## 1 Introduction  
Military service members (SMs) are required to wear combat helmets in training at all times regardless of the type of exercise and weapon system used in order to reduce or eliminate head injuries due to the blunt force trauma. The need for improved ballistic protection led to the development of technologically advanced helmets that outperform their steel-based analogs used in WWI to the Vietnam War era [1]. The invention of high-performance fiber-reinforced polymer-matrix composites resulted in weight reduction, while vastly improving the ballistic protection of modern combat helmets [1, 2]. The Kevlar-based Advanced Combat Helmet (ACH) has been used since the mid-2000s by combat troops, and more recently, the Enhanced Combat Helmet (ECH) was introduced in 2013. It is made of Dyneena(r) HB80, an ultra-high-molecular-weight polyethylene (UHMWPE) reinforced with carbon fibers, a thermoplastic that improved ballistic performance and lower density compared to Kevlar [3]. While these helmets were designed for ballistic protection, the recent focus on blast overpressure (BOP) exposure from high-explosives and heavy weapon systems has resulted in questions concerning overpressure inside the helmet [4]. To date, research on blast wave propagation inside of the combat helmets is based almost exclusively on data from numerical simulations [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. Advances in medical diagnostic techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) has facilitated the development of high-fidelity, anatomically accurate head models [19; 20; 21; 22]. Progress in these fields stimulated the development of high-fidelity computational methods that helped identify mechanisms governing blast loading of the brain without a helmet. The finite element method (FEM) based numerical simulations demonstrated that in the early stages of the blast wave interaction with the head, the intracranial pressure (ICP) is increased (compressive wave) at the coup (impact) side, which is accompanied by the development of the underpressure (tensile wave) on the counter-coup side [8; 18; 19; 23; 24]. This mechanism is universal and independent of the direction of the blast wave propagation, which was confirmed not only for the front impact but also for the blast waves directed at the head from the side, back, top, or the bottom [13; 15; 23]. Studies that evaluated the propagation of the blast wave inside of helmets identified the under-wash effect, where the elevated pressure develops under the helmet [5; 6; 11; 16; 25] at the back of the head (for frontal impact). A similar mechanism was also confirmed for the back and side incident direction when the ACH was used in simulations [14; 16; 25]. These reports indicate that the magnitude of the under-wash-induced overpressure on the head exceeds the primary shockwave. However, the correlation between the surface pressure build-up and the corresponding ICP increase is not well researched [5; 25]. The blast wave propagation inside combat helmets is a complex problem, with the pressure focusing under the helmet dependent on several factors: (1) the shape of the helmet, (2) orientation relative to the source of the blast wave, and (3) the suspension system (i.e., number of pads, their distribution under the helmet and material properties). The mounting evidence suggests an association between occupational low-level blast exposure and acute adverse neurological effects [26], and related serum biomarkers changes [27; 28]. The role of the helmet in the development of these effects is unclear at this time. Thus, it is essential that work evaluating the performance of the modern helmet is conducted at relevant blast overpressures (BOPs). Using anatomically accurate headform, we assessed the effect of the four modern combat helmets on the pressure accumulation under the helmet.