Head and helmet motion during running, jumping and exposure to whole-body vibration

Abstract

An understanding of the mechanical responses of the head and helmet is important to minimise adverse effects of helmet motion on health and comfort. Little is known of the interaction between head and helmet when running or jumping and during exposure.to whole-body vibration. This study was conducted to investigate the frequency, magnitude and dominant directions of motion occurring at the head and helmet under these conditions and to gain an understanding of the mechanical relationship between head and helmet based upon their modal characteristics. Three experiments were conducted to measure the frequency response of the head and helmet whilst running, jumping and during exposure to whole-body vibration. The experiments showed that both translational and rotational motion of the head and helmet can occur whilst running or jumping and when exposed to whole-body vibration, with the dominant motion occurring in the mid-sagittal plane. When running and jumping the greatest amount of motion, in all directions of motion, occurred at the excitation frequency. In these conditions the head and helmet moved together in the vertical direction at the excitation frequencies. In the fore-aft direction relative motion was observed between head and helmet at the excitation frequencies when running and jumping. The effect of helmet mass and distribution of mass on the helmet was investigated during exposure to whole-body vibration. The study showed that at frequencies between 3 and 5 Hz motion of the head was reduced when a helmet was placed on the head. This Seat-to-head occurred in all transmissibilities, in all directions, decreased further as helmet mass increased. The seat- to-head transmissibility was also influenced by the distribution of mass on the helmet. Seat-to-head fore-aft and vertical transmissibilities decreased as additional helmet mass was moved lower and further forward on the helmet. motion in the mid-sagittal directions of plane. An experimental modal analysis identified three modes of vibration of the neck- head system and four modes of vibration of the neck-head-helmet system. A finite element lumped parameter model of the neck-head-helmet system was also developed and predicted the four dominant mode shapes that were obtained in the experimental modal analysis. The variability in seat-to-head and seat-to-helmet transmissibilities was thought to be attributable to resonance behaviour of the system. The extra mode of vibration obtained in the neck-head-helmet system was a pitch mode of the helmet on the head. Its natural frequency was controlled by helmet mass and the stiffness of the skin coupling at the fore- head. This implies that this mode was an independent mode of the helmet on the head.

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