Many studies have documented the thorax as the most likely body region for adults to sustain injuries in the rear seat in frontal crashes. This study explored options for accurately assessing the risk of thoracic injuries for rear-seat occupants in frontal test programmes by evaluating the sensitivity of various thoracic injury metrics to force limiting, pretensioning and belt position during loading. Nineteen sled tests were conducted using the HIII 5th female dummy while varying force limiting, pretensioning and shoulder belt position. Sternum deflection, thorax acceleration, electro-optical rib deflections and pressure mat thorax group loads all showed sensitivity to both force-limiting and pretensioning. Sternum deflection and electro-optical rib deflection, however, also showed a strong linear correlation with belt position. Sternum deflection for the standard belt and 3-kN load limiter and pretensioner belt, respectively, reported 2.7% and 2.3% reductions for every 5 mm of upward belt movement, which could have implications for crash test programmes when comparing the relative protection of restraint systems. The pressure sensor provided accurate dynamic belt positions. Thus, measurement of belt position with sternum deflection for the 5th female may provide testing organisations with information to interpret vehicle crash test sternum deflection results. The pressure sensor thorax group loads had strong correlations with sternum deflection when the belt position was constant, but showed little sensitivity to belt position indicating the sensor provides relevant information on load magnitude and distribution irrespective of belt position.