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The interface between the socket and residual limb can have a significant effect on the performance of a prosthesis. Specifically, knowledge of the rotational stiffness of the socket-residual limb (S-RL) interface is extremely useful in designing new prostheses and evaluating new control paradigms, as well as in comparing existing and new socket technologies. No previous studies, however, have examined the rotational stiffness of S-RL interfaces. To address this problem, a math model is compared to a more complex finite element analysis, to see if the math model sufficiently captures the main effects of S-RL interface rotational stiffness. Both of these models are then compared to preliminary empirical testing, in which a series of X-rays, called fluoroscopy, is taken to obtain the movement of the bone relative to the socket. Force data are simultaneously recorded, and the combination of force and movement data are used to calculate the empirical rotational stiffness of elbow S-RL interface. The empirical rotational stiffness values are then compared to the models, to see if values of Young's modulus obtained in other studies at localized points may be used to determine the global rotational stiffness of the S-RL interface. Findings include agreement between the models and empirical results and the ability of persons to significantly modulate the rotational stiffness of their S-RL interface a little less than one order of magnitude. The floor and ceiling of this range depend significantly on socket length and co-contraction levels, but not on residual limb diameter or bone diameter. Measured trans-humeral S-RL interface rotational stiffness values ranged from 24-140 Nm/rad for the four subjects tested in this study.