Comparative data of thoracic compression response between live vs. post mortem human subjects (PMHS) has been reported, but the live subject tests are often at low deflections and include the effects of muscle tensing. Novel technology has been developed that overcomes several of these limitations. Specifically, a load cell and accelerometer has been integrated into a clinical monitor-defibrillator to measure chest compression and applied force during live human cardio-pulmonary resuscitation (CPR). The sensor is interposed between the hands of the person administering CPR and the sternum of the patient. The objective of this study was to compare the thoracic force-deflection measured during adult CPR to that measured during hubbased loading of adult PMHS. CPR represents a unique setting in which to study the mechanics of the chest as the thorax is loaded to a maximum chest deflection similar to that seen in a frontal crash environment and the effects of muscle tensing are minimized. PMHS and CPR data were modeled with a progressive spring in parallel with a viscous damper. Statistical comparison of the model parameters used to describe the compliance of the thorax during large compressions revealed that under the specific loading conditions tested, chest compressions during CPR generated less force at equivalent deflections than the PMHS. Specifically, the spring force at 40 mm deflection was 286 N for the CPR data and 588 N for the PMHS. Differences in area of load application and thoracic structures engaged, however, may partially explain these differences. In addition, the chest response during CPR demonstrated more hysteresis than the PMHS suggesting that the viscoelastic nature of the thorax is more pronounced when tissues are naturally perfused and substantial tissue autolysis has not begun. This study presents fundamental biomechanical data on the mechanical response of the adult thorax that increases our understanding of any potential differences between PMHS and chest response obtained during CPR before substantial tissue autolysis begins. We contend that the thoracic compliance measured during CPR may continue to offer important force-deflection estimates in the future.