General-purpose mechanical computer simulation software (Working Model 3D; MSC.Working Knowledge, San Mateo, Calif) was used to develop a model of a 3-year-old child stair fall. This rigid-body software is commonly used to study the motion and interaction of mechanical linkages. Various 3-D geometric shapes, joints, and mechanical constraints, eg, springs and dampers, are available in the software to construct body segments or the fall environment. Inertial properties of each model segment, eg, mass, center of gravity, and mass distribution, can also be prescribed. Surface characteristics, including friction and elasticity, which influence the contact between segments and the fall environment, also can be specified in the model. Forces and torques of specified magnitude and direction can be applied to the model segments and are used to establish initial conditions of various body segments. These parameters, with the specified inertial properties of the model segments and their surface characteristics that influence interaction with the fall environment, dictate the motion and response of the model. Simulations are controlled through equations of motion defined by principles of physics and are not constrained by animation. Measurements of force, rotational acceleration, linear acceleration, velocity, and self-defined parameters can be measured and recorded for each segment throughout the simulation event.