temp

Blood

Chemistry

O2, CO2 Conservation

Heat Conservation

Stroke Vol

Heart Rate

Arterial

Resistance

Cardiac Control

Arterial

Resistance Control

Frequency

Drive

Breathing Control

Shivering

Sweating

Thermal Control

Blood Flow

Distribution

Circulation

Air Flow

Distribution

Ventilation
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

Causal Links of Trauma-Caused Brain Injury

It is generally accepted that TBI is caused by local 

mechanical stresses in the brain that disrupt neural 

cells and tissues (primary injury), which, in turn, 

leads to functional disruption through metabolic and 

biochemical processes (secondary cascade), resulting in the clinically observed neuropsychological

consequences. Although the physical, chemical, and 

neurological processes are not rigorously quantified, 

there is ample evidence that the primary injuries lead

to the neurological consequences that are seen in patients with TBI.

Mechanically caused primary injury has been 

observed in animal models for both impact and

blast trauma. In blast animal models, astrocyte and 

microglial responses have been seen,84,85 as well as 

changes in apoptotic mechanisms.86 Histological studies have detected degenerating neurons in the cerebral

cortex and hippocampus of rats 1 and 5 days after

blast exposure.87,88 Evidence of ultrastructural and

functional changes following blast neurotrauma has 

also been observed.88,89 Chemical alterations leading 

to motor and cognitive dysfunction follow diffuse

TBI in rats.90,91 In isolated tissue tests, injury to cells 

resulting from the application of rapid mechanical 

forces has been observed in endothelial cells,92 in 

axons,93–96 and in cortical astrocytes and neurons.97

Evidence of tension as a regulator of axonal activity

has also been observed.98 In addition to tension and 

shear-induced cellularinjury, there is also evidence of

cellular damage from direct pressure effects. Cavitation, which has been shown to occur at moderate head 

acceleration, leads to large local pressures (thousands 

of atmospheres) upon vapor bubble collapse.99 This 

mechanism has been confirmed in lithotripsy studies.100 Depolymerization of spindle microtubules has 

been observed at very high static pressures (680 atm)

and in ultrasound exposures (low-pressure amplitudes, but rapid fluctuations). 

The blast-induced primary mechanical damage 

to cellular structures starts a secondary cascade of 

effects, including ion channel disruption, apoptosis, 

and necrosis. These secondary processes lead to 

functional and metabolic dysfunctions that are observable in advanced magnetic resonance imaging

(MRI). Quantification of the link between observable

microscopic damage and MRI parameters is the fourth 

link in the predictive chain. Observation of secondary

brain injury, in the form of biochemical alterations, has 

been observed in blast injury.101–104 More importantly, 

however, it is well known that secondary injury is a

complex cascade of physiological events caused by

the residual metabolic, biochemical, endocrinological, 

and immunological alterations initiated by primary 

injuries.105,106

These secondary effects are readily observable with

magnetic resonance spectroscopy (MRS). MRS is a 

powerful in vivo method to detect neurochemicals

within the brain that are relevant to brain processes.

The most widely used methods are 1H-MRS and 31P-MRS that detect compounds with hydrogen and 

phosphorus. Recent studies have shown that the

absolute concentrations or ratios of concentrations, 

particularly N-acetylaspartate (NAA), correlate with 

neuropsychological tests, cognitive dysfunction, and

early detection and longitudinal change.107 MRS has 

provided correlates with the following:

•	 cognitive decline in the aging brain108; 

	 •	 cognitive outcomes in children109; 

	 •	 the Wechsler Memory Scale110,111; 

	 •	 genetically basedWilliams syndrome112,113;and 

	 •	 diseases such as bipolar disorder,114,115

Alzheimer’s,116,117 and schizophrenia.118

MRS has correlated with posttraumatic stress disorder (PTSD) in combat veterans,119–121 combat veterans

with evidence of biochemical change,122 prisoners of 

war,123 and nonprisoners of war.124 In some studies, 

however, patients with PTSD did not score differently

on attention, memory, or learning tests,125 nor did they 

show lower levels of NAA.126 Proton MRS also has 

mood disorders.127–133 MRS imaging has correlated with 

functional independence measure,134 Glasgow Outcome Scale, and neuropsychological performance.135

Blast Traumatic Brain Injury Mechanisms

Once mechanical stresses have created primary cellular injury in the brain tissue, subsequent biochemical, metabolic, and neurological processes will lead 

to neurocognitive dysfunctions and adverse clinical

outcomes. The unique challenge of blast TBI is to determine how the external stressors (mechanical, chemical,

and electrical) created by exposure to explosions create and amplify those of primary cellular injury. The 

Predictive Model of the TBI project will build on the 25

years of blast model experience and tools to determine

a quantitative connection. The following sections frame

the nature and magnitude of the blast stressors that are 

likely to be involved in blast TBI.

Characterization of Blast Exposure

 Blast injury is initiated by the mechanical loading 

of the blast wave on the body, with secondary effects

of the toxic gas and electromagnetic pulse released
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Blast Injury: Translating Research Into Operational Medicine

by the explosive. The strongest blast waves for which

there is a high probability of survival have a peak

pressure of ~20 atm and a duration of a few milliseconds. Using CFD and data compiled in the Blast 

Overpressure Database, there is adequate information

to characterize blast threats faced by US warfighters 

and to select conditions that will be meaningful for 

laboratory investigation. Characterizing the blast

effects is the first step in developing an end-to-end

predictive model.

IEDs can span an explosive energy range from the

equivalent of a few pounds of TNT (trinitrotoluene) in

pipe bombs to thousands of pounds in truck bombs. 

Distances from these explosions to the warfighter can

also vary, and the blast loading can be amplified by

reflections inside enclosures. The ranges of interest 

for studying TBI, however, are those exposures that

will not otherwise produce severe injury or death.

The biomechanically based INJURY software, used 

throughout the military to estimate blast injury for all 

levels from occupational exposure limits to survivability estimates,136–138 is used to select exposure conditions

that are in a meaningful range (Figure 10-18). Based on 

INJURYanalyses and confirmed by the extensive blast

overpressure data repository, it is concluded that the

strongest blast waves for which there is a high probability of survival have a peak pressure of ~20 atm (as

described above) and a duration of a few milliseconds.

Pathway of Internal Disturbances 

Acceleration of the brain from direct blast loading 

to the head or a surge of blood into the brain from 

blast loads to the torso can produce pressure excursions, flow pulses, and mechanical stress far outside 

the normal physiological range that can cause tissue 

and cellular damage. Furthermore, temporary disruption of oxygen delivery from immediate neurological

response and electrical disruption of cellular processes 

from the explosion-generated electromagnetic pulse

may further compound these primary injuries. Modeling these pathways is a critical part of building a 

Predictive Model and designing protective solutions.

Acceleration Effects. A blast wave travels at ~300

m/s, so one side of the head/body is affected a fraction of a millisecond before the other. This differential 

pressure can accelerate the head, and simulations 

show that, at the maximum survivable peak values,

the head can be accelerated up to 300g’s. Jaycor has 

determined that the threshold for mild TBI, based on 

sports injury reconstruction,139 is 50g’s, so that head 

acceleration from blast is certainly a potential injury 

pathway. Acceleration also introduces a hydrostatic 

pressure gradient in the brain, with a pressure difference equal to the acceleration x density of the brain 

x half the width of the brain. For a 300-g acceleration, 

the brain could experience regions of compression as

Fig. 10-18. INJURY correlation of lethality. BOP: blast overpressure; Wtot: Wsum corrected for the number of exposures. Probability of Lethality

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Logistic regression for lethality

95% confidence bands

BOP data with 10 sample bin size

0.001 0.010 0.100 1.000

Wtot
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

great as 3 atm on the impact (coup) side. Accelerations 

greater than 80g’s are capable of lowering pressure 

in the brain to a point wherein cavitation occurs on

the opposite (contrecoup) side. Collapse of the vapor

bubbles can lead to large pressures in excess of 1,000

atm,116 which can cause considerable local damage. 

This phenomenon has been demonstrated in surrogate 

models and in mathematical simulations,140,141 and is 

one of the primary injury mechanism candidates in 

head impact.140–146 Simulations further show that small 

flexure of the skull can extend the cavitation to large

areas of the brain.141 The biomechanical FEM of the 

head provides a way of translating external loading

into stress in the brain tissue (Figure 10-19). 

Vascular Surge. Distortion of the thorax and abdomen is observed under blast loading, arising from

the presence of air-containing organs in the torso. In 

the thorax, the inward-moving chest wall produces a

compression wave in the lung that not only does damage to the lung tissue, but can also exert a compressive

force on the heart. The application of blast loading to 

the abdomen produces a direct coupling, through the 

abdominal contents, to the diaphragm, leading to an 

upward motion that also exerts a force on the heart—a

“blast punch to the gut.” Both of these processes can

lead to a surge in blood flow. Brain injury from blast 

animal models, whose heads were protected from the 

direct blast,100 might be explained by this mechanism.

Finite element analysis shows that a 20-atm blast loading against the torso produced a small, volumetric

surge of blood from the heart of about 0.2 mL as a 

result of the load transmitted to the heart through the 

lungs; but, it also produced a 10-mL surge as a result 

of loading delivered through the abdomen (Figure 10-

20). Assuming the normal distribution of blood flow, 

about 2 mL of the surge will be delivered to the brain.

When this volumetric surge reaches the brain, the sudden increase of blood volume produces a subsequent

pressure increase. Simulations using the L-3/Jaycor 

head model estimate that an intracranial pressure of 

nearly 10 atm can result. Combined with the increased 

crushing effect of personal body armor, this mechanical 

pathway is being studied in the project.

Mechanical trauma caused by the blast waves, blast

winds, or secondary impacts is most likely the primary 

cause of TBI. Nonetheless, there are auxiliary effects

that are associated with explosives that could augment

or aggravate the mechanical injury.

Hypoxic Effects. It is commonly observed that animals exposed to blast suffer disruptions in ventilation

and cardiac output for 10 s following blast exposure

and reductions of these outputs that persist for many 

minutes. In addition, explosions produce noxious

gases that have other acute effects on these processes.

Fig. 10-19. Deformations of the skull coupled with head 

acceleration can create regions of high stress concentration 

within the brain.

In our studies, the effect of toxic gas on oxygenation

of the blood has shown that brain ischemia effects can 

occur in minutes under acute conditions. This process 

has been quantified with the DPM.147

Electromagnetic Pulse. The overpressure of the

explosion accelerates hot, ionized gases in the fireball

to a velocity exceeding the ambient speed of sound,

and the rapid motion of charged particles results in 

a radiated electromagnetic pulse (EMP). Soloviev et

al148,149 provide a summary of field data collected from

conventional high explosives (1–5 kg) and a theoretical model that can be used for extrapolation. They

conclude that, in the far field of these explosions, the

electric field component of the EMP acts as an electric 

quadrupole—that is, the strength decreases as the 

fourth power of distance—with strength proportional 

to the explosive weight.

It is well established that significant thermal effects 

are produced from continuous, incident power levels

greater than 100 mW/cm2. At this flux of energy, the

body cannot dissipate the heat rapidly enough, and tissue temperatures rise, leading to a wide range of physiological effects and damage.Afew tests have looked at

nonthermal effects on cerebral tissues of short-duration 

pulsed exposures (perhaps like repeated blast expo-
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Blast Injury: Translating Research Into Operational Medicine

sure) or modulated high-frequency exposures. Servantie et al150 saw changes in the electroencephalogram in 

rats at pulsed exposures: 0.1-μs pulses, average power

5 mW/cm2; and 500 to 600 pulses/s, for 10 days. Bawin 

et al151–154 observed changes in Ca2+ release in chicken 

cerebral tissue after 10 minutes of exposure to 0.75

mW/cm2 of modulated 450 MHz radiation, which is an 

energy dose of 450 mW-s/cm2. At 10 m from a 100-kg 

explosive, the blast wave is predicted to produce only

1% lethality, whereas Soloviev’s relation predicts that

the EMP energy dose will be about 500 mW-s/cm2. 

Therefore, there is a possibility that blast-generated 

EMP, under the right circumstances, may augment the 

Ca2+ channel disruption that is initiated by primary 

mechanical injury.

Biomechanical and Physiological Response Models

The Predictive Model of the blast TBI project will

build on Jaycor’s integrated finite element models of 

blast and trauma responses. Additionally, the parallel 

work to integrate FEM and DPM will greatly assist 

the blast TBI project. Surrogate large and small animal 

testing and clinical examination of civilian and military

patients with TBI will provide quantitative data for

each causal link and a validation of the overall predictive methodology.

The research partnership between USAMRMC 

and NHTSA has advanced finite element models of

the head that combine anatomical geometry with 

the mechanical properties of the tissues to provide

a powerful calculation tool. This partnership has 

spurred a number of studies into dynamics of the 

entire brain,155 tissue properties,156–161 and individual

cells.94,162 Because of the availability of high-resolution

anatomy and dynamic tissue properties, the ability to 

determine local mechanical stresses within the brain 

is well advanced.

As in decades past, the USAMRMC/MOMRP foresight of developing fundamental mathematical tools to

understand blast/trauma injury may allow an issue of 

national importance to be addressed quickly, with the 

potential for developing both a scientific understanding and a strategy for prevention and mitigation.

Fig. 10-20. Schematic diagram of mechanical response 

model for binding structures in neurological cells.

SUMMARY

For more than 25 years, MOMRP has been a world 

leader in the study of blast-related injury. This research 

has produced insights, data, and predictive models

that set standards for human exposure, guide the design of protective systems, and allow quick responses

to the questions generated in a rapidly changing battle 

environment.

The work of MOMRP continues to address the 

new blast issues of the 21st century. Together with its 

partners, MOMRP is researching injury to the central 

nervous system; protection of current body armorfrom

emerging thermobaric weapons; and prediction of 

performance decrement, both physical and cognitive,

under the stress of blast and other trauma.

The decision (made decades before) to use mathematical modeling as a lens to focus the understanding 

of data; to guide future experiments; and to project

understanding to complex, real-life scenarios on the

battlefield has resulted in a capability that is responsive to new challenges. Furthermore, the ability to

simulate biomechanical and physiological aspects of 

the body under traumatic circumstances can be applied to interests of health, safety, and performance 

of soldiers.
294

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