The construction of multibody biomechanical models for impact is discussed here with the emphasis on the formulation aspects. First the relations between the human or the dummy anthropometric data and the rigid bodies in the model are presented. The motion restrictions between the different anatomical segments of model can be defined as kinematic joints, suitable to represent mechanical joints of dummies or a simplified kinematics of human joints, or as contact/sliding pairs, which are used to describe realistic human like anatomical joints. Another particular aspect of biomechanical models is the representation of the range of motion of the anatomical joints. This is achieved either by setting proper contact pairs between the adjacent anatomical segments or by setting resisting muscle forces or resisting moments that develop when the relative orientation between the segments reach critical values. Another fundamental aspect of the models is the ability to represent the contact geometries and the contact forces with realism. In fact, the outcome of all injury indexes predictions is strongly dependent on the quality of the representation of the contact. Contact models suitable to be used in biomechanical models, to represent the contact between anatomical segments of the biomechanical model or between these and external objects, are presented and discussed emphasizing the requirements to develop more advanced biomechanical models. The current biomechanical models either do not include muscle actions or, at the most, include a reflexive muscle contraction. It is suggested here that for the case of standing passengers it is important to include in the biomechanical models muscle models that allow for the representation of the muscle voluntary contractions and joint stiffening. It is also suggested that the evaluation of the models leading to the identification of such actions can be done by using techniques similar to those used in the evaluation of muscle force sharing in different human motions.