Modelling and experimental characterisation of mechanical vibration propagation in the upper limb when using rotary handheld machinery

Outline of reasons and objectives
Today, in France, approximately 11% of salaried workers are exposed to vibration transmitted to the upper limb. Prolonged exposure to high levels of vibration can lead to a certain number of pathologies. To try and protect workers, daily exposure to vibration is governed by regulations and is assessed in compliance with a measurement standard that currently does not guarantee sufficient protection for workers if we consider vascular disorders such as Raynaud’s phenomenon. Our ultimate aim is to provide knowledge enabling certain physiopathological effects of vibration on
the digital vascular system to be better taken into account in the standardised method for assessing exposure to vibration. This study therefore aims to measure and to predict the mechanical effects of the vibration transmitted to the hand-finger system and to assess the influence of certain factors on vibration propagation (gripping force, pushing force, amplitude of the vibration, etc.).

Approach
At the scale of the hand, an experimental setup was implemented using a scanning laser vibrometer, which made it possible to estimate the local biomechanical characteristics at the surface of the dorsal face of a hand gripping a vibrating handle. At the scale of the phalange, a test setup was built to measure the biodynamic response of the vibrated pre-stressed phalange. In addition, an appropriate numerical mathematical model was developed for simulating the same experimental conditions, and for calculating the thermo-mechanical magnitudes inside the biological tissues of the finger, such data being difficult to access by experimentation. All of the measurements were taken on a group of 20 volunteers, under a biological research protocol.

Main results
The local biomechanical responses at the surface of the hand were spatially heterogeneous. They highlighted zones of high vibration absorption (soft tissues) and zones of low vibration absorption (metacarpals). Compared with the pushing forces or thrust forces, the gripping forces were of predominant significance in vibration transfer between the wrist and the hand. The measurements showed that the phalanges had mechanical behaviour similar to that of certain elastomers with a flexibility that varied little, up to 125 Hz, and then high stiffening above that frequency. For the numerical model, a new mechanical behaviour relationship, based on physiological considerations, was devised.
The associated parameters were then identified (test/calculation error less than 8%).

Discussion
As regards prevention, the measurement methodologies developed can be used to characterise the vibration attenuation performance of anti-vibration solutions (gloves, and handles). Charts giving contours for dissipated power as a function of acceleration level and frequency of vibration have been established. They offer a potential avenue to explore for changing the standard for measuring exposure to vibration. From a scientific viewpoint, the numerical model designed and validated in this study constitutes the first stage in a multi-scale strategy whose ultimate objective is to predict certain effects of vibration on deregulation of basal arterial vasoconstriction by taking
into account the coupling between mechanics, biology, and physiology.