In vehicle crash tests, an unbelted occupant's kinetic energy is absorbed by the restraints such as an air bag and/or knee bolster and by the vehicle structure during occupant ride-down with the deforming structure. Both the restraint energy absorbed by the restraints and the ride-down energy absorbed by the structure through restraint coupling were studied in time and displacement domains using crash test data and a simple vehicle-occupant model. Using the vehicle and occupant accelerometers and/or load cell data from the 31 mph barrier crash tests, the restraint and ride-down energy components were computed for the lower extremity, such as the femur, for the light truck and passenger car respectively. Relationships between ride-down efficiency (μ, ratio of ride-down energy to initial occupant kinetic energy) and occupant response in terms of femur deceleration and load are then studied for the unbelted driver occupant in the air bag equipped light truck and passenger car respectively. Restraint and structural design parameters affecting μ in a crash test were evaluated using a simple vehicle-occupant model. Trend analysis using closed-form relationships is subsequently performed. μ is found to be closely related to two parameters, the vehicle dynamic crush and restraint stiffness. In the practical design ranges of those two parameters, an increase in either results in an increase in ride-down efficiency. However, only an increase of ride-down efficiency due to an increase in dynamic crush results in the lower occupant response. Although an increase in restraint stiffness results in an increase in higher ride-down efficiency, it also tends to increase the occupant response.