The mechanical properties of human soft tissue are crucial for impact biomechanics, rehabilitation engineering, and surgical simulation. Validation of these constitutive models using human data remains challenging and often requires the use of non-invasive imaging and inverse finite element (FE) analysis. Post-processing data from imaging methods such as tagged magnetic resonance imaging (MRI) can be challenging. Digital image correlation (DIC), however, is a relatively straightforward imaging method. DIC has been used in the past to study the planar and superficial properties of soft tissue and excised soft tissue layers. However, DIC has not been used to non-invasive study of the bulk properties of human soft tissue in vivo. Thus, the goal of this study was to assess the use of DIC in combination with FE modelling to determine the bulk material properties of human soft tissue. Indentation experiments were performed on a silicone gel soft tissue phantom. A two camera DIC setup was then used to record the 3D surface deformation. The experiment was then simulated using a FE model. The gel was modelled as Neo-Hookean hyperelastic, and the material parameters were determined by minimising the error between the experimental and FE data. The iterative FE analysis determined material parameters (micro=1.80kPa, K=2999kPa) that were in close agreement with parameters derived independently from regression to uniaxial compression tests (micro=1.71kPa, K=2857kPa). Furthermore the FE model was capable of reproducing the experimental indentor force as well as the surface deformation found (R(2)=0.81). It was therefore concluded that a two camera DIC configuration combined with FE modelling can be used to determine the bulk mechanical properties of materials that can be represented using hyperelastic Neo-Hookean constitutive laws.

The mechanical properties of the skin are important in many applications, but are not well understood. This paper presents a method for measuring the mechanical properties of human skin in vivo using digital image correlation, with a finite element model that was used to optimize the material properties to obtain the best match with the model data. The skin was modelled as an Ogden hyperelastic membrane, with a tension field wrinkling model and an initial stretch identified as an additional material parameter, and the boundary conditions were the measured load and the displacements around the edge of the region of interest. Fast, reliable convergence was obtained using a Hager–Zhang non-linear conjugate gradient solver. A stochastic optimization procedure was used to identify the material parameters. Good estimates of the material parameters could be obtained from the displacement field at a single time point. Typical material parameters were μ = 10 Pa, α = 26, and an initial strain of 0.2. These parameters were not unique; the stochastic optimization procedure gave good global convergence and an indication of the overall uncertainty in the identification of the results. It is argued that the use of the DIC technique, which generates very large amounts of data, also gave a clearer picture of the overall uncertainty.

The dart thrower’s motion is a wrist rotation along an oblique plane from radial extension to ulnar flexion. We report an in-vivo study to confirm the use of the dart thrower’s motion during activities of daily living. Global wrist motion in ten volunteers was recorded using a three-dimensional optoelectronic motion capture system, in which digital infra-red cameras track the movement of retro-reflective marker clusters. Global wrist motion has been approximated to the dart thrower’s motion when hammering a nail, throwing a ball, drinking from a glass, pouring from a jug and twisting the lid of a jar, but not when combing hair or manipulating buttons. The dart thrower’s motion is the plane of global wrist motion used during most activities of daily living. Arthrodesis of the radiocarpal joint instead of the midcarpal joint will allow better wrist function during most activities of daily living by preserving the dart thrower’s motion.

The accuracy of partial weightbearing was assessed in six healthy volunteers and 23 patients who had sustained either a fracture of a lower limb or surgery. They were trained to weightbear partially using the conventional bathroom scale method and were assessed in a gait laboratory using force platforms. The amount of weight exerted on the involved limb during three-point crutch walking was determined. Four of six volunteers exerted a mean of 27% of body weight more than required. The remaining two volunteers exerted a mean of 8.5% of body weight less than required. Of the 23 patients, 21 exerted a mean of 35.3% of body weight more than that prescribed and two patients exerted a mean of 11.97% of body weight less than that prescribed. In both groups there was little relationship between the weightbearing prescribed and actual weightbearing. None of the patients or volunteers was able to reproduce the extent of partial weightbearing for which they were trained using the bathroom scale method, confirming that this technique of instructing patients in partial weightbearing is inaccurate.