The Effects of Military Body Armor on Isometric and Isokinetic Knee Behaviors

Abstract

OCCUPATIONAL APPLICATIONS Despite the proven effectiveness of new-generation body armor against ballistic threats, small arms, and improvised explosive devices, such body armor has become a source of concern for warfighters' performance and musculoskeletal injuries. Prolonged periods of performing physical activities, with versus without body armor, were found to alter knee neuromuscular behavior. With body armor, the maximum strength of the knee flexors and the extensor-to-flexor strength ratio were found, respectively, to reduce and increase than in conditions without body armor. Such changes in knee neuromuscular behavior are known to adversely affect physical performance and risk of knee injury. Quantitative information related to changes in knee neuromuscular behavior induced by body armor can be used in the design of injury risk assessment tools, physical training programs that decrease or correct a particular imbalance, and to establish return to service (i.e., post injury) standards for dismounted warfighters. TECHNICAL ABSTRACT Background: Knee injuries among military service members have a substantially high prevalence. Dismounted warfighters often must navigate diverse environments while wearing body armor. Purpose: Given the suggested role of body armor mass on a warfighter's performance and risk of injury, the immediate and prolonged effects of body armor on the neuromuscular behavior of knee flexors and extensors were investigated. Methods: Body armor-induced changes in selected measures related to isometric and isokinetic behaviors of the knee were quantified using a commercial dynamometer. This was done in two testing sessions, with and without body armor, by assessing neuromuscular behaviors of the knee before and after participants completed a battery of basic and military-inspired tests, as well as a 45-minute brisk walking protocol. Results: Completing tests with body armor versus no armor was found to cause a greater change (i.e., reduction of ∼10 N·m) in maximum isometric strength of the knee flexors. It also was found to be associated with a significant increase in conventional/functional strength ratios. While there was no significant difference in maximum isometric knee extensor strength between genders at baseline, females demonstrated a significant reduction (∼16 N·m) following completion of tests regardless of the body armor condition. Conclusion: In general, the prolonged duration of wearing body armor was found to cause changes in the neuromuscular behavior of knee flexors and extensors that have been suggested in the literature to be associated with a higher risk of injury and reduced warfighter performance.