The research focuses on the application of the human body model in assessment of possible seat belt induced injuries in a frontal crash setup. The THOMO model is used as a reference (www.thomo.eu). Modifications applied to the model material data mimic aging or disease, leading to weakening of the thoracic structure of an occupant. A method of estimating chest injuries basing on rib fractures is used to verify the influence of seat belt settings on the occupant. The results show increased susceptibility of the aging thorax to the rib fractures caused by the seat belt loading. A reduction of forces exerted by the belt leads to reduction of the amount of the injuries and a different pattern of fractures. The results obtained are referred to other studies.