INVICTA Study: Uncovering Blast Exposure’s Impact on Special Operations Forces

USU’s five-year INVICTA study examines the subtle, yet significant effects of low-level blast overpressure exposure on service members’ neurological functions, including memory, gait, sensory processing, and brain health.

The five-year INVICTA study examines how low-level blast overpressure exposures affect brain

structure and function in Special Operator heavy weapon trainees and Range Safety Officers

(RSOs). This research is crucial for military medicine, as it directly impacts the health, safety, 

and readiness of those who serve. (U.S. Army photo by Capt. Joseph Legros)

April 10, 2025 by Hadiyah Brendel

For years, the hidden toll of low-level blast overpressure exposure on service members’ brains went unexplored. However recent studies have begun to shed light on the neurological impact of these exposures.

The five-year INVICTA study, led by Drs. Michael Roy and David Keyser at the Uniformed Services University (USU), examines how these exposures affect brain structure and function in Special Operator heavy weapon trainees and Range Safety Officers (RSOs). This research is crucial for military medicine, as it directly impacts the health, safety, and readiness of those who serve.

Measuring the Impact: INVICTA’s Rigorous Methodology

The Investigating Training Associated Blast Pathology, or INVICTA, study measures the blast exposure that military Special Operators encounter during heavy weapons training. “We’re looking at the three positions that Special Operators who are directly exposed to blast during heavy weapons training hold during live fire exercises –  the gunner who is firing the weapon, the assistant gunner who is shoulder-to-shoulder with them and equally close to the weapon, and the instructor who has traditionally been on the other shoulder of the gunner – to see what kind of impact low-level energy can cause over time,” explains Roy. 

Roy is the Deputy Director of the Military TBI Initiative (MTBI2) as well as the Director of the Division of Military Medicine. Keyser is the Deputy Director of Traumatic Brain Injury Research Program. Both serve as principal investigators on the study. 

To achieve precise measurements, the study uses advanced blast gauges placed strategically on participants’ chests, shoulders, and helmets. The gauges measure peak overpressure (the highest pressure achieved when the weapon is fired) and impulse (the time over which blast exposure occurs). The gauges measure the peak pressure in pounds per square inch (psi) and impulse in psi-seconds. Understanding these measurements and their effects is essential for refining safety thresholds, enabling more effective risk management strategies.

Assessing Brain Function: Key Tests and Findings

The INVICTA study employs a comprehensive suite of tests to thoroughly assess brain function. The study used three main tests to examine the subtle effects of blast exposure on the brain: a word list recall task to evaluate memory, the AccWalker™ system for gait analysis, and the Brain Gauge test for tactile discrimination. 

The word recall test reveals significant decline in short-term memory in both Special Operators and RSOs 30 minutes post-training, with relatively rapid recovery among the former. However, RSOs tested near the end of their multi-year tour show a greater change in their performance, taking weeks, rather than days, to recover. "That’s a pretty remarkable difference in terms of both the magnitude and the duration of the impact,” Roy emphasizes.

Memory is not the only area affected. Gait analysis, using a phone strapped to the thigh during walking, reveals a “robotic” gait pattern post-exposure. The tactile discrimination task, which measures the ability to detect subtle vibrations between the index and middle finger, indicates sensory processing changes. Blood samples reveal changes in specific biomarkers released by the brain after injury, which may suggest inflammation and potential damage to brain cells. 

These findings underscore the critical role of ongoing research in military medicine, driving advancements in risk management to strengthen the overall readiness of service members.

The INVICTA study exemplifies USU's commitment to advancing mission-critical research

that directly enhances warfighter lethality and readiness. (U.S. Army Photo by Sgt. Henry 

Villarama)

Training Protocol Changes and Future Applications  

The INVICTA study has already prompted tangible changes in training protocols for these units, demonstrating the immediate impact of this research. “We saw some of them starting to step back further,” Roy noted, describing how instructors adjusted their positioning to minimize blast exposure. Stepping back one or three meters significantly reduced blast exposure levels. One instructor standing shoulder-to-shoulder with gunners displayed readings of six to eight psi, almost double the Department’s interim blast overpressure exposure safety guideline of four psi, used as a threshold to require initiation of appropriate risk management actions. After stepping back one meter, the psi dropped to approximately 3.5. At three meters, the psi was less than 0.5.

Looking ahead, researchers aim to develop a risk stratification measure to identify individuals at high risk to blast exposure. The scoring system will use study assessments, prior exposure history, and other metrics to inform military leaders’ training and deployment decisions. This tailored approach has the potential to transform military protection strategies, enabling personalized risk management based on each service member’s unique exposure history and susceptibility.

Protecting Service Member Brains: INVICTA’s Legacy

As the INVICTA study reaches its five-year conclusion, its legacy is poised to be transformative. Existing risk management strategies reduce blast exposure using measures such as advanced simulators, enhanced protective equipment, weapon modifications, and refined training protocols; the INVICTA study provides critical data to further refine these approaches to safeguard service members, while sustaining warfighter lethality.

“Our military service members are the most valuable tool that we have,” Roy states. “Taking proactive steps to reduce the risks of exposure-related injuries isn’t just a safety measure—it’s an investment in our lethality, readiness, and mission” he added, emphasizing the critical importance of this research. 

Keyser echoes the importance of safeguarding brain health and the potential for personalized medicine to optimize interventions. INVICTA’s data, along with findings from their other ongoing studies, is producing “a data set so rich we’re going to be able to look at it for years to come,” he states. This in-depth information will contribute to a deeper understanding of why individual service members experience varying degrees of impact from blast exposure, paving the way for more targeted and effective protective strategies. 

Researchers at USU are already building on the study’s findings to conduct three follow-on investigations, which expand on INVICTA’s methodology and data to further explore blast exposure effects in other military occupations, such as breaching and explosive ordnance disposal.

The INVICTA study exemplifies USU's commitment to advancing mission-critical research that directly enhances warfighter lethality and readiness. This research is shaping the future of military medicine by informing new protective measures and deepening our understanding of blast exposure's impact on service members. Ultimately, this vital work safeguards the Force’s greatest asset: its personnel and their most powerful weapons—their brains.