The effect of weight and weight distribution on upper extremity muscular fatigue during static rifle aiming

Abstract

Modern weapon-mounted technologies, such as novel sighting devices, detachable grenade launchers, and telescopic butt stocks, add considerable weight to weapons and alter the center of mass (COM). Currently, few objective studies have investigated the effects of these alterations on soldiers’ rifle handling. The purpose of this study was to examine the effect of weight and weight distribution on upper extremity muscular fatigue during static rifle aiming. Custom testing rigs were designed to allow the weight and COM of the in-service C7A2 assault rifle to be altered using supplementary weights and an adjustable rail system. Seven rifle configurations, each of which represented the weight and COM of a potential future design, were assessed. Soldiers performed static rifle holds with each of the rifle configurations. Upper extremity muscle activity was monitored using surface electromyography (EMG) and a subjective scale of perceived exertion. Dependent measures included muscular activity level (integrated EMG), rate of muscular fatigue (slope of the median power frequency), and subjective ratings of perceived exertion (Borg CR10 scale). The body areas exhibiting the most muscular strain throughout testing were also subjectively assessed using a body map. The results revealed that the muscle activation levels of the supporting arm were substantially greater (>30% increase) when the COM was shifted forward 7cm. This objective finding was supported by the soldiers’ subjective perceptions of muscle effort. Conversely, muscle activity levels did not make evident the effects of added weight; although this may have been largely due to the fact that muscle activity from the lower back was not captured. Rates of muscle fatigue were found to be greater in the anterior deltoid than other extremity muscles and fatigue levels were greater in the supporting arm than the trigger arm. However, using rates of fatigue it was not possible to differentiate between weight conditions and COM positions. Despite the limitations of this study, it has provided a starting point toward developing a standardized protocol for assessing muscular demands during rifle aiming.