This paper describes the conversion of a crash analysis program from its original batch program form with awkward input to an efficient, user-oriented interactive tool. The program simulates a vehicle occupant with a two dimensional, seven link mathematical model restrained by a seat belt and shoulder harness. A nonlinear finite element capability was added to enable modeling of a seat which would interact realistically with the occupant. A new differential equation solver was developed which achieved a sixty per cent reduction in the computer time required for the transient response analysis. The modified program incorporates user aids such as free-field data input and an on-line data edit capability. Output was reformatted to provide user-selected time history and occupant configuration plots as well as readable printout.