Tag Archives: military

Top Gun Trauma: the Effects of Ejecting From a Fighter Jet on the Spine

The need for speed places fighter pilots in electrifying yet dangerous situations. When things go wrong during flight, pilots must consider ejecting, a terrifying choice. Ejection is a last resort due to the large compressive forces and the high wind speeds that can cause many different serious injuries, including spinal injuries. Approximately 20-30% of people who survive ejection endure spinal fractures. Understanding the dangers of flight that servicemembers face increases awareness of the military lifestyle within the civilian population and is critical in finding solutions to lessen the severity of injury.

During ejection, the rocket-propelled ejection seat thrusts the pilot upward out of the aircraft. The pilot experiences around 18 g-forces (18 times your bodyweight)! The acceleration from the thrust of the seat, peaking at 140 to 160 m/sec2, compresses the spine vertically, loading the thoracic and lumbar spinal regions seen below. 

Anatomy of the spine
Photo from Patel et al., Pediatric Practice: Sports Medicine, 2009

The large rate of loading causes spinal fractures that can be either unstable and require surgery due to the movement of vertebrae or stable and treated with a brace. Thoracolumbar (lower back) fractures can be modeled using a variety of methods. One study applied axial loads of 5.2 kN (1,169 lbs) on two different spines from cadavers with a peak acceleration of around 20 g to simulate ejection. The resulting fractures for both specimens were on the L1 vertebrae, and one fracture was stable while the other was unstable. Another study constructed a drop tower and subjected 23 lumbar spines (T12-L5) to axial forces between 2.1 (472 lbs) and 7.3 kN (1,641 lbs) and accelerations between 8 and 23 g. Data analysis produced injury probability curves, which showed a 95% chance of injury with an acceleration of 20 g. The larger loads and accelerations also correlated with lower-level injuries (L4 and L5 vertebrae). 

One study modeled ejection using the finite element method, which can mathematically model the spine’s response to forces, and imaging software to investigate the effect of posture on spine injury severity. 

Software model of thoracolumbar spines in normal and relaxed postures.
Modified from Du et al., Int. J. Numer. Meth. Biomed. Engng., 2014

Thoracolumbar spines in normal and relaxed postures shown in the image above were simulated with an acceleration peak of 15 g for 0.2 sec. The relaxed posture correlates with increased stress on the endplate (the region between a vertebrae and an intervertebral disc), as the relaxed posture increases anterior flexion (forward bending) of the spine that is then increased by compression. Sitting straight up could help decrease the chance of injury during ejection. 

Ejection is a harsh reality that some pilots face. But as dangerous as ejecting is, ejection seats have a 92% survival rate, and sustaining a spinal injury is worth keeping your life. One B-1 Bomber crew member who ejected over the Indian Ocean said, “I lost a full inch in height.” It’s the price service members pay to dominate the skies and fly faster than the speed of sound. 

For more information, check out this retrospective study of French forces and this analysis of accident reports from the Royal Air Force

The Weight of Combat: Are powered exoskeletons the solution to heavy combat loads?

Have you ever wondered how much weight a soldier carries in a combat zone?

Military servicemembers, particularly those in physically demanding roles such as infantry, are routinely required to carry heavy combat loads ranging from 25- to over 100-lbs. This load potentially includes weapons, ammunition, body armor, food, sleeping equipment, and other necessities for the mission. Consider that these loads are often carried for hours or even days at a time in both deployed and non-deployed environments and it becomes clear that these loads take a physical toll on those who bear them.

The physiological demands of these loads often lead to servicemember injury or discomfort both during and after their time in service. The most common musculoskeletal injuries resulting from carrying heavy combat loads include increased lower back pain and injuries to the knee, ankle, and spinal cord. Such injuries lead to acute and chronic effects over the servicemembers’ lifetimes, increased military healthcare costs, and decreased military readiness.

While it would be advantageous to decrease both the weight of the combat load as well as the frequency of weight-bearing events, the reality of modern warfare gives little hope to these suggestions. However, there is another solution: external, electrically powered exoskeletons to aid with carrying combat loads.

American defense and technology company Lockheed Martin is currently developing a prototype exoskeleton for military use – the ONYX exoskeleton. Two prior-service soldiers are shown performing common physical tasks under load – walking up a steep incline and walking up flights of stairs – while aided by the exoskeleton. Both soldiers involved in the test indicated a high level of comfort with the exoskeleton as well as improved weight-bearing ability using the ONYX exoskeleton. Check out the video to learn more:

Powered exoskeletons come with drawbacks, namely mobility/comfort issues and the need for a mobile, long-lasting power source. While the devices may perform well in a laboratory or controlled setting, reliability in the field will require durable materials and electronics. Additionally, while Lockheed-Martin’s ONYX exoskeleton is designed to reduce load on the wearer’s knees and quadriceps muscles, it gives no such support to the lower back or other parts of the body. This shift in load distribution throughout the body may have unintended consequences and potentially lead to further injury. A 2006 study by researchers at Loughborough University in the UK found that existing military load carriage systems result in gait and posture changes (head on neck angle, trunk angle, etc.) which lead to muscle tensions that increase one’s risk for injury.

A figure visualizing the angles made by the head, torso, and legs when walking
Image taken from Attwells et al., Ergonomics, 2006.

Thus, while there have been many improvements in robotic and soft electronics technology in recent years, powered exoskeletons have much to prove before they see time in service.

What do you think – are powered exoskeletons going to be commonplace on the battlefields of tomorrow, or are they a passing fad?

For more information, check out the following articles from the Army Times and Breaking Defense on the ONYX exoskeleton.