The objective of the present study was to determine the biomechanics of the human thorax in a simulated frontal impact. Fourteen unembalmed human cadavers were subjected to deceleration sled tests at velocities of nine or 13 m/s. Air bag - knee bolster, air bag - lap belt, and air bag - three-point belt restraint systems were used with the specimen positioned in the driver's seat. Two chest bands were used to derive the deformation patterns at the upper and lower thoracic levels. Lap and shoulder belt forces were recorded with seatbelt transducers. After the test, specimens were evaluated using palpation, radiography, and a detailed autopsy. Thoracic trauma was graded according to the Abbreviated Injury Scale based on autopsy findings. Peak thoracic deformations were normalized with respect to the initial chest depth to facilitate comparison between the specimens.Results indicated that under any restraint combination, regional differences exist in the deformation response between the upper and lower thoracic levels. The air bag - knee bolster tests indicated more uniform compressions of the thorax (based on chest band contours), demonstrated greater maximum lower chest deflections, produced fractures in the lower region of the thorax due to steering wheel contact, allowed greater hip and torso excursion, and produced significant steering wheel and column loading with permanent deformations. The air bag - lap belt experiments indicated uniform compressions of the thorax (chest band contours), produced minimal fractures, allowed greater torso excursion but less hip excursion, and produced significant steering wheel and column loading with residual deformations. The air bag - three-point belt system tests indicated high localized compressions of the thorax (chest band contours), produced multiple rib fractures consistent with shoulder belt loading, allowed less hip and torso excursion, and produced virtually no steering wheel and column loading. Based on the contours of thoracic deformation, kinematics and injury patterns, the biomechanical response of the human thorax is different between air bag - three point belt loading compared to the air bag - knee bolster/lap belt restraint combination.