Abstract : Kevlar helmets provide the soldier with basic ballistic and impact protection. However, the helmet has recently become a mounting platform for devices such as night-vision goggles, drop down displays, weapon-aiming systems, etc. Although designed to enhance soldier performance, these systems increase the mass of the helmet and typically shift the position of the helmet's center of mass forward. The effects of changing the mass properties of the helmet on head and neck forces and moment on neck muscle activity and fatigue are well documented for aviators and soldiers in vehicles. No research to date has been focused on the effects of helmets of varying mass and mass distribution on head and neck forces and moments during a combat foot soldier's physical activities. Physical demands on the combat foot soldier are substantially different from those on aviator or soldiers in vehicles. Therefore, changing the mass properties of the helmet likely has different effects on combat foot soldiers than on aviators. The study answers the militarily relevant question of what forces and moments on the head and neck are associated with currently used Army helmets and night vision goggles during combat physical activities of the ground soldier.