One critical aspect in the design of an exoskeleton is its goodness of fit on the human body, preventing the generation of high pressures, which are commonly related to the perceived comfort. In this study, a multibody model of both the exoskeleton and the human body was set up, with the aim of providing a numerical tool for the ergonomic assessment of these devices. Contact force parameters were tuned to agree with experimental data found in literature, reporting peak pressure at the shoulder for a load carrying system. Then the multibody model was used to simulate a specific action (stoop) and it was validated against experimental data. Both strap tension required for the stability of the exoskeleton system and the relative displacement of the device during a 90° trunk flexion were assessed. The model was able to predict the actual system behaviour, with deviations of 7.6% and 8.9% for the predicted strip tension and displacement, respectively.