Current efforts to prevent injury to children in car accidents involve the use of pediatric anthropomorphic test devices (ATDs) which are designed based on data from adult post-mortem human subjects (PMHS) and animal surrogates, rather than from data obtained directly from the pediatric population. In this study, the forcedeflection characteristics of the pediatric and adult shoulder were measured directly using a combination of optical motion capture, resistive loading, and electromyography (EMG). The right shoulder of nine adult volunteers and ten pediatric volunteers was quasi-statically displaced using a hand-held force applicator in both medial and posteromedial directions. Each subject had reflective markers placed on the upper right arm, both acromions, the manubrium, and both epicondyles of the right elbow. The motions of the reflective markers were tracked using an eight-camera Vicon motion capture system. Surface EMG electrodes were applied to the latissimus dorsi, upper trapezius, anterior deltoid, posterior deltoid, biceps brachii, and pectoralis major to measure the level of muscle activity during loading. Three to five tests were performed for each loading direction and in both relaxed and tensed states. The resulting force-deflection curves were normalized and then shoulder stiffness was calculated. Shoulder stiffness in the medial direction could not be obtained since less than 2 mm of shoulder deflection was recorded in the medial loading direction prior to the data being truncated due to subject tilting. The shoulder stiffness in the posteromedial direction was found to be 3.8 N/mm for the 50th male, 2.4 N/mm for the 10 year old age group, and 3.7 N/mm for the 6 year old group in the relaxed condition. In the tensed condition, posteromedial shoulder stiffness was found to be 9.7 N/mm for the 50th male, 4.1 N/mm for the 10 year old age group, and 5.0 N/mm for the 6 year old age group. Statistical analyses were performed and it was found that adults had a significantly higher shoulder stiffness than the children. Tensed shoulder stiffness was found to be greater than relaxed shoulder stiffness for all age groups (p < 0.001).