The dynamic behaviour of dummy occupants was investigated in a series of simulated vehicle crash tests on a Hyge sled. The performance of a spatial measurement system is described which was used to monitor dummy trajectories during the tests. This system analysed the images from three video cameras to generate the 3-dimensional coordinate data for targets on the dummy and vehicle at 200 points per second. A comparison was made of the trajectories of Hybrid II and III dummies with respect to seat belt position. Seat structure was also investigated and occupant submarining was an important phenomenon measured with the new system.