Authors: Alexander T. McDaniel and Lindsey H. Schroeder
School of Health and Applied Human Sciences, University of North Carolina Wilmington, Wilmington, NC, USA
Lindsey H. Schroeder, Ed.D., LAT, ATC, CES
601 S. College Dr.
Wilmington, NC 28403-5956
Alexander T. McDaniel, DHSc, LAT, ATC, CSCS is an Assistant Professor of Exercise Science in the School of Health and Applied Human Sciences at the University of North Carolina Wilmington in Wilmington, NC. His research interests focus on primary prevention strategies for mild traumatic brain injury among military and athletic populations as well as the reduction of stress, anxiety, and depression in various demographic populations through functional training.
Lindsey H. Schroeder, Ed.D., LAT, ATC, CES is an Assistant Professor of Athletic Training in the School of Health and Applied Human Sciences at the University of North Carolina Wilmington in Wilmington, NC. In addition to concussion and mild traumatic brain injury research, her research interests also focus on work-life balance, job satisfaction, and retention issues in athletic training. Dr. Schroeder is an alumnus of the United States Sports Academy.
Clinical Review: The Identification of Potential Risk Factors of Mild Traumatic Brain Injury among the Military Paratrooper and a Potential Intervention Strategy
For combat readiness, active duty United States military service members have high physical fitness demands placed on them that rival even the most well-trained athletes. The key to maintaining a high level of performance is to prevent common injuries. The purpose of this manuscript is to increase awareness regarding mild traumatic brain injury due to parachute landing and identify a neck strengthening intervention to potentially reduce the risk of injury. The review of literature will provide information regarding the pathophysiology of a mild traumatic brain injury from parachute landing, provide an in-depth review of the anatomy of cervical spine musculature, and the biomechanics of how force is attenuated during a whiplash effect upon impact on the head or body. A review of potential strengthening techniques and the muscles to be targeted is indicated as well as a discussion of potential intervention protocols aimed at the reeducation of mild traumatic brain injury risk from parachute landing.
Keywords: neck strengthening program, parachute landfall training, concussion, neuromuscular training
Each year, traumatic brain injuries (TBI) contribute to a substantial number of deaths and permanent disabilities. The Centers for Disease Control and Prevention (CDC) estimates that in 2014, the number of emergency department visits, hospitalization, or deaths from TBI to be 2.88 million in the United States (3). About 75% of TBIs that occur each year are concussions or other forms of mild TBI (22). Direct medical costs and indirect costs of TBI, such as lost productivity, totaled an estimated $60 billion in the United States in 2000 (11).
Active duty military service members are some of the most fit individuals due to the requirement that they “maintain physical readiness through appropriate nutrition, health, and fitness habits. Aerobic capacity, muscular strength, muscular endurance, and desirable body fat composition form the basis for the DoD [Department of Defense] Physical Fitness and Body Fat Programs” (8). Since the mid-2000s, public health and health-care communities have become aware of the increased rates of TBI among active duty United States military personnel. In response to these public health concerns, Congress passed the Traumatic Brain Injury Act of 2008, which requires the CDC and the National Institutes of Health (NIH) in consultation with the Department of Defense (DoD) and the Department of Veterans Affairs (VA) to make recommendations about the development and improvement of TBI diagnostic tools and treatments (28). From 2000 to date, there have been 413,858 traumatic brain injuries recorded by the Armed Forces Health Surveillance Branch. Of these traumatic brain injuries, 82.8% were classified as mild (7). Civilian and military reports have been released estimating costs related to TBI, demonstrating the significant economic impact of these injuries and their outcomes. In 2008, the RAND Corporation estimated TBI costs resulting from Operation Enduring Freedom, which showed that in the first year following injury, costs were estimated to be $27,260 to $32,760 per case for mild TBI and up to $408,520 for those with moderate to severe injury (28).
Included in the report to Congress from the NIH, was the recommendation to improve TBI prevention (4). Specifically, the NIH noted evidence-based strategies exist, and it is imperative that there is strict adherence to those practices. The purpose of this document is to review the evidence-based literature to support the development of a cervical spine muscle strengthening program to prevent TBI in active duty military. This document will highlight previous research outlining the pathophysiological information to support the need for a neck-strengthening program. The information from this document will be utilized to develop a four phase clinical study; 1) validate a novel isometric neck strength assessment tool, 2) compare a traditional neck strengthening program to a novel neck strengthening protocol utilizing a new tool on the market, TopSpin
260, 3) to validate the TopSpin 360 in a 16 week strength program
to determine its peak physiological response from the neck musculature, and 4) examine the effects of the neck strengthening protocol among military populations.
Paratroopers specifically train to prevent injuries such as a TBI. Parachute Land Fall Training was developed to avoid injuries that may be considered a consistent job hazard. Even with improvements to paratrooper exosystems, the T-11 hardware and parachute system, and improved landing techniques, both the landing technique and parachute system have not fully prevented injuries observed among airborne soldiers. Airborne paratroopers remain among the highest of all members of the military to sustain musculoskeletal injuries during training operations, such as closed head injuries. The incidence of mild TBI experienced at Fort Bragg, 82nd Airborne Division, by paratroopers are more than double that of non-paratrooper personnel (15). Parachute injuries, especially those involving mild TBI, result in decreased training time, unnecessary medical costs, and a reduction in overall unit effectiveness. Most importantly mild TBIs will minimize a soldier’s capabilities and readiness. In severe cases, they can have long-lasting or permanent health impact.
Mild Traumatic Brain Injury
Mild traumatic brain injury (TBI), otherwise known as a concussion, occurs from traumatic biomechanical forces placed on the head and neck affecting the brain that results in a pathophysiological process altering function (21). Altered brain functions experienced during a mild TBI may include neuro-cognitive impairments, disturbance of vision, and vestibular-cochlear disruptions. The altered brain functions experienced during a mild TBI often occur rapidly once initial impact is felt, and symptoms or impairments of neurological function return to normal within a few weeks of initial onset (27). This altered brain function is thought to be due in part by a chemical imbalance and energy crisis (13).
Pathophysiology of Mild Traumatic Brain Injury
The etiology of a mild TBI involves a force applied to the body, head, or neck that results in a rapid acceleration of the brain overcoming the absorption properties of cerebral spinal fluid (20). The subsequent result is a collision between the brain and the inner skull, causing injury (20). The symptoms experienced during a mild TBI are a result of the physiological changes occurring in the brain. The physiological effect of mild TBI involves a complex set of altered metabolic process, chemical homeostatic imbalances, and both intra- and extracellular ionic fluctuations within the brain (20). The rotational and linear forces can produce a shearing and elongation effect of the neurons that can be attributed to the altered state of consciousness (27). An energy crisis or metabolic inhibition can occur within the brain after impact and is due in part to the ionic fluctuations, specifically potassium and sodium. The subsequent result is a decrease in mitochondrial function and adenosine triphosphate (ATP) production (13). Rapid onset of symptoms is usually resolved within several weeks; however, long-term impairments may occur if repeated injury or increased cognitive function occurs prior to full recovery from the previous mild TBI (20).
Short- and Long-term Implications of Mild Traumatic Brain Injury
As previously described, effects of a mild TBI often resolve within a few weeks of initial onset (21). The relatively quick resolutions of symptoms occur due to the body’s capability to restore chemical balance and homeostasis because of the brain staving off cellular death and permanent structural damage (20). Short-term effects or symptoms of mild TBI include loss of consciousness for less than 30 seconds, confusion, amnesia, headache, dizziness, blurred vision, attention difficulty, nausea and vomiting, drowsiness, and behavioral disorders (13, 21, 27). If medical management of mild TBI is inadequately implemented or a subsequent impact to the body, head, or neck occur, the risk of post-concussive or second-impact syndrome increases. In other words, long-term impairments may continue to affect the patient and can lead to permanent damage or in severe cases, death (27). Furthermore, return to activity or cognitive function too soon can result in persistent symptoms lasting 3 months to a year (24).
Biomechanics of Mild Traumatic Brain Injury
A mild TBI occurs from an impact to the body, head, or neck, resulting in a forceful acceleration or deceleration, subsequently producing collision of the brain against the inner skull (12). The impact location directly affects the linear, angular, and rotation acceleration experienced by the head and brain (1). There seems to be a consensus that the forces produced during impact must overcome specific characteristics of an individual for a mild TBI to be sustained (12). Such defense or preventative characteristics include cervical spine muscular strength, the capability to anticipate an impending impact, disparity in cerebrospinal fluid, and protective equipment external to the body (12). Nevertheless, if forceful accelerations are above the threshold for the defensive characteristics stated above and occur at a rotational and angular vector, a whiplash mechanism can result in a shearing force of the brainstem and collision of the brain (27).
Cervical Spine Anatomy and Movement
The cervical spine musculature and boney anatomy are involved in the attenuation of force and movement of the head during impact and, if strengthened, may help to reduce rotational and linear forces associated with mild TBI (1). Cervical spine musculature related to force attenuation include the sternocleidomastoid, splenius capitus, semispinalis capitus, longus capitus, anterior and posterior rectus capitus, trapizius, multifundus, erector spinae, and the levator scapula (26). The bony anatomy is comprised of seven vertebrae, which articulate with one another via the facet joints. There are four facets on each vertebra that allow the cervical vertebra column to move in a chain-linkage-like manner (26). The movements occurring at the cervical spine are rotation of the head, lateral flexion, cervical flexion and extension, and capital extension and flexion (26). The sternocleidomastoid and the trapezius have been indicated as the two main muscles that help to attenuate and stabilize anterior to posterior and posterior to anterior movements in the sagittal plane of the head (10). The muscular involvement for coronal plane lateral flexion is controlled by the sternocleidomastoid and the deep muscles of the posterior and lateral neck. These muscles have been shown to be especially important for anticipatory cervical muscle activation (10). Coronal plane lateral flexion has been indicated as a more injurious motion of the head and neck due to the combined rotational motion, resulting in shearing of the brain upon impact of the inner skull (10). The role of the cervical spine musculature in preventing mild TBI is important as it functions as an anticipated dynamic stabilizer during forceful impact to the body or head (10, 14). The eccentric load placed on the cervical spine musculature will result in attenuation and absorption of the force applied so that the brain undergoes minimal to no contact with the inner skull. Thus, strengthening the cervical spine, shoulder complex, and upper back musculature may help prevent the incidence of mild TBI (10).
Cervical Strength and Size Influence
Since the early 2000s, within the field of athletics, specifically American football, the role of cervical spine musculature has been indicated to be a strong predictor as a risk factor for the incidence of mild TBI (18). Absorption and attenuation of the forces applied to the head and neck upon impact are critical for the prevention of the brain affecting the inner skull (1). Furthermore, the cervical dynamic muscle reaction that occurs upon impact tends to be stronger for individuals who are aware of an impending collision. In other words, the awareness of impact can increase the amount of force that may be absorbed and attenuated (20). The head and neck are two separate segments, yet they act synergistically to perform the movements of the head as previously described. Thus, as the neck stiffens preparing for impact, the head decelerates more rapidly, which may increase when cervical strength and girth size are greater (10). Thus, the improvement of cervical spine musculature may improve the ability for the head and neck to attenuate and absorb impact forces that should in turn result in a decrease of the whiplash mechanism of movement (10).
Head Acceleration and Threshold of Force
Head acceleration is an important factor when attempting to determine the threshold or minimum amount of force required sustaining a mild TBI. Most mild TBIs that have been measured in vivo with an impact sensing system occurring during American football have resulted from an impact exhibiting approximately 90 grams to 150 grams of force on the head (10). However, the lowest mild TBI reported occurred with only a 31.8-gram force of impact on the head (10). The low force of impact on the head measured indicated the impact threshold of force may be low, and the amount of acceleration needed to produce this amount of force is also low. Coronal lateral flexion movements tend to produce a higher shearing force in the brain than that of sagittal plane flexion and extension movements. The theory most often accepted for the difference in planar movement effects is explained by the amount of musculature on the anterior and posterior side versus the lateral aspects of the neck and the shape of the cervical vertebra and skull boney anatomy (10, 14). Nevertheless, head acceleration and the minimum amount of force resulting in a possible mild TBI remains low; therefore, a resistance training program designed to strengthen muscles around the neck and educate about the importance of bracing the neck and head for impact seems to be critical for decreasing the incidence of mild TBI (10).
Anticipatory Neck Activation
Head kinematic response to an impact force is more traumatic to an individual who is unaware and does not have the opportunity to brace for impact (10). The anticipatory activation of neck and shoulder musculature has resulted in a great resistance to the minimum amount of force required to sustain a mild TBI (14). Furthermore, individuals with smaller, weaker necks tend to have a lower minimum threshold of force, even with anticipatory neck activation (10). Thus, if neck strength can be increased and neck stiffness or early onset of muscle activation occur prior to impact, then the severity of the acceleration and attenuation of force may occur (26).
Mild Traumatic Brain Injury and the Paratrooper
The military warfighter is an inherently dangerous profession within most all aspects of its job description. Wartime deployment and training operations result in bodily injury for the military warfighter. The military paratrooper is a subclass of the warfighter and involves unique training as compared with the normal army solider (16). During training operations, paratroopers perform hundreds of jumps a year (16). The paratrooper performs training operations in full gear, often carrying weight that ranges between 100 pounds to 300 pounds or over, and depending on humidity and air temperature, the paratrooper falls at speeds around 15 mph to 20 mph (16). The landing and impact on the ground result in excessive bodily forces that may result in injury. Furthermore, among all Department of Defense active duty soldiers, parachuting injuries are ranked the sixth highest leading cause of hospitalizations (16). In 2014, Knapik et al. (16) found the amount of injuries that occurred were with the two most common parachute units, the T-10D and T-11 parachute systems. Data were gathered about parachute injuries from 2010 to 2013 with the 82nd Airborne Division at Fort Bragg, North Carolina (16). From 2010 to 2013, there were 131,747 parachute jumps completed during training operations, resulting in 1,101 total injuries, and mild TBI was the second most prevalent injury to occur (16).
Paratrooper Parachute Land Fall Training
The paratrooper warfighter must complete Airborne School at Fort Benning, GA (9). Airborne training school has a three-part training operation separated into three one-week sections: ground week, tower week, and jump week (9). The purpose of the three-week training operation is to provide soldiers with the basic knowledge and training to perform parachute jumping and landing as safely as possible. Within the three-week training operations, warfighters review a 520-page document that covers static line parachuting techniques (9). Warfighters will learn and apply the five points of performance,which was developed to promote safe jumping and landing (9). The five points of performance include (a) proper exit, check body position, and count; (b) check canopy and gain controls; (c) sharp lookout at all time and compare rate of decent; (d) prepare to land; and (e) land (9). The paratrooper begins preparation to land at approximately 250 feet and should position in the direction of oncoming wind to slow decent to a speed optimal for a quick and safe landing (9). During parachute land falling, the paratrooper is taught to tuck the chin to the chest and tense the neck and upper back muscles bracing for impact (9). Depending on the wind direction and draft, the paratrooper can either perform a side, front, or back landing, all of which require the same neck posture and muscle activation (9). Upon impact, the jumper should bend at the knees, allowing his/her calves, buttock, and lower back to contact the ground consecutively, absorbing the load (9). In theory, the forces placed on the neck and head should be distributed throughout the body, allowing for the paratrooper to land safely.
Paratrooper Health and Fitness Standards
The Army Basic Airborne Course (BAC) is required for all individuals who are interested in Special Forces or any type of elite United States (U.S.) military force (9). Therefore, there are strict fitness standards that must be met for military personnel be accepted into Airborne training. The Army Physical Fitness Test and the Flexed Arm Hang assessment are the two fitness requirements necessary for acceptance into Airborne training (6). The Army Physical Fitness Test and the Flexed Arm Hang have a requirement that the individual to perform to failure pushups and sit-ups, followed by a timed two-mile run and a 20-second arm flexed hang from a high bar, a basic pull-up position (6). To pass, the soldier must complete a minimum of 42 pushups for men and 19 for women, 53 sit-ups for men and women, and the two-mile run in a time of 15 minutes and 54 seconds for men and 18 minutes and 54 seconds for women (6). Failure to pass either the flexed arm hang test or physical fitness test will result in disqualification for airborne training.
Cervical Spine Muscular Strength
Improvements in neck strength and the ability to react to an impact to reduce acceleration experienced at the head have been identified as potential indicators for mediation of a traumatic brain injury (1, 10, 25, 29). Currently, there is no research identifying the neck musculature activation during the military parachute landing. However, examining athletic impact research can be of useful value when determining the amount of impact and load attenuated through the body, resulting in head acceleration leading to brain injury. The player impact during a football contest may be like the head acceleration in a multiplanar and rotational direction to that of the impact experienced during a parachute landfall. During an impact in football, the head accelerates in a multi-directional path, resulting in a head velocity that is high enough to overcome the absorption properties of cerebral spinal fluid, resulting in a traumatic impact of the brain on the inner portion of the skull (23, 26). Thus, the neck musculature should be contracted prior to impact anticipating the contact and should be able to generate a contraction to counter the force impact force and displacement of the brain to avoid inner skull contact (14). To determine the amount of neck strength required, it is necessary to identify an estimated minimum injury threshold for sustaining a mild TBI.
It should be noted that neck strength alone is not indicated to reduce the incidence of mild TBI among athletic individuals, only when coupled with an anticipatory muscle activation was when a decrease in mild TBI incidence experienced (10). Several studies have been conducted within the field of athletics, specifically American football, in which there was an attempt to identify a minimum neck strength threshold to counter the force sustained during impact to the body (2, 23). In 2006, Pellman and Viano (23), determined a linear acceleration force threshold to produce a mild TBI injury was on average 98 grams of impact force. However, in 2010, Broglio et al. (2) identified a lower minimum threshold range of 70 grams to 75 grams of impact force necessary to result in a mild TBI. Force mechanics was assessed in a linear acceleration, which may not translate well to the multi-directional nature of the force mechanics experienced during a parachute landfall (2). However, the minimum threshold identified can be helpful in the development of strength training protocols to improve neck strength. The linear acceleration of the head is often accompanied by a rotational acceleration during impact, and thus, assessment of linear force alone does not provide an accurate minimum threshold for impact (2, 26).
Potential Strengthening Techniques
Strengthening techniques for neck musculature have traditionally focused on four-way neck strength machines (19). The four-way neck strength machine does improve overall neck strength. However, the four-way neck machines have not been effective in improving muscle activation upon impact, only overall neck strength (19). The contraction speed with the four-way neck machines is slow, and a static resistance and only trains the neck in linear planes, neglecting rotational and angular motions (19). Although, in 2014, Collins et al. (5) identified a 5% decrease in concussion for every pound of neck strength increase. Therefore, the four-way neck strength machine should be combined with neuromuscular training and multi-directional and multi-speed training that is within a velocity range to safely produce neck musculature activation. Neuromuscular training involves activation of neck musculature throughout multiple planes of movement dynamically, which will better mimic the motor recruitment necessary to absorb high-magnitude impacts (10, 26).
Neck Musculature to be Targeted
A combination of isometric, isotonic, and dynamic movements involving neuromuscular activation should be implemented during training (10, 26). Furthermore, in 2017, Lee et al. (17) identified mobility exercises for neck musculature are often overlooked and should be a part of neck strengthening programs. The muscles to be targeted in the neck include the deep neck flexors, extensors, and muscles of the upper back (17). Training sessions should target the above-mentioned muscles in a specific manner to mimic how the body will experience the impact, in other words, promoting sport-specific training (17). In addition, training should be long-term with multiple sessions occurring weekly, including a combination of weight training and neuromuscular perturbation training with an emphasis of proper periodization program design to avoid overtraining and special attention should be given to the amount of load placed on the neck to avoid injury during training sessions (17).
The NIH provided congress with a report which contained recommendations to improve the health of the U.S. military. Primary prevention strategies to reduce the incidence of TBI were among the top of the recommendations presented to congress (4). This report was specifically designed for the Airborne paratroopers as they remain among the highest of all members of the military to sustain musculoskeletal injuries during training operations, such as closed head injuries. For instance, with respects to the incidence of mild TBI, paratroopers are more than double that of non-paratrooper personnel at Fort Bragg who experience a mild TBI injury (15).
A neck strengthening program should be implemented for the military paratrooper. The program should be developed from evidence-based research involving the biomechanics of the head and neck complex during body impact or collision, and the subsequent muscle activation that is involved in slowing down the acceleration experienced by the head and neck. The goal of the neck strengthening program should be to improve the strength of the muscles surrounding the neck and upper back and improve upon the stretch reflex muscle activation. The improvement of muscle activation will improve neuromuscular activity, thereby, allowing the paratrooper to optimally attenuate the forces experienced during body-to-ground collision (10).
There are some key aspects regarding neck strengthening and the potential to reduce the incidence of mild TBI among paratroopers that were highlighted. First, the amount of force experienced to produce a whip lash mechanism and a subsequent injury to the head can be relatively low for an individual who has untrained musculature, in vivo studies indicated this threshold to be at or lower than 31.8 grams of force impact to the head (10). Furthermore, head and neck acceleration in the parachute landing is like that of the impact experienced by an athlete during a contact sport athletic contest. The head accelerates in a multi-directional path, resulting in a high enough velocity to overcome the absorption properties of cerebral spinal fluid, thereby increasing the risk of brain impact on the inner portion of the skull (23, 26).
Thus, the key take-away from the neck-strengthening program is to improve strength in a proprioceptive manner. Training should focus on neuromuscular activation in multiple plans of motion with a high degree of contraction rate. The neuromuscular activation and strength training techniques should elicit the stretch reflex to better train the paratrooper and mimic the conditions experienced during parachute landing.
APPLICATIONS IN SPORT
The military paratrooper is considered a physical intense and high injury risk job with an increased incidence rate of mild TBI from parachute landing (15). As such, this author recommends primary prevention strategies be implemented to decrease the risk of mild TBI injury from parachute landing and improve military readiness. Parachute injuries, especially those involving mild TBI, result in decreased training time, unnecessary medical costs, and a reduction in overall unit effectiveness. Current efforts to decrease injury from parachute injury have been minimally successful in reducing mild TBI, resulting in the need for improvements in primary prevention efforts. Therefore, implementation of this neck strengthening program could potentially decrease the incidence of mild TBI and improve military readiness. Furthermore, from a cost-benefit analysis viewpoint, there is great potential to reduce health care costs for the military resulting from mild TBI.
The recommendations by the authors involve a research plan that will examine a potential neck strengthening protocol to address the needs of the military paratrooper. First, an adequate isometric neck strength assessment tool should be validated to determine the optimal strength required to provide stability to the joints of the cervical spine. Secondly, the completion of a clinical study examining the effectiveness of a novel neck strengthening protocol compared to that of a traditional neck strengthening program will provide the researchers an indication as to what is the optimal training protocol. The purpose would be to identify if the neck strengthening program should include activation of proprioceptive neuromuscular strength training and whether it is efficacious in nature and does in fact reduce the incidence of mild TBI. Lastly, it is recommended to evaluate the implementation and institutionalization of the neck strengthening program at Fort Bragg, NC and Fort Benning, GA where military paratroopers conduct parachute training.
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