Hand-arm transmitted vibrations: biomechanical characterisation and modelling of gripping

In France, approximately 2.6~million workers are exposed to hand-transmitted vibrations. In the long term, such exposure may result in the onset of vascular, neurological, osteoarticular, or musculoskeletal disorders. The use of rotary machinery emitting vibrations at frequencies above 50 Hz has been demonstrated to induce vibration-induced secondary Raynaud's syndrome. However, the standardised weighting filter used to assess the legal daily exposure limit underestimates the hazardousness of vibrations at these high frequencies. The mechanisms underlying the development of these disorders remain only partially understood. In order to achieve a more comprehensive understanding of the issue, it is necessary to investigate the entire deleterious process, from the transmission of vibrations from the machine through biological tissues, to the short-term physiological response of the body, and ultimately to the onset of chronic symptoms. The objective of this thesis was to experimentally characterise certain quantities influenced by grip force, with the aim of incorporating them into a finite element model of the hand. This model was designed to predict stress and strain fields induced by gripping and exposure to vibrations. The mechanical behaviour of the first dorsal interosseous muscle (FDIM) was assessed in a cohort of volunteers using quasi-static indentation tests, dynamic mechanical analysis, and shear wave ultrasound elastography. The results demonstrated that gripping, through muscle contraction, increased the stiffness and anisotropy of the FDIM. Furthermore, the muscle exhibited hyperelastic behaviour under large deformations, with mechanical properties that varied depending on the level of grip. The material's viscoelastic properties were also influenced, especially in the 20-300~Hz frequency range, where higher grip forces led to increased mechanical energy dissipation. An experimental setup was constructed to quantify the contact pressure exerted by the long fingers as a function of grip and push levels. A dedicated methodology was developed to analyse the pressure maps, with the aim of determining the contribution of each individual finger. Measurements conducted on a few volunteers revealed substantial inter-individual variability. Nevertheless, grip level appeared to have a relatively limited effect on the overall pressure distribution. The index and middle fingers accounted for approximately 70\% of the total applied pressure, whereas the ring and little fingers contributed in a more secondary manner. These data supported the development of a finite element model of the hand capable of simulating gripping by reproducing the experimentally observed pressure distribution. The model incorporated intrinsic muscles, whose mechanical behaviour was described using isotropic hyperelastic constitutive laws, with parameters dependent on the grip level. The feasibility of kinematic control was demonstrated through the use of mechanical joints, whose axes of rotation were derived from an analysis of the curvature of the articular surfaces. Increasing grip force resulted in elevated stress levels within the fingers and muscles, with variations between digits that did not follow a clear or consistent pattern. Finally, gripping simulations were used to perform a modal analysis of the constrained hand, revealing that tissue stiffening-caused by muscle contraction and contact pressure-alters the natural frequencies of the fingers and muscles.