Man-in-the-loop simulation uses a person in the control loop to provide feedback to the system operations. Proper operator cueing must be provided to ensure a realistic response. Real-time computer graphics and dynamics both play dominant roles in providing these necessary cues. Dynamics simulation of modern vehicles requires a multi-body non-linear approach for acceptable fidelity of motion. A vehicle can be modeled as a set of linked rigid bodies, whose connections are described by a graph. Real-time constraints on the computation of non-linear dynamics equations require the development of naturally parallel recursive algorithms, whose organization closely follows the system graph. Significant speed-up can be accomplished using these parallel algorithms.