This two part study investigates differences in thorax deflections between volunteers and dummies when they are dynamically loaded by diagonal shoulder belts and it shows how internal measurements at the Hybrid III dummy's sternum relate to external compressions at various points of the rib cage. Test results reveal that the thorax of the Hybrid III dummy, when loaded by a diagonal belt, is somewhat stiffer than that of volunteers for both tensed and relaxed conditions. The thorax of volunteers under dynamic belt loading deflects underneath the belt in an action similar to that of a flat rigid plate being pushed into the thorax with increasing deflection towards the lower part of the ribcage, while the dummy's deflected profile assumes a parabolic curvature.
The Hybrid III dummy's thorax deflection gage underestimates the compressions which are administered externally to the thorax by small area loading probes except in those cases in which the loading device is aligned with the sensitive axis of the deflection gage. The internal deflection gage, however, overestimates externally produced rib cage compressions when loading occurs by large area test probes which are concentric with the sensitive axis of the deflection gage. Angled compressions by the large area test probe are underestimated by the dummy's internal deflection gage measurements.