In this study an anatomically accurate 3D Finite Element (FE) model of the lower limbs was developed from axial cryosection images of the Visible Man (VM). The relative position of the lower limbs of a subject in standing and sitting positions was acquired with a laser scanner. A subset of these data points were used as control points in a novel application of the Host Mesh Fitting (HMF) technique, where the generic model geometry was morphed to subject data in the standing position, and then this subject-specific model was articulated to the seated posture. The gluteus maximus muscle and a portion of the skin mesh of the customised model were selected to provide a framework with which to examine the mechanics of sitting. Passive material properties were taken from the literature and were implemented in two two-parameter Mooney-Rivlin models to assess the response of the anatomical models to applied forces and pressures. The average deformation of the skin mesh was 0.77+/-1.525 mm which resulted in a maximum von Mises stress of 3.98 kPa. The average deformation of the gluteus maximus mesh was 2.69+/-0.6 mm which produced a maximum von Mises stress of 43 kPa. The results of the von Mises stress distribution support the theory that the highest stress occurs in the region immediately beneath the ischial tuberosities. The results of this research confirm previous conclusions reached using geometrically less complex models and the application of customisation to nonlinear mechanics provides a novel avenue to quantitatively assess office chair design and to analyse the mechanics of sitting.