The Mechanobiology and Mechanophysiology of Military-Related Injuries

In this chapter, a review of the current state-of-the-art in techniques, efforts and ideas in the area of modeling skeletal injuries in military scenarios is provided. The review includes detailed discussions of the head, neck, spine, upper and lower extremity body regions. Each section begins with a description of the injury taxonomy reported for military scenarios for a particular body region and then a review of the computational modeling follows. In addition, a brief classification of modeling methods, tools and codes typically employed is provided and the processes and strategies for validation of models are discussed. Finally, we conclude with a short list of recommendations and observations for future work in this area. In summary, much work has been completed, however, there remains much to do in this research area. With continued efforts, modeling and simulation will continue to provide insight and understanding into the progression and time course of skeletal injuries in military scenarios with a high degree of spatial and temporal resolution. However, more work is needed to improve mechanistic-based modeling of injury mechanisms, such as fracture, and increase the inclusion of bio-variability into simulation frameworks.

Military static-line parachuting is a highly tactical and hazardous activity, with a well-documented injury risk. Due to the high impact forces and rapid rate of loading when a parachutist lands, injuries most frequently occur to the lower limbs and the trunk/spine, with ankle injuries accounting for between 30 and 60 % of all parachuting injuries. Although military static-line parachuting injuries can be sustained at any time between the paratrooper attempting to leave the aircraft until they have landed and removed their harness, most injuries occur on landing. Throughout the world, various landing techniques are taught to paratroopers to reduce the risk of injury, by enabling parachute landing forces to be more evenly distributed over the body. In this chapter, we review research associated with static-line military parachuting injuries, focusing on injuries that occur during high-impact landings. We summarize literature pertaining to strategies for military paratroopers to land safely upon ground contact, especially when performing the parachute fall landing technique. Recommendations for future research in this field are provided, particularly in relation to the parachute fall landing technique and training methods. Ultimately, any changes to current practice in landing technique, how it is taught, and whether protective equipment is introduced, should be monitored in well controlled, prospective studies, with the statistical design accounting for the interaction between the variables, to determine the effect of these factors on injury rates and paratrooper performance. This will ensure that evidence-based guidelines can be developed, particularly in relation to landing technique and how this is trained, in order to minimize injuries associated with landings during military static-line parachuting in subsequent training and tactical operations.

Loads carried by the Warfighter have increased substantially throughout recorded history, with the typical U.S. ground Soldier carrying external loads averaging 45 kg during recent conflicts. Carrying heavy loads is one potential source of injury that has been researched from a performance and epidemiological perspective. This chapter focuses on the biomechanics of military load carriage, primarily focusing on lower extremity joint stresses and potential overuse injury mechanisms that may be associated with carrying a load. Studies into the biomechanics of load carriage have documented motion-related differences such as increased step rate, decreased stride length, and more trunk lean with increases in pack-borne loads. Ground reaction forces have been found to increase proportionately with loads up to 40 kg. However, there is a paucity of literature on the relationship between load carriage and biomechanical mechanisms of overuse injury. Findings of recent studies will be presented which add mechanistic information to increased stresses on the lower extremity. Efforts to model injury mechanisms require continued biomechanical measurements in humans while carrying occupationally-relevant loads in order to be validated. In addition to lab-based biomechanics data needed to further explore the mechanistic relationship between load magnitude and injury, technologies should be exploited to accurately quantify stresses related to load carriage the field.

Since the beginning of recorded history, soldiers have been required to carry arms and equipment on their bodies. Recently, the weight of these loads has substantially increased due to improvements in weapons and personal protection. As soldier loads increase there are increases in energy cost, altered gait mechanics, increased stress on the musculoskeletal system, and more rapid fatigue, factors that may increase the risk of injury. Passive surveillance of injuries experienced by soldiers on load carriage missions and surveys of hikers and backpackers indicate that foot blisters, stress fractures, compression-related paresthesias (brachial plexus palsy, meralgia paresthetica, digitalgia paresthetica), metatarsalgia, knee problems, and back problems are among the most common load carriage-related maladies. This article discussed these injuries providing incidences, rates, symptoms, mechanisms, and risk factors, and provides evidence-based preventative measures to reduce injury risk. In general, lighter loads, improving load distribution, using appropriate physical training, selecting proper equipment, and using specific techniques directed at injury prevention will facilitate load carriage. An understanding of injury mechanisms and implementation of appropriate prevention strategies will provide service members a higher probability of mission success.

The most common reason of medical evacuation for non-combat related injuries appears to be related to the musculoskeletal system. This is reported during both military deployments as well as during basic combat training. The most common cause of non-combat musculoskeletal injuries appear to occur from overuse, generally as a result of physical training. Overuse injuries are considered an outcome of the overtraining syndrome, which is considered a continuum of negative adaptations to training. Symptoms appear when the training stimulus has reached the point where the intensity and or volume of training have become too excessive, coupled with inadequate rest and recovery. These are issues that are quite common within the military during both training and deployment. During periods of deployment additional physiological stresses such as the environment (altitude, cold and heat), and nutritional and sleep deprivation may pose significant challenges on the health and performance of the soldier. This is often manifested during sustained combat operations, in which the ability to provide rest and recovery become secondary to the mission’s objectives. This chapter will focus on the frequency, mechanism and risks associated with overuse injuries reported during both military training and deployment.

Stress fractures (SFs) are of the most common and potentially serious overuse injuries. Many athletes, naïve exercisers, and military recruits who are engaged in frequent and repetitive activity may suffer a SF; the most common site for SF is the tibia. SF is regarded as fatigue fracture—when training yields bone strains in a range where the micro-damage formation in the bone exceeds the ability of a remodeling process to repair it and ultimately this cumulative tissue damage might result with a spontaneous fracture. The registry of SFs among athletes is incomplete, but in military recruits the incidence of SFs range between 5 and 12 % (female soldiers are 2–10 times more prone to SFs compared to their male counterparts). Recovery from a SF is primarily achieved by halting any load bearing activities and on rest. This might be detrimental to athletes and military recruits, as results in loss of training days and consequently a reduction in physical capacity. The ample risk factors for SFs can be categorized as internal factors depending on the individual (e.g. gender, bone geometry) and external factors (e.g. training volume). It follows that in many cases SFs are preventable. Recruits engaged in a reasonable level of physical activity, especially impact exercise in the years prior to joining the military, and also maintain adequate nutrition, may lower their risk for SFs. Yet, several fundamental issues in regard to SFs are still left unresolved. For example, how muscle forces provide a protective effect against SFs, how many cycles (i.e. steps or strides) can an individual perform before he or she will be at a risk of suffering a SF, or is it necessary to implement prophylactic interventions in order to protect those who are identified at a greater risk? New experimental tools and improved computational modeling frameworks for investigating and better addressing the above questions that are reviewed in this chapter can be used to improve the knowledge on the etiology and prevention of SFs.

An increasing number of women are serving in militaries around the world. Overuse musculoskeletal injuries (OMI) are common with military activities in both sexes but are more common in female soldiers, in part because of differences in whole body and tissue-level biomechanics. Sex-based differences in whole body biomechanics such as stride length, knee valgus, and others may help explain differences in OMI risk. Further, tissue-level sexual dimorphisms in body composition, muscle, and bone, also contribute to the higher risk of OMI in female soldiers. Understanding these biomechanical differences will help militaries tailor preventative measures towards female soldiers at high risk of OMI.

Traumatic brain injury is relatively common in military and law enforcement activities, despite ongoing improvements in head protection gear and in medical aid procedures and evacuation equipment in battlefield and conflict scenarios. In this chapter, we provide the relevant anatomical and physiological background which is relevant for understanding the occurrence and consequences of a traumatic brain injury and its subcategories. Next, we review the biomechanics of traumatic brain injury, and describe biomechanical injury criteria and thresholds. Finally, we introduce the concepts of modelling brain injuries by means of finite element techniques which consider the biomechanical properties of the head and neck tissues. The possible applications of such computational modelling and simulations, particularly for developing and testing military head-protection equipment, are discussed as well.

Eye injury accounts for approximately 15 % of battlefield injuries worldwide. Most eye injuries are not life-threatening and are not a high priority on the battlefield. However, these injuries can result in a severe loss of function, diminished quality of life, and decreased career opportunities for military personnel following completion of service. The complexity of the eye is similar to the complexity of the brain in that trauma to the eye results in primary acute injuries with secondary and long term sequelae leading to visual dysfunction. To date, the biomechanics and resulting injury of ocular trauma has been a fairly underrepresented area of research. This has changed as a result of the recent wars and conflicts, but there is still a lot that is unknown or not well understood. This chapter will provide an overview of ocular anatomy, types and causes of ocular injury found in the military, and a review the state of knowledge in biomechanics of ocular trauma from blunt impact and blast exposure. Careful evaluation of the current literature will identify urgent areas of focus, establish guidelines for future biomechanics research, and lead to the development of better strategies for the detection, assessment, and prevention of ocular injury in military personnel.

Contemporary military environments almost invariably require the use of personal protective clothing and equipment, and the burden accompanying its use can sometimes challenge the integrated regulation of critical physiological variables, pushing some individuals to the limits of regulation. Indeed, it is not uncommon for work to be prematurely terminated due to cardiovascular insufficiency. In such states, operational capability is reduced. In this Chapter, four topics will be addressed, including the impact of battle-dress uniforms, ballistic protection, undergarment moisture management, and chemical and biological protection. The principal emphasis is upon thermal and cardiovascular regulation in the person-clothing-environment system. For battle-dress uniforms, body heat storage is modelled using thermodynamics algorithms, with a three-dimensional summary presented to identify combinations of work rates and thermal exposures that yield positive heat storage. When ballistic protection is considered, one must evaluate both the impact of the added mass and the impediment it presents for dry and evaporative heat exchanges. Various moisture management practices are being marketed to address these matters. However, evidence will be presented that these do not offer measurable thermoregulatory or perceptual benefits when used beneath battle dress and ballistic protection in operational simulations. Finally, the most stressful scenario relates to protecting individuals from chemical, biological and radiological challenges. Indeed, working in such encapsulating ensembles can only be tolerated for short durations without supplementary cooling.