Background: Repeated sub-concussive blast exposure (RSCBE) may occur in military training and operational settings, which can negatively affect neurological function. However, most studies to date have examined RSCBE in relatively small sample sizes. The INVestigating TraIning AssoCiated BlasT PAthology (INVICTA) Study was designed to address this challenge by enrolling a large cohort of Service Members exposed to RSCBE, as well as a control cohort. Balance (i.e., neuromotor) tests are commonly used to assess neurological function, which was accomplished in this study with a dynamic balance test via custom smartphone app named AccWalker. This preliminary analysis examined changes in neuromotor performance after RSCBE in this undiagnosed population.
Methods: This repeated-measures longitudinal study design enrolled two groups with RSCBE as part of a training program: (1) Special Operators (or trainees) who shot heavy weapons as part of their training and (2) Range Safety Officers (RSOs) present when the heavy weapons were shot. Participants were included in the blast exposed group if their blast gauges registered 4 psi or greater two or more times during the training. Participants completed the AccWalker neuromotor task before (baseline) and after (6-hr, 72-hr, 2-wks, and 3-months) RSCBE. Two control groups were also enrolled: (1) training controls [Special Operators (or trainees) who participated in the training, but did not shoot heavy weapons] and (2) naïve controls (active-duty Service Members who did not participate in the training). The AccWalker neuromotor task consisted of two trials of a 70s step-in-place task with eyes closed while the smartphone was attached to the participants’ thigh. Variability in stepping (i.e., neuromotor performance) was quantified as the Coefficient of Variation (CV) of the peak thigh flexion angle (CV PeakFlex %). A linear mixed model with a random intercept was used to examine group differences.
Results: A total of 110 participants qualified for analysis (i.e., two or more timepoints): 55 blast exposed Special Operators (or trainees), 28 blast exposed RSOs, 16 training controls, and 11 naïve controls. For the blast exposed Special Operators (or trainees), a significant decline was observed from baseline (M=6.79, SE=0.22) to 6-hr (M=6.08, SE=0.22) and remained suppressed at 72-hr (M=6.16, SE=0.23), after which they rebounded to values not different from baseline at the 2-wks and 3-month timepoints. Similarly for RSOs, a significant decline was observed from baseline (M=6.73, SE=0.28) to 6-hr (M=6.03, SE=0.29), but then rebounded to values not different from baseline at the 72-hr and 2-wks. However, a decline was again observed at 3-months (M=5.93, SE=0.31). For both control groups, no difference was observed from baseline across any of the post-RSCBE timepoints.
Conclusions: In this preliminary report, short-term neuromotor dysfunction was observed in Special Operators and RSOs after RSCBE. The data show a decline in movement variability, which reflects a more robotic form of stepping. While this may appear to be a more stable (and thus better) form of movement, current thought suggests otherwise. Human movement requires the flexibility to make micro (and sometimes macro) adjustments in real time to maintain balance due to the ever-changing location of the body’s center of mass, irregular terrain, etc. These variable factors require a continuous monitoring of temporal/spatial aspects of the movement and constant updates to the movement pattern to maintain upright stance and/or to adhere to the constraints of the task (i.e., attempt to step at a consistent pace). From a neuromotor performance perspective, this results in small variations from step-to-step. However, a subclinical neurological insult—such as RSCBE—can reduce the body’s ability to make the necessary micro-adjustments, hence producing a more robotic-like pattern. This dysfunction is short-lived, with neuromotor performance rebounding to baseline levels within 72 hours of RSCBE for RSOs and 2-wks for Special Operators. It should be noted that this study is ongoing and it will be important to test the replicability of these findings with a larger sample size in the future.
Funding: This work was supported by a grant from the Defense Health Agency to Michael J. Roy (HU0001-19-2-0119) and a subcontract to Christopher K. Rhea (HJF subcontract 5361).
