Pressure ulcers are a major problem worldwide believed to affect over 5% of all hospital in-patients, and countless others in the community at large. Many types of Patient Support Systems (PSS) are sold as pressure ulcer prevention equipment, but no consensus exists as to their mechanical efficacy. The use of human volunteers to assess the mechanical properties of PSS introduces non-repeatability and variability in results which cannot give a statistically significant difference in performance between systems. Mechanical testing without human volunteers provides faster evaluations of PSS, with improved precision and repeatability. An instrumented articulated anthropometric phantom has been developed to investigate the distortion of simulated soft body "tissues" of the buttock and sacral areas due to precise and repeatable static loading on a PSS. The weight of the phantom can be adjusted to 50, 70 and 90 kg and can be applied with the torso inclined at 0 degrees, 45 degrees and 80 degrees. Validation of the phantom by measuring interface pressure using a force sensing array mat indicates that the phantom represents realistic physiological loading conditions. The new method of measuring the distortion of the "artificial tissues" provides a highly selective ranking of PSS.