Measuring and modelling human response to foot-transmitted vibration exposure

Abstract

Foot-transmitted vibration (FTV) occurs when a worker is exposed to vibration through the feet and can occur when operating vibrating equipment such as bolters, jumbo drills, or crushers, or standing to operate mobile equipment such as locomotives and forklifts. Exposure to FTV has been linked to the development of vibration-induced white feet, a vascular disorder with reduced circulation to the toes causing blanching. Vibration research has been focused on whole-body vibration (WBV) and hand-arm vibration, with FTV being lumped in to standing WBV. This research includes, but is not limited to, resonant frequency identification, development of international standards governing safe exposure limits, personal protective equipment design, and model development. It is the intention of this research to initiate research specifically for FTV. The first step to preventing harmful exposure is to identify the resonant frequencies at different anatomical locations on the foot (Objective 1). The resonance of 24 anatomical locations on the foot was identified for 21 participants, where the most notable differences in the average peak frequency occurred between the toes (range: 99-147Hz), midfoot (range: 51-84Hz), and ankle (range: 16-39Hz). As workers do not normally stand in a completely natural position, it was equally important to measure how altering the location of the centre of pressure (COP) changes resonance and the transmissibility of vibration through the foot (Objective 2). The resonance at the same 24 anatomical locations was identified when the COP was pushed forward (towards toes) and backward (towards heels). Generally, resonance at the measurement location increased when the COP was concentrated to a particular portion of the foot. The third objective of this research was to reduce the measurements at 24 anatomical locations, from the first two objectives, down to a representative subset (Objective 3). Multiple correspondence analysis was conducted on the peak transmissibility magnitude in order to assess structure displacement leading to increases in potential injury risk. Transmissibility results were analysed based on two magnitude thresholds: at 2.0 indicating 100% amplification of the input signal, and at 2.5 indicating 150% amplification. Results indicate that transmissibility measurements at the nail bed of first phalange, head of first metatarsal, head of second metatarsal, and the lateral malleolus may be sufficient to effectively measure foot-transmitted vibration when participants changed their COP location from natural, forward and backward. Then a K-means analysis was conducted to minimize the anatomical locations necessary to capture the transmissibility response from 10 to 200 Hz, and using the reduced locations, a lumped-parameter model was designed and validated (Objective 4). Three locations (the nail of the big toe, the third metatarsal, and the lateral malleolus) were found to be sufficient for summarizing FTV transmissibility modulus. A three segment, four degrees-of-freedom lumpedparameter model of the foot-ankle system (FAS) was designed to model the transmissibility response at three locations when exposed to vertical vibration from 10 to 60 Hz. Reasonable results were found at the ankle, midfoot, and toes in the natural standing position and forward COP. However, when the COP is backward, the model does not sufficiently capture the transmissibility response at the ankle. Determining the resonant frequencies of the FAS is important for the prevention of vibration-induced injury. Resonance needs to be incorporated into the design of equipment, tools (e.g. anti-vibration drills, isolated platforms), and personal protective equipment (e.g. antivibration insoles or boots) can be modified to reduce vibration at the frequencies where tissue resonance occurs. These findings could also inform the development of new international standards for measuring/reducing exposure to FTV.