This paper presents a series of tests utilizing ten post-mortem human surrogates (PMHS) to study the effective stiffness (keff) of the thorax at realistic restraint loading rates (~1 m/s) under four loading conditions (distributed load, diagonal belt, 4-point belt, and hub). Subjects were grouped into four subgroups:​ younger males (n = 2, age ≤ 54 years), younger females (n = 2, age ≤ 58 years), older males (n = 3, age ≥ 75), and older females (n = 3, age ≥ 79). It is shown that keff is strongly dependent on the loading condition, with the lowest keff corresponding to the hub loading condition (keff = 4,750 N/ 100% deflection). The highest keff was measured with the distributed loading condition (3.1 times hub keff), followed by the 4-point belt (3.0), and the diagonal belt (2.1). The effect of age was small compared to the influence of size, but the older subjects exhibited slightly higher keff than the younger subjects of similar size, indicating a slight trend toward increasing keff as a person ages.