Mechanical characterisation of hand first dorsal interosseous muscle during gripping.

Prolonged exposure to vibrations from handheld tools can result in various disorders. Understanding how vibrations propagate through the hand is a key area of research involved in preventing these disorders. Within this, it is essential to grasp the mechanical behaviour of hand intrinsic muscles, especially as their properties may change during gripping due to muscular contraction. In order to achieve this objective, a homemade setup was elaborated with a view to measuring the mechanical characteristics of the first dorsal interosseous (FDI) muscle, which is located between the thumb and index finger. The apparatus featured quasi-static indentation, dynamic mechanical analysis (DMA), and shear wave elastography to investigate muscle hyperelastic, viscoelastic, and anisotropic properties, respectively. Measurements were conducted on 27 volunteers with grip instructions ranging from 0 to 40% of their maximal grip strength. The use of repeated measures analysis of variance and the computation of cross-correlations between the proposed measurement techniques unveiled that grip forces significantly modulate the mechanical behaviour of the FDI muscle. In addition, our results emphasised that the FDI muscle stiffened as grip force increased, primarily in the direction longitudinal to muscle fibres. The muscle static stiffness also rose non-linearly with the indenter penetration, thus exhibiting the hyperelastic behaviour of living tissues. The muscle dynamic stiffness was found to be strongly reshaped by vibration frequencies. It remained roughly constant up to around 100 Hz (when no grip), then climbed steeply. Grip force revealed its greatest influence on the muscle dynamic stiffness for vibrations with frequencies ranging from 20 Hz to approximately 300 Hz: the greater the grip force, the higher the dynamic stiffness. Furthermore, the FDI muscle was shown to exhibit a four- to six-fold increase in mechanical power dissipation between 20 Hz and 80 Hz when the handle was gripped at the maximum tested force, in comparison to no grip. Elevating grip forces increased vibration dissipated power within the hand muscles, thereby arguably leading to what are possibly the foremost vibration-induced risks.